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In view of the initial success of the complete active space (CAS) based size-extensive state-specific multireference perturbation theory (SS-MRPT) [J. Phys. Chem. A 103, 1822 (1999)] for relatively diverse yet simple chemically interesting systems, in this paper, we present the computation of the potential energy curves (PEC) of systems with arbitrary complexity and generality such as HF, H2O, H2S, C2, and N2 molecules. The ground states of such systems (and also low-lying singlet excited states of C2) possess multireference character making the description of the state difficult with single-reference (SR) methods. In this paper, we have considered the Mo\ller–Plesset (MP) partitioning scheme [SS-MRPT(MP)] method. The accuracy of energies generated via SS-MRPT(MP) method is tested through comparison with other available results. Comparison with FCI has also been provided wherever available. The accuracy of this method is also demonstrated through the calculations of NPE (nonparallelism error) and the computation of the spectroscopic constants of all the above mentioned systems. The quality of the computed spectroscopic constants is established through comparison with the corresponding experimental and FCI results. Our numerical investigations demonstrate that the SS-MRPT(MP) approach provides a balanced treatment of dynamical and non-dynamical correlations across the entire PECs of the systems considered. [ABSTRACT FROM AUTHOR]

A modified version of the optothermal technique has been used to measure photofragment angular distributions resulting from the vibrational predissociation of Ar–CO2 from both members of the (1001)/(0201) Fermi diad. These angular distributions show resolvable structure which can be assigned to individual final states of the resulting fragments. The emphasis of the present paper is on the role of the vibrational degrees of freedom of the CO2 fragment as depositories for the excess energy. The dissociation energy (D0) of the complex has been determined to be 166±1 cm-1. This result is used in conjunction with the spectroscopic data already available in the literature to refine two previously reported two-dimensional Ar–CO2 potential energy surfaces. [ABSTRACT FROM AUTHOR]

Part III. Presents a modified energy switching approach to potential energy surface for the water molecule. Increase of the size of the potential matrix; Incorporation of the proper dissociation limits in all channels pertaining to the excited [sup 1]A' electronic state; Analysis of the loci of conical intersections.

The photochemistry of the water molecule has revealed a wealth of quantum phenomena, which arise from the involvement of several coupled electronic states with very different potential energy surfaces. Most recently, dissociation from single rotational levels of its C1B1 state near 124 nm has been attributed to a vibronically coupled decay via the lower Ã-state surface, despite a large vertical energy gap of 2.8 eV. Similar conclusions have been reached for subsequent experimental data for D2O. The present paper presents further experimental data for HOD and for both the H+OD(X) and D+OH(X) products. Unlike the cases for H2O and D2O, the vibrational populations for hydroxyl products do not follow a smooth distribution with v(OH/OD). In particular, for OH there is a clear alternation in population for all the strong peaks, with odd v favoured over even v. These experimental data are analysed using new MRCI+Q calculations, which have been used to generate potential surfaces and associated non-adiabatic matrix elements for transition from the adiabatic C-state to lower unbound potential surfaces; and hence, to guide dynamical calculations using time-dependent wavepackets. It is concluded that although there is a minor contribution from the C→ à decay route, the major route follows C→ 1A2 → Ã. This is mediated through two regions of near degeneracy of the elusive 1A2 surface with C for short bonds ca. 0.8 Å; and between 1A2 and à with long bonds ≥2 Å, thereby bridging the vertical energy gap. The striking population alternation for the D+OH(X) products is attributed to dynamic symmetry breaking on the 1A2 surface. [ABSTRACT FROM AUTHOR]

Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H3+. The underlying Born-Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born-Oppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41 655 ab initio points is presented which gives a standard deviation better than 0.1 cm-1 when restricted to the points up to 6000 cm-1 above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The ro-vibrational transition frequencies for H3+, H2D+, and HD2+ are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute ro-vibrational energy levels, which makes explicit allowance for the inclusion of non-adiabatic effects, reproduces all the known ro-vibrational levels of the H3+ isotopologues considered to better than 0.2 cm-1. This represents a significant (order-of-magnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of non-adiabatic effects in the H3+ isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of H3+ resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16 000 cm-1, and (c) results suggest that we can predict accurately the lines of H3+ towards dissociation and thus facilitate their experimental observation. [ABSTRACT FROM AUTHOR]

A previous study of diatomic molecules revealed that variational second-order density matrix theory has serious problems in the dissociation limit when the N-representability is imposed at the level of the usual two-index (P,Q,G) or even three-index (T1,T2) conditions [H. Van Aggelen et al., Phys. Chem. Chem. Phys. 11, 5558 (2009)]. Heteronuclear molecules tend to dissociate into fractionally charged atoms. In this paper we introduce a general class of N-representability conditions, called subsystem constraints, and show that they cure the dissociation problem at little additional computational cost. As a numerical example the singlet potential energy surface of Be B+ is studied. The extension to polyatomic molecules, where more subsystem choices can be identified, is also discussed. [ABSTRACT FROM AUTHOR]

Subsequent to our spectroscopic detection of the HBX (X=F, Cl, Br) free radicals (S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)), the electronic spectrum of the à 2A″Π-X 2A′ system of the fluoroborane (HBF) radical in the 600–745 nm region has been studied in detail using the pulsed discharge jet technique. The band system involves a linear-bent transition between the two Renner–Teller components of what would be a 2Π state at linearity. Using the results of our theoretical study of the ground and excited state vibrational energy levels and 11B–10B isotope shifts (see the companion paper), the vibrational quantum numbers of the bands in the laser-induced fluorescence (LIF) spectra have been assigned. Rotational and vibrational analyses of the LIF and wavelength resolved emission spectra have been carried out, from which the linear excited state and the bent ground state equilibrium configurations have been confirmed. The ground state molecular geometry of HBF has been determined as r0(BH)=1.214(2) Å, r0(BF)=1.303 4(5) Å, and θ=120.7(1)°. Based on high-level ab initio calculations and symmetry considerations, predissociation of the excited state into H(2S)+BF(1Σ+) on the ground state potential energy surface is identified as the cause of the breaking off of fluorescence in the LIF spectra. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of a HX (X=Cl,F) molecule adsorbed on a hexagonal ice surface at T=0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures. [ABSTRACT FROM AUTHOR]

The neural network (NN) procedure to interpolate ab initio data for the purpose of molecular dynamics (MD) simulations has been tested on the SiO2 system. Unlike other similar NN studies, here, we studied the dissociation of SiO2 without the initial use of any empirical potential. During the dissociation of SiO2 into Si+O or Si+O2, the spin multiplicity of the system changes from singlet to triplet in the first reaction and from singlet to pentet in the second. This paper employs four potential surfaces. The first is a NN fit [NN(STP)] to a database comprising the lowest of the singlet, triplet, and pentet energies obtained from density functional calculations in 6673 nuclear configurations. The other three potential surfaces are obtained from NN fits to the singlet, triplet, and pentet-state energies. The dissociation dynamics on the singlet-state and NN(STP) surfaces are reported. The results obtained using the singlet surface correspond to those expected if the reaction were to occur adiabatically. The dynamics on the NN(STP) surface represent those expected if the reaction follows a minimum-energy pathway. This study on a small system demonstrates the application of NNs for MD studies using ab initio data when the spin multiplicity of the system changes during the dissociation process. [ABSTRACT FROM AUTHOR]

