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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]

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]

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]

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]

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]

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]

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]

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]