We have calculated new potential-energy surfaces for the lowest two spin-aligned 4A′ states of the Li3 trimer. This calculation shows a seam of conical intersections between these states resulting from the extra symmetry of the system when the atoms are in a collinear arrangement. This seam is especially important because of its proximity to the three-body dissociation limit of the system; ultracold scattering calculations and the bound-state energies of the system will be affected by the presence of this conical intersection. In this paper we discuss the calculation of the potential-energy surface and the location of the conical intersection seam. [ABSTRACT FROM AUTHOR]

Two recent papers presented calculations of the highly excited vibrational states of ozone [J. Chem. Phys. 119, 6512 (2003); 119, 6554 (2003)]. The nature and energies of these states may hold the key to the anomalous isotopic distribution of ozone in the atmosphere. Even though the same potential energy surface of Babikov et al. [J. Chem. Phys. 119, 2577 (2003)] was used in both calculations, the number of bound van der Waals states reported below dissociation differed significantly. In order to resolve the issue we present here the results of an independent computation of all the bound vibrational states of [sup 16]O[sup 16]O[sup 16]O and [sup 16]O[sup 16]O[sup 18]O up to dissociation. Our methods differ from both earlier calculations since we use hyperspherical coordinates and a direct product discrete variable representation of the Hamiltonian. The results of present work support the existence of several van der Waals states for J=0 on this potential energy surface. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this letter we present preliminary results of full six dimensional quantum dynamics calculations for dissociative adsorption of a hydrogen molecule on a Cu(111) surface. We utilize the time-dependent wave-packet approach to simulate the dissociation process on a full dimensional LEPS potential energy surface which has incorporated the latest ab initio data [Hammer et al. Phys. Rev. Lett. 73, 1400 (1994)]. We use a novel partitioning of the angular momentum operator in the split-operator method so that a direct product DVR can be rigorously implemented. The most interesting observation in the present rigorous quantum dynamics study is the site-averaged effect, i.e., the averaged dissociation probability of the four dimensional calculations over the three symmetric impact sites strongly resembles the exact dissociation probability of the six dimensional calculations. In accord with the low dimensional calculations, initial vibrational excitation of H2 effectively reduces the translational threshold energy. The rotational orientation effect observed in the four dimensional studies remains in the present full dimensional dynamics with the cartwheel orientation yielding dramatically lower dissociative efficiency than the energetically equivalent helicopter orientation. We focus on normal incident scattering. The diffractive scattering and more detailed results will be presented in a later paper.. [ABSTRACT FROM AUTHOR]

This is the first of a series of papers in which we investigate the unimolecular dissociation of hydroperoxyl. Using the DMBE IV potential energy surface [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)], in the present study 726 bound states of HO2(X) up to the H+O2 dissociation threshold are calculated in an attempt to access the extent of the coupling between the modes of the system. The first approach involves an analysis of the nodal structure of the wave functions. While the wave functions for the lowest states are regular and assignable, the degree of mixing and complexity rapidly increases with energy. The wave functions close to the dissociation threshold are mostly irregular without any clear cut nodal structure and fill the entire coordinate space available. Nevertheless, a small number of regular states, that are associated with large excitation in the O2 stretching coordinate and no or only little excitation in the other modes, are found even at high energies. The second approach used to study the degree of intramolecular coupling is an analysis of the energy spectrum. The nearest neighbor level spacing distribution, which probes the short-range correlation, as well as the Σ2 and Δ3 statistics, which are sensitive to the long-range correlations in the spectrum, are investigated and compared to the distributions predicted for regular and irregular spectra. Both of these approaches indicate that the system is almost totally irregular with a Brody parameter of about 92%. In addition, the sum of states at a particular energy, which is extremely important in all statistical models for unimolecular dissociation, is approximately calculated from the volume of classical phase space and found to be in excellent agreement with the exact quantum mechanical result. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The laser induced fluorescence (LIF) spectrum of jet-cooled NO2 in the energy range from 16 300 cm-1 up to the dissociation limit at 25 130.6 cm-1 was measured with an energy resolution of 0.4 cm-1, and vibronic interaction was discussed through a feature state assignment and a hierarchical analysis. By convoluting the spectrum, the feature states representing bending excited levels in the 2B2 state were identified. The hierarchical level structure just below the dissociation limit was interpreted in terms of a stepwise intramolecular vibronic energy redistribution (IVR) caused by the anharmonic couplings within the 2B2 state followed by the vibronic and rovibronic couplings between the 2B2 and 2A1 states. In a higher resolution (∼0.04 cm-1) measurement the transitions to the rovibronic eigenstates just (0–55 cm-1) below the dissociation limit were resolved. The observed vibronic level density having b2 symmetry, ρvib(b2)=1.6/cm-1, in this energy region is derived from the observed peak density, ρpeak=9.6/cm-1, by assuming a strong K mixing. The observed large peak density was ascribed to the large anharmonisity of the ground state potential energy surface near the dissociation limit. The statistical analyses applied to this eigenstate spectrum showed an extremely strong correlation among these eigenstates, indicating the complete IVR. The present results of the statistical analyses near the dissociation limit support the statistical behavior in the dissociation dynamics just above the dissociation limit investigated in our previous paper [J. Chem. Phys. 99, 254 (1993)]. [ABSTRACT FROM AUTHOR]

Quasi-classical trajectory (QCT) calculations are used to study state-specific ro-vibrational energy exchange and dissociation in the O2 + O system. Atom-diatom collisions with energy between 0.1 and 20 eV are calculated with a double many body expansion potential energy surface by Varandas and Pais [Mol. Phys. 65, 843 (1988)]. Inelastic collisions favor mono-quantum vibrational transitions at translational energies above 1.3 eV although multi-quantum transitions are also important. Post-collision vibrational favoring decreases first exponentially and then linearly as Δv increases. Vibrationally elastic collisions (Δv = 0) favor small ΔJ transitions while vibrationally inelastic collisions have equilibrium post-collision rotational distributions. Dissociation exhibits both vibrational and rotational favoring. New vibrational-translational (VT), vibrational-rotational-translational (VRT) energy exchange, and dissociation models are developed based on QCT observations and maximum entropy considerations. Full set of parameters for state-to-state modeling of oxygen is presented. The VT energy exchange model describes 22 000 state-to-state vibrational cross sections using 11 parameters and reproduces vibrational relaxation rates within 30% in the 2500.20 000 K temperature range. The VRT model captures 80 × 106 state-to-state ro-vibrational cross sections using 19 parameters and reproduces vibrational relaxation rates within 60% in the 5000.15 000 K temperature range. The developed dissociation model reproduces state-specific and equilibrium dissociation rates within 25% using just 48 parameters. The maximum entropy framework makes it feasible to upscale ab initio simulation to full nonequilibrium flow calculations. [ABSTRACT FROM AUTHOR]

Dissociations after the à 1B1→B 1A1 photoexcitation of SiH2, SiHD, and SiD2 were studied to investigate excited-state dynamics and effects of the initial vibrational state. The cross section (σ) for the photodissociation relative to SiH2(B)→Si(1D)+H2 and the rovibrational population of the H2 fragment were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces (PESs) of the à and B electronic states and the transition dipole surfaces, which were reported in our previous paper [J. Chem. Phys. 122, 144307 (2005)]. The photodissociation spectrum consists of a broadband and a number of sharp peaks. For SiH2 and SiD2, the sharp peaks correspond to the resonance structure of the vibrational levels of the B state and the broadbands are nearly independent of the photon energy. The broadband for SiHD increases steeply with the photon energy above 30 000 cm-1. The flux leaving the computational grid for SiH2 and SiD2 consists of at least two components, whereas that for SiHD consists of only a faster component. These large isotope effects were discussed based on the valley to the dissociation channel on PES and the difference in the position of the initial wave packet for three isotopomers. [ABSTRACT FROM AUTHOR]

This paper reports measurements of the homogeneous predissociation of the Li[sub 2] F [sup 1]Σ[sub g][sup +] state due to electrostatic interaction with the E [sup 1]Σ[sub g][sup +] state. Ab initio potential energy curves have been calculated for both states which in the adiabatic representation show two avoided crossings. However, predissociation was not previously predicted. Our experimental results show that the three isotopomers [sup 7]Li[sub 2], [sup 6]Li[sup 7]Li, and [sup 6]Li[sub 2] all strongly predissociate above the 2s+3s atomic limit. We report high resolution measurements of linewidths for a large number of F-state levels spread across the 2600 cm[sup -1] energy region between the 2s+3s and 2p+2p atomic limits, which yield systematic information regarding the rotational, vibrational, and isotopomer dependence of the predissociation rate. An experimental RKR potential energy curve for the F state is derived and used to calculate predissociation rates whose trends show good agreement with the experimental values. This paper presents the first complete data set of observations on the predissociation rate of a diatomic molecule and its variation with v and J when the interaction with the perturbing state takes place at two distinct internuclear distances. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A trajectory surface-hopping study of collisions of H+2 (v)+He for v=0, 3, 6, and 10 has been carried out on the two lowest potential-energy surfaces at relative collision energies of 3.1, 5, and 10 eV. The diatomics-in-molecules (DIM) surfaces of Whitton and Kuntz, suitably modified at large internuclear distances, were used in the calculations. The probability for hopping between the two surfaces was calculated using the Demkov formalism. Both total cross sections and velocity vector distributions are reported. The reactive cross sections to give HeH++H were not affected by the accessibility of the excited potential surface. By comparison, the results for collision-induced dissociation (CID) to give He+H++H were quite revealing. In an earlier paper we have shown that it is possible to distinguish the two (nearly) degenerate product states in CID. The present work shows that between 33% and 45% of the CID products appear in the excited electronic state. The H+ velocity distributions are quite different in the two product states. The results obtained here are compared with other theoretical and experimental work on this system. [ABSTRACT FROM AUTHOR]

When a chemical bond is broken in a direct dissociation reaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10-13 s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as ‘‘transition states.’’ Femtosecond transition-state spectroscopy (FTS) is a real-time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real-time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s). [ABSTRACT FROM AUTHOR]

In this paper we report the results of a classical trajectory study of collinear H+H2 on the PK2 surface at fixed total energies slightly above the dissociation threshold. Two energies, 4.85 and 5.00 eV, were investigated. The method of Andrews and Chesnavich was used to resolve the reagents phase space into nonreactive, reactive, and dissociative regions. A striking feature of the phase space banding structure is that it is remarkably simple. Also, all trajectories exhibit regular and smooth behavior with respect to changes in initial condition. No evidence was found of either periodic trajectories or of ‘‘snarled’’ trajectories. Some general properties of the dynamics are discussed, including a suggested explanation as to why all trajectories in one of the reactive bands cross the symmetric stretch line three times. Some suggestions for future work, including model studies which could provide information on phase space banding in three-dimensional systems, are also given. [ABSTRACT FROM AUTHOR]

Examines the theoretical calculation of the trajectory surface hopping (TSH) for collisions of Ar++H2 and Ar++HD on three low potential energy surfaces. Determination of location and probability of hops between surfaces with the Parlant and Gislason algorithm; Dissociation of excited ArD+ products; Propagation of trajectories with the TSH method.

Thermal dissociation reaction into polyatomic molecules on the ground state no-barrier potential energy surface is studied by classical molecular dynamics simulations of N2O4⇋2NO2. A phase space surface ET=Veff(Rl)>=0 is identified as the transition state (TS), where ET is the sum of the potential and kinetic energies of interfragment motion and Veff(Rl) is the orbital angular momentum-dependent effective barrier. By dividing the motion of the system into fragments’ vibrational (V), rotational (R), and interfragment (T) modes, where the T mode is composed of translational (TT) and orbital (TL) modes, a scheme of reactive energy transfer for fragmentation is presented. The present energy condition for the TS is in accord with the one of phase space theory (PST). The observed photofragmentation rates of NCNO and CH2CO which increase with energy slower than predicted by PST suggest that intrareactant energy redistribution may influence the rate. Dissociation is found to occur by energy redistribution among T-R-V modes followed by the one among TT-TL-R modes, which determine the product vibrational and rotational distributions, respectively. This scheme supports separate statistical ensemble method in reproducing the nascent distributions from unimolecular photofragmentation at excess energies above the vibrational threshold. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We develop a theory and the method of its application for chemical dynamics in systems, in which the adiabatic potential energy hyper-surfaces (PES) are densely quasi-degenerate to each other in a wide range of molecular geometry. Such adiabatic electronic states tend to couple each other through strong nonadiabatic interactions. Technically, therefore, it is often extremely hard to accurately single out the individual PES in those systems. Moreover, due to the mutual nonadiabatic couplings that may spread wide in space and due to the energy-time uncertainty relation, the notion of the isolated and well-defined potential energy surface should lose the sense. On the other hand, such dense electronic states should offer a very interesting molecular field in which chemical reactions to proceed in characteristic manners. However, to treat these systems, the standard theoretical framework of chemical reaction dynamics, which starts from the Born-Oppenheimer approximation and ends up with quantum nuclear wavepacket dynamics, is not very useful. We here explore this problem with our developed nonadiabatic electron wavepacket theory, which we call the phase-space averaging and natural branching (PSANB) method [T. Yonehara and K. Takatsuka, J. Chem. Phys. 129, 134109 (2008)], or branching-path representation, in which the packets are propagated in time along the non-Born-Oppenheimer branching paths. In this paper, after outlining the basic theory, we examine using a one-dimensional model how well the PSANB method works with such densely quasi-degenerate nonadiabatic systems. To do so, we compare the performance of PSANB with the full quantum mechanical results and those given by the fewest switches surface hopping (FSSH) method, which is known to be one of the most reliable and flexible methods to date. It turns out that the PSANB electron wavepacket approach actually yields very good results with far fewer initial sampling paths. Then we apply the electron wavepacket dynamics in path-branching representation and the so-called semiclassical Ehrenfest theory to a hydrogen molecule embedded in twelve membered boron cluster (B12) in excited states, which are densely quasi-degenerate due to the vacancy in 2p orbitals of boron atom [1s22s22p1]. Bond dissociation of the hydrogen molecule quickly takes place in the cluster and the resultant hydrogen atoms are squeezed out to the surface of the cluster. We further study collision dynamics between H2 and B12, which also gives interesting phenomena. The present study suggests an interesting functionality of the boron clusters. [ABSTRACT FROM AUTHOR]

We report state-to-state quasi-classical trajectory rate constants for the D + H2 reactive collision, using the accurate H3 global potential energy surface of Mielke et al. [J. Chem. Phys. 116, 4142 (2002)]. High relative collision energies (up to ≈56 000 K) and high rovibrational levels of H2 (up to ≈50 000 K), relevant to various astrophysical media, are considered. The HD product molecule is formed in highly excited rovibrational states, over a wide collision energy range. The collision-induced dissociation channel (often overlooked in fully quantum reaction dynamics calculations) is found to be significantly populated, even at collision energies as low as 1500 K. [ABSTRACT FROM AUTHOR]

In recent years, ozone and its isotopologues have been a topic of interest in many fields of research, due to its importance in atmospheric chemistry and its anomalous isotopic enrichment—or the so-called “mass-independent fractionation.” In the field of potential energy surface (PES) creation, debate over the existence of a potential barrier just under the dissociation threshold (referred to as a “potential reef”) has plagued research for some years. Recently, Dawes and co-workers [Dawes, Lolur, Li, Jiang, and Guo (DLLJG) J. Chem. Phys. 139, 201103 (2013)] created a highly accurate global PES, for which the reef is found to be replaced with a (monotonic) “plateau.” Subsequent dynamical calculations on this “DLLJG” PES have shown improved agreement with experiment, particularly the vibrational spectrum. However, it is well known that reaction dynamics is also highly influenced by the rovibrational states, especially in cases like ozone that assume a Lindemann-type mechanism. Accordingly, we present the first significant step toward a complete characterization of the rovibrational spectrum for various isotopologues of ozone, computed using the DLLJG PES together with the ScalIT suite of parallel codes. Additionally, artificial neural networks are used in an innovative fashion—not to construct the PES function per se but rather to greatly speed up its evaluation. [ABSTRACT FROM AUTHOR]

A new approach for preventing zero-point energy (ZPE) violation in quasi-classical trajectory (QCT) simulations is presented and applied to H2CO “roaming” reactions. Zero-point energy may be problematic in roaming reactions because they occur at or near bond dissociation thresholds and these channels may be incorrectly open or closed depending on if, or how, ZPE has been treated. Here we run QCT simulations on a “ZPE-corrected” potential energy surface defined as the sum of the molecular potential energy surface (PES) and the global harmonic ZPE surface. Five different harmonic ZPE estimates are examined with four, on average, giving values within 4 kJ/mol—chemical accuracy—for H2CO. The local harmonic ZPE, at arbitrary molecular configurations, is subsequently defined in terms of “projected” Cartesian coordinates and a global ZPE “surface” is constructed using Shepard interpolation. This, combined with a second-order modified Shepard interpolated PES, V, allows us to construct a proof-of-concept ZPE-corrected PES for H2CO, Veff, at no additional computational cost to the PES itself. Both V and Veff are used to model product state distributions from the H + HCO → H2 + CO abstraction reaction, which are shown to reproduce the literature roaming product state distributions. Our ZPE-corrected PES allows all trajectories to be analysed, whereas, in previous simulations, a significant proportion was discarded because of ZPE violation. We find ZPE has little effect on product rotational distributions, validating previous QCT simulations. Running trajectories on V, however, shifts the product kinetic energy release to higher energy than on Veff and classical simulations of kinetic energy release should therefore be viewed with caution. [ABSTRACT FROM AUTHOR]

Methane dissociation is a prototypical system for the study of surface reaction dynamics. The dissociation and recombination rates of CH4 through the Ni(111) surface are calculated by using the quantum instanton method with an analytical potential energy surface. The Ni(111) lattice is treated rigidly, classically, and quantum mechanically so as to reveal the effect of lattice motion. The results demonstrate that it is the lateral displacements rather than the upward and downward movements of the surface nickel atoms that affect the rates a lot. Compared with the rigid lattice, the classical relaxation of the lattice can increase the rates by lowering the free energy barriers. For instance, at 300 K, the dissociation and recombination rates with the classical lattice exceed the ones with the rigid lattice by 6 and 10 orders of magnitude, respectively. Compared with the classical lattice, the quantum delocalization rather than the zero-point energy of the Ni atoms further enhances the rates by widening the reaction path. For instance, the dissociation rate with the quantum lattice is about 10 times larger than that with the classical lattice at 300 K. On the rigid lattice, due to the zero-point energy difference between CH4 and CD4, the kinetic isotope effects are larger than 1 for the dissociation process, while they are smaller than 1 for the recombination process. The increasing kinetic isotope effect with decreasing temperature demonstrates that the quantum tunneling effect is remarkable for the dissociation process. [ABSTRACT FROM AUTHOR]

Quasi-classical trajectory (QCT) calculations are used in this work to calculate state-specific N2(X1Σ) + O(3P)→2N(4S) + O(3P) dissociation and N2(X1Σ) + O(3P)→NO(X2II) + N(4S) exchange cross sections and rates based on the 13A" and 13A' ab initio potential energy surface by Gamallo et al. [J. Chem. Phys. 119, 2545-2556 (2003)]. The calculations consider translational energies up to 23 eV and temperatures between 1000 K and 20 000 K. Vibrational favoring is observed for dissociation reaction at the whole range of collision energies and for exchange reaction around the dissociation limit. For the same collision energy, cross sections for 3 = 30 are 4 to 6 times larger than those for the ground state. The exchange reaction has an effective activation energy that is dependent on the initial rovibrational level, which is different from dissociation reaction. In addition, the exchange cross sections have a maximum when the total collision energy (TCE) approaches dissociation energy. The calculations are used to generate compact QCT-derived state-specific dissociation (QCT-SSD) and QCT-derived state-specific exchange (QCT-SSE) models, which describe over 1 x 106 cross sections with about 150 model parameters. The models can be used directly within direct simulation Monte Carlo and computational fluid dynamics simulations. Rate constants predicted by the new models are compared to the experimental measurements, directQCTcalculations and predictions by other models that include: TCE model, Bose-Candler QCT-based exchange model, Macheret-Fridman dissociation model, Macheret's exchange model, and Park's two-temperature model. The new models match QCTcalculated and experimental rates within 30% under nonequilibrium conditions while other models under predict by over an order of magnitude under vibrationally-cold conditions. [ABSTRACT FROM AUTHOR]

We have performed calculations on the dissociative chemisorption of H2 on un-reconstructed and reconstructed Au(111) with density functional theory, and dynamics calculations on this process on un-reconstructed Au(111). Due to a very late barrier for dissociation, H2 + Au(111) is a candidate H2-metal system for which the dissociative chemisorption could be considerably affected by the energy transfer to electron-hole pairs. Minimum barrier geometries and potential energy surfaces were computed for six density functionals. The functionals tested yield minimum barrier heights in the range of 1.15-1.6 eV, and barriers that are even later than found for the similar H2 + Cu(111) system. The potential energy surfaces have been used in quasi-classical trajectory calculations of the initial (v,J) state resolved reaction probability for several vibrational states v and rotational states J of H2 and D2. Our calculations may serve as predictions for state-resolved associative desorption experiments, from which initial state-resolved dissociative chemisorption probabilities can be extracted by invoking detailed balance. The vibrational efficacy ηv=0→1 reported for D2 dissociating on un-reconstructed Au(111) (about 0.9) is similar to that found in earlier quantum dynamics calculations on H2 + Ag(111), but larger than found for D2 + Cu(111). With the two functionals tested most extensively, the reactivity of H2 and D2 exhibits an almost monotonic increase with increasing rotational quantum number J. Test calculations suggest that, for chemical accuracy (1 kcal/mol), the herringbone reconstruction of Au(111) should be modeled. [ABSTRACT FROM AUTHOR]

A four-component (4c) relativistic state specific multireference coupled cluster (4c-SSMRCC) method has been developed and applied to compute the ground state spectroscopic constants of Ag2, Cu2, Au2, and I2. The reference functions used in these calculations are obtained using computationally inexpensive improved virtual orbital-complete active space configuration interaction scheme. Rigorous size-extensivity and insensitivity towards the intruder state problem make our method an interesting choice for the calculation of the dissociation energy surface. To the best of our knowledge, this study is the first implementation of the SSMRCC within the relativistic framework. The overall agreement of our results, employing the smallest model space, with both theoretical and experimental reference values indicates that the 4c-SSMRCC method can be fruitfully used to describe electronic structures and associated properties of systems containing heavy elements. We observe a relativistic bond stabilization for the coinage metal dimers while the I-I bond is weakened by the relativistic effects. [ABSTRACT FROM AUTHOR]

We present the first measurement of a bound-state spectrum of the NO-He complex. The recorded spectrum is associated with the first overtone transition of the NO moiety. The IR absorption is detected by exciting the vibrationally excited complex to the Ã-state dissociation continuum. The resulting NO(A) fragment is subsequently ionized in the same laser pulse. We recorded two bands centered around the NO monomer rotational lines, Q11(0.5) and R11(0.5), consistent with an almost free rotation of the NO fragment within the complex. The origin of the spectrum is found at 3724.06 cm-1 blue shifted by 0.21 cm-1 from the corresponding NO monomer origin. The rotational structures of the spectrum are found to be in very good agreement with calculated spectra based on bound states derived from a set of high level ab initio potential energy surfaces [Kłos et al. J. Chem. Phys. 112, 2195 (2000)]. [ABSTRACT FROM AUTHOR]

A method for calculations of rotational-vibrational states of triatomic molecules up to dissociation threshold (and scattering resonances above it) is devised, that combines hyper-spherical coordinates, sequential diagonalization-truncation procedure, optimized grid DVR, and complex absorbing potential. Efficiency and accuracy of the method and new code are tested by computing the spectrum of ozone up to dissociation threshold, using two different potential energy surfaces. In both cases good agreement with results of previous studies is obtained for the lower energy states localized in the deep (~10 000 cm-1) covalent well. Upper part of the bound state spectrum, within 600 cm-1 below dissociation threshold, is also computed and is analyzed in detail. It is found that long progressions of symmetric-stretching and bending states (up to 8 and 11 quanta, respectively) survive up to dissociation threshold and even above it, whereas excitations of the asymmetric-stretching overtones couple to the local vibration modes, making assignments difficult. Within 140 cm-1 below dissociation threshold, large-amplitude vibrational states of a floppy complex O…O2 are formed over the shallow van derWaals plateau. These are assigned using two local modes: the rocking-motion and the dissociative-motion progressions, up to 6 quanta in each, both with frequency ~20 cm-1. Many of these plateau states are mixed with states of the covalent well. Interestingly, excitation of the rocking-motion helps keeping these states localized within the plateau region, by raising the effective barrier. [ABSTRACT FROM AUTHOR]

A new accurate global potential energy surface for the ground electronic state of ozone [R. Dawes et al., J. Chem. Phys. 139, 201103 (2013)] was published fairly recently. The topography near dissociation differs significantly from previous surfaces, without spurious submerged reefs and corresponding van derWaals wells. This has enabled significantly improved descriptions of scattering processes, capturing the negative temperature dependence and large kinetic isotope effects in exchange reaction rates. The exchange reactivity was found to depend on the character of near-threshold resonances and their overlap with reactant and product wavefunctions, which in turn are sensitive to the potential. Here we present global "three-well" calculations of all bound vibrational states of three isotopic combinations of ozone (48O3, 16O2 18O, 16O2 17O) for J = 0 and J = 1 with a focus on the character and density of highly excited states and discuss their impact on the ozone isotopic anomaly. The calculations were done using a parallel symmetry-adapted Lanczos method with the RV3 code. Some comparisons were made with results obtained with the improved relaxation method implemented in the Heidelberg multi-configuration time-dependent Hartree code. [ABSTRACT FROM AUTHOR]

We present a detailed theoretical approach to investigate the laser-induced dissociation dynamics of a triatomic molecule on its electronic excited state in full dimensional case. In this method, the time evolution of the time-dependent system is propagated via combined the split operator method and the expansion of Chebyshev polynomials (or short-time Chebyshev propagation) and the system wave functions are expanded in terms of molecular rotational bases. As an example of the application of this formalism, the dissociation dynamics of H3+ → H²+ + H induced by ultrashort UV laser pulses are investigated on new Born-Oppenheimer potential energy surfaces. Our numerical results show that the signals of dissociation products will be easier to observe as the increasing of field strength. Driving by a 266 nm laser beam, the calculated central value of kinetic-energy-release is 2.04 eV which shows excellent agreement with the experimental estimation of 2.1 eV. When the H3+ ion is rotationally excited, the spatial distribution of product fragments will become well converged. [ABSTRACT FROM AUTHOR]

A fully ab initio-based potential energy surface is first reported for the ground electronic state of the C3 radical using the double many-body expansion (DMBE) method. The DMBE form so obtained mimics the full set of energies calculated at the multireference configuration interaction level of theory with chemical accuracy. To account for the incompleteness of the one- and Nelectron bases, the calculated external correlation energies have been scaled prior to the fitting procedure via DMBE-scaled external correlation method. Furthermore, the novel potential energy surface reproduces accurately dissociation energies, diatomic potentials, long-range interactions at all asymptotic channels, and the correct topological behavior at the region of 4 conical intersections with the partner state of the same symmetry near equilateral triangular geometries due to combined Jahn-Teller (E' ⊗ e') plus pseudo-Jahn-Teller [(E' + A'1) ⊗ e'] interactions. Rovibrational calculations have also been performed, unveiling a good match of the vibrational spectrum of C3 for 53 calculated levels. The present DMBE form is, therefore, commended for both spectroscopic and reaction dynamics studies, some also performed in the present work. [ABSTRACT FROM AUTHOR]

Accurate modeling of high-temperature hypersonic flows in the atmosphere requires consideration of collision-induced dissociation of molecular species and energy transfer between the translational and internal modes of the gas molecules. Here, we describe a study of the N2 + N2 → N2 + 2N and N2 + N2 → 4N nitrogen dissociation reactions using the quasiclassical trajectory (QCT) method. The simulations used a new potential energy surface for the N4 system; the surface is an improved version of one that was presented previously. In the QCT calculations, initial conditions were determined based on a two-temperature model that approximately separates the translationalrotational temperature from the vibrational temperature of the N2 diatoms. Five values from 8000 K to 30 000 K were considered for each of the two temperatures. Over 2.4 × 109 trajectories were calculated. We present results for ensemble-averaged dissociation rate constants as functions of the translational-rotational temperature T and the vibrational temperature Tv. The rate constant depends more strongly on T when Tv is low, and it depends more strongly on Tv when T is low. Quasibound reactant states contribute significantly to the rate constants, as do exchange processes at higher temperatures. We discuss two sets of runs in detail: an equilibrium test set in which T = Tv and a nonequilibrium test set in which Tv < T. In the equilibrium test set, high-v and moderately-low-j molecules contribute most significantly to the overall dissociation rate, and this state specificity becomes stronger as the temperature decreases. Dissociating trajectories tend to result in a major loss of vibrational energy and a minor loss of rotational energy. In the nonequilibrium test set, as Tv decreases while T is fixed, higher- j molecules contribute more significantly to the dissociation rate, dissociating trajectories tend to result in a greater rotational energy loss, and the dissociation probability's dependence on v weakens. In this way, as Tv decreases, rotational energy appears to compensate for the decline in average vibrational energy in promoting dissociation. In both the equilibrium and nonequilibrium test sets, in every case, the average total internal energy loss in the dissociating trajectories is between 10.2 and 11.0 eV, slightly larger than the equilibrium potential energy change of N2 dissociation. [ABSTRACT FROM AUTHOR]

Accurately modeling the chemisorption dynamics of N2 on metal surfaces is of both practical and fundamental interest. The factors that may have hampered this achievement so far are the lack of an accurate density functional and the use of approximate methods to deal with surface phonons and non-adiabatic effects. In the current work, the dissociation of molecular nitrogen on W(110) has been studied using ab initio molecular dynamics (AIMD) calculations, simulating both surface temperature effects, such as lattice distortion, and surface motion effects, like recoil. The forces were calculated using density functional theory, and two density functionals were tested, namely, the Perdew-Burke-Ernzerhof (PBE) and the revised PBE (RPBE) functionals. The computed dissociation probability considerably differs from earlier static surface results, with AIMD predicting a much larger contribution of the indirect reaction channel, in which molecules dissociate after being temporally trapped in the proximity of the surface. Calculations suggest that the surface motion effects play a role here, since the energy transfer to the lattice does not allow molecules that have been trapped into potential wells close to the surface to find their way back to the gas phase. In comparison to experimental data, AIMD results overestimate the dissociation probability at the lowest energies investigated, where trapping dominates, suggesting a failure of both tested exchange-correlation functionals in describing the potential energy surface in the area sampled by trapped molecules. [ABSTRACT FROM AUTHOR]

Using ab initio molecular dynamics (AIMD) calculations, we investigate the role of the van der Waals (vdW) interaction in the dissociative adsorption of N 2 on W(110). Hitherto, existing classical dynamics calculations performed on six-dimensional potential energy surfaces based on density functional theory (DFT), and the semi-local PW91 and RPBE [Hammer et al. Phys. Rev. B 59, 7413 (1999)] exchange-correlation functionals were unable to fully describe the dependence of the initial sticking coefficient on the molecular beam incidence conditions as found in experiments. N 2 dissociation on W(110) was shown to be very sensitive not only to short molecule-surface distances but also to large distances where the vdW interaction, not included in semilocal-DFT, should dominate. In this work, we perform a systematic study on the dissociative adsorption using a selection of existing non-local functionals that include the vdW interaction (vdW-functionals). Clearly, the inclusion of the non-local correlation term contributes in all cases to correct the unrealistic energy barriers that were identified in the RPBE at large molecule-surface distances. Among the tested vdW-functionals, the original vdW-DF by Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)] and the ulterior vdW-DF2 give also an adequate description of the N2 adsorption energy and energy barrier at the transition state, i.e., of the properties that are commonly used to verify the quality of any exchange-correlation functional. However, the results of our A I M D calculations, which are performed at different incidence conditions and hence extensively probe the multi-configurational potential energy surface of the system, do not seem as satisfactory as the preliminary static analysis suggested. When comparing the obtained dissociation probabilities with existing experimental data, none of the used vdW-functionals seems to provide altogether an adequate description of the N2/W(110) interaction at short and large distances. [ABSTRACT FROM AUTHOR]

The singlet electronic potential energy surfaces for the simplest Criegee intermediate CH2OO are computed over a two-dimensional reduced subspace of coordinates, and utilized to simulate the photo-initiated dynamics on the S2 (B) state leading to dissociation on multiple coupled excited electronic states. The adiabatic electronic potentials are evaluated using dynamically weighted stateaveraged complete active space self-consistent field theory. Quasi-diabatic states are constructed from the adiabatic states by maximizing the charge separation between the states. The dissociation dynamics are then simulated on the diabatically coupled excited electronic states. The B<-X electronic transition with large oscillator strength was used to initiate dynamics on the S2 (B) excited singlet state. Diabatic coupling of the B state with other dissociative singlet states results in about 5% of the population evolving to the lowest spin-allowed asymptote, generating H2CO (X¹A1) and O (¹D) fragments. The remaining ~95% of the population remains on repulsive B state and dissociates to H2CO (a³A") and O (³P) products associated with a higher asymptotic limit. Due to the dissociative nature of the B state, the simulated electronic absorption spectrum is found to be broad and devoid of any vibrational structure. [ABSTRACT FROM AUTHOR]

A new, full (nine)-dimensional potential energy surface and dipole moment surface to describe the NO+(H2O) cluster is reported. The PES is based on fitting of roughly 32000 CCSD(T)-F12/aug-cc-pVTZ electronic energies. The surface is a linear least-squares fit using a permutationally invariant basis with Morse-type variables. The PES is used in a Diffusion Monte Carlo study of the zero-point energy and wavefunction of the NO+(H2O) and NO+(D2O) complexes. Using the calculated ZPE the dissociation energies of the clusters are reported. Vibrational configuration interaction calculations of NO+(H2O) and NO+(D2O) using the MULTIMODE program are performed. The fundamental, a number of overtone, and combination states of the clusters are reported. The IR spectrum of the NO+(H2O) cluster is calculated using 4, 5, 7, and 8 modes VSCF/CI calculations. The anharmonic, coupled vibrational calculations, and IR spectrum show very good agreement with experiment. Mode coupling of the water "antisymmetric" stretching mode with the low-frequency intermolecular modes results in intensity borrowing. [ABSTRACT FROM AUTHOR]

In this work, we present a density functional theory study of the structure and stability of neutral and positively-charged coronene C24Hq+ . In particular, we have investigated (i) adiabatic and vertical ionization potentials up to charge q = 9, (ii) the corresponding infrared spectra, and (iii) dissociation energies and potential energy surfaces for several hydrogen loss channels: sequential H+H, H+H+, H++H, H++H+, and direct H12 . In particular, we have investigated (i) adiabatic and vertical ionization potentials up to charge q = 9, (ii) the corresponding infrared spectra, and (iii) dissociation energies and potential energy surfaces for several hydrogen loss channels: sequential H+H, H+H+, H++H, H++H+, and direct H2 and H+2 . We have found that the stability of positively-charged coronene is extremely high as a consequence of the molecule's capability to redistribute the charge all over the structure. The computed dissociation energies and fragmentation barriers show that there is competition between different hydrogen loss channels and that the relative importance of these channels depends on the charge of the molecule. From a careful analysis of the potential energy surface we conclude that the channel with the lowest barrier corresponds to the loss of H2 from neutral, singly-, doubly-, and triply-charged coronene, H+ 2 from quadruply-charged coronene and H++H+ from quintuply-charged coronene. [ABSTRACT FROM AUTHOR]

The six-dimensional quantum dynamics calculations for the dissociative chemisorption of HCl on Au(111) are carried out using the time-dependent wave-packet approach, based on an accurate PES which was recently developed by neural network fitting to density functional theory energy points. The influence of vibrational excitation and rotational orientation of HCl on the reactivity is investigated by calculating the exact six-dimensional dissociation probabilities, as well as the four-dimensional fixed-site dissociation probabilities. The vibrational excitation of HCl enhances the reactivity and the helicopter orientation yields higher dissociation probability than the cartwheel orientation. A new interesting site-averaged effect is found for the title molecule-surface system that one can essentially reproduce the six-dimensional dissociation probability by averaging the four-dimensional dissociation probabilities over 25 fixed sites. [ABSTRACT FROM AUTHOR]

We investigate the analytical representation of potentials of mean force (pmf) using the Morse/long-range (MLR) potential approach. The MLR method had previously been used to represent potential energy surfaces, and we assess its validity for representing free-energies. The advantage of the approach is that the potential of mean force data only needs to be calculated in the short to medium range region of the reaction coordinate while the long range can be handled analytically. This can result in significant savings in terms of computational effort since one does not need to cover the whole range of the reaction coordinate during simulations. The water dimer with rigid monomers whose interactions are described by the commonly used TIP4P model [W. Jorgensen and J. Madura, Mol. Phys. 56, 1381 (1985)] is used as a test case. We first calculate an 'exact' pmf using direct Monte Carlo (MC) integration and term such a calculation as our gold standard (GS). Second, we compare this GS with several MLR fits to the GS to test the validity of the fitting procedure. We then obtain the water dimer pmf using metadynamics simulations in a limited range of the reaction coordinate and show how the MLR treatment allows the accurate generation of the full pmf. We finally calculate the transition state theory rate constant for the water dimer dissociation process using the GS, the GS MLR fits, and the metadynamics MLR fits. Our approach can yield a compact, smooth, and accurate analytical representation of pmf data with reduced computational cost. [ABSTRACT FROM AUTHOR]

High-energy electron transfer dissociation (HE-ETD) on collisions with alkali metal targets (Cs, K, and Na) was investigated for CH3X+ (X = Cl, Br, and I) ions by a charge inversion mass spectrometry. Relative peak intensities of the negative ions formed via HE-ETD strongly depend on the precursor ions and the target alkali metals. The dependency is explained by the exothermicities of the respective dissociation processes. Peak shapes of the negative ions, especially of the X- ions, which comprise a triangle and a trapezoid, also strongly depend on the precursor ions and the target alkali metals. The trapezoidal part of the I- peak observed with the Na target is more dominant and much broader than that with the Cs target. This dependence on the targets shows an inverse relation between the peak width and the available energy, which corresponds to the exothermicity assuming formation of fragment pair in their ground internal states. From a comparison of the kinetic energy release value calculated from the trapezoidal shape of I- with the available energy of the near-resonant level on the CH3I potential energy curve reported by ab initio calculations, the trapezoidal part is attributed to the dissociation to CH3 + I(2P3/2) via the repulsive 3Q1 state of CH3I, which is not dominant in the photo-dissociation of CH3I. The observation of trapezoid shape of the CH2I- peak with the Cs target indicates spontaneous dissociation via repulsive potential from the 3R2 Rydberg state, although the correlation between the 3R2 Rydberg state and relevant repulsive states has not been reported by any theoretical calculation. [ABSTRACT FROM AUTHOR]

We present a detailed theoretical study of the stability of the gas-phase diatomic dications SnF2+, SnCl2+, and SnO2+ using ab initio computer calculations. The ground states of SnF2+, SnCl2+, and SnO2+ are thermodynamically stable, respectively, with dissociation energies of 0.45, 0.30, and 0.42 eV. Whereas SnF2+ dissociates into Sn2+ + F, the long range behaviour of the potential energy curves of SnCl2+ and SnO2+ is repulsive and wide barrier heights due to avoided crossing act as a kind of effective dissociation energy. Their equilibrium internuclear distances are 4.855, 5.201, and 4.852 a0, respectively. The double ionisation energies (Te) to form SnF2+, SnCl2+, and SnO2+ from their respective neutral parents are 25.87, 23.71, and 25.97 eV. We combine our theoretical work with the experimental results of a search for these doubly positively charged diatomic molecules in the gas phase. SnO2+ and SnF2+ have been observed for prolonged oxygen (16O-) ion beam sputtering of a tin metal foil and of tin (II) fluoride (SnF2) powder, respectively, for ion flight times of about 10-5 s through a magnetic-sector mass spectrometer. In addition, SnCl2+ has been detected for 16O- ion surface bombardment of stannous (tin (II)) chloride (SnCl2) powder. To our knowledge, SnF2+ is a novel gas-phase molecule, whereas SnCl2+ had been detected previously by electron-impact ionization mass spectrometry, and SnO2+ had been observed before by spark source mass spectrometry as well as by atom probe mass spectrometry. We are not aware of any previous theoretical studies of these molecular systems. [ABSTRACT FROM AUTHOR]

We present results of configuration-interaction (CI) computations of wavefunctions and of properties of the first two singlet states, X1Σ+ and A1Π, of the, as yet unobserved, BeMg polar molecule, for internuclear distances in the range [2.5-1000] Å. The X1Σ+ state is very weakly bound, (De = 469.4 cm-1 at Re = 3.241 Å), whereas the A1Π state, which correlates with the excited dissociation channel [Mg KL3s3p1Po + Be 1s22s2 1S], is bound rather strongly (De = 19 394 cm-1 (55.5 kcal/mol) at Re = 2.385 Å). The X1Σ+ state supports 12 vibrational levels, for which vibrationally averaged dipole moments, <μ>υ, were obtained, while 71 vibrational levels were found for A1Π. For the level (X1Σ+), <μ>0 = 0.213 D. The υ″ = 7 and 8 X1Σ+ vibrational levels are found to have the highest probability to be reached via emission from the lowest lying vibrational levels of A1Π. The work had a dual outcome: First, it explored consequences of different choices of the state-specific reference 'Fermi-sea' space ('active' space), which is required for the construction and execution of the multiconfigurational 'complete active space self-consistent field' calculations and the subsequent multi-reference CI calculations. In this context, comparisons with results on the weakly bound ground states of the homonuclear Be2 and Mg2 molecules were made. Second, it produced reliable data for the short- as well as the long-range parts of the potential energy curve (PEC). Such information is relevant to analyses concerning cold and ultra-cold Physics and Chemistry. For example, accurate fits to the X1Σ+ PEC, which was computed to nano-Hartree accuracy, with account for basis-set-superposition error, produced the C6 and C8 dispersion coefficients as 364.3 ± 1.1 a.u. and 28 000 ± 500 a.u., respectively. The result for C6 is in excellent agreement with that of Derevianko et al. [At. Data Nucl. Data Tables 96, 323 (2010)], (364 ± 4 a.u.), that was obtained in the framework of the theory of long-range interactions and many-body calculations on the constituent atoms. On the other hand, our result for C8 differs from that of Standard and Certain [J. Chem. Phys. 83, 3002 (1985)] by about 7000 a.u. [ABSTRACT FROM AUTHOR]

A model for taking into account surface temperature effects in molecule-surface reactions is reported and applied to the dissociation of H2 and D2 on Cu(111). In contrast to many models developed before, the model constructed here takes into account the effects of static corrugation of the potential energy surface rather than energy exchange between the impinging hydrogen molecule and the surface. Such an approximation is a vibrational sudden approximation. The quality of the model is assessed by comparison to a recent density functional theory study. It is shown that the model gives a reasonable agreement with recently performed ab initio molecular dynamics calculations, in which the surface atoms were allowed to move. The observed broadening of the reaction probability curve with increasing surface temperature is attributed to the displacement of surface atoms, whereas the effect of thermal expansion is found to be primarily a shift of the curve to lower energies. It is also found that the rotational quadrupole alignment parameter is generally lowered at low energies, whereas it remains approximately constant at high energies. Finally, it is shown that the approximation of an ideal static surface works well for low surface temperatures, in particular for the molecular beams for this system (Ts = 120 K). Nonetheless, for the state-resolved reaction probability at this surface temperature, some broadening is found. [ABSTRACT FROM AUTHOR]

A global single-sheeted double many-body expansion potential energy surface is reported for the first excited triplet state of NH3. It employs an approximate cluster expansion of the molecular potential that utilizes previously reported functions of the same family for the triatomic fragments. Four-body energy terms have been calibrated from extensive accurate ab initio data so as to reproduce the main features of the title system. A new switching function formalism has been reported to approximate the true multisheeted nature of NH3(3A2′′) potential energy surface, thus allowing the correct behavior at the NH2(2A″) + H(2S) and NH2(4A″) + H(2S) dissociation limits. The resulting fully six-dimensional potential energy function reproduces the correct symmetry under the permutation of identical atoms, and predicts the correct behavior at all dissociation channels while providing a realistic representation at all interatomic separations. The major attributes of the NH3 double many-body expansion potential energy surface have also been characterized, and found to be in good agreement, both with the calculated ones from the raw ab initio energies and the theoretical results available in the literature. [ABSTRACT FROM AUTHOR]

Based on different ab initio electronic structure calculations (CI-R12 and Gaussian Geminals) of the Born-Oppenheimer electronic energy EBO of H3+ from high to highest quality, we build up a potential energy surface which represents a highly reliable form of the topology of the whole potential region, locally and globally. We use the CI-R12 method in order to get within reasonable CPU-time a relatively dense grid of energy points. We demonstrate that CI-R12 is good enough to give an accurate surface, i.e., Gaussian Geminals are not absolutely necessary. For different types of potential energy surface fits, we performed variational calculations of all bound vibrational states, including resonances above the dissociation limit, for total angular momentum J = 0. We clarify the differences between different fits of the energy to various functional forms of the potential surface. Small rms-values (<1 cm-1) of the fit do not provide precise information about the interpolatory behaviour of the fit functions. [ABSTRACT FROM AUTHOR]

The X2Σ1/2+, A2Π1/2, A2Π3/2, and B2Σ1/2+ potential energy curves and associated dipole matrix elements are computed for M + Ng at the spin-orbit multi-reference configuration interaction level, where M = K, Rb, Cs and Ng = He, Ne, Ar. Dissociation energies and equilibrium positions for all minima are identified and corresponding vibrational energy levels are computed. Difference potentials are used together with the quasistatic approximation to estimate the position of satellite peaks of collisionally broadened D2 lines. The comparison of potential energy curves for different alkali atom and noble gas atom combinations is facilitated by using the same level of theory for all nine M + Ng pairs. [ABSTRACT FROM AUTHOR]