Search

Quasiclassical trajectories have been run for N 2 A 3 Σ u + + H 2 S collisions allowing for electronic transitions through surface hopping. An accurate multi-sheeted representation of the HN2 potential energy surface has been employed. For the rate constant at room temperature, we obtain 9.85 ± 0.16 × 10 - 10 cm 3 s - 1 , a value fivefold larger than the recommended experimental one. Unprecedently, we investigate the temperature dependence of the rate constant, and obtained an essentially flat Arrehenius plot. We estimate that the NH yield is at least one order of magnitude lower than the experimental upper limit.

The multiple energy switching scheme [J. Chem. Phys. 119 (2003) 2596] has been used to improve the double many-body expansion (DMBE II) potential energy surface of C 3 near its linear global minima by morphing it with an accurate Taylor-series expansion [J. Chem. Phys. 144 (2016) 044307]. The final ES form attains the accuracy of the local form in reproducing the rovibrational spectrum of C 3 while keeping unaltered all key attributes of the original DMBE II, namely conical intersection seams and dissociative channels. The ES form is therefore commended for adiabatic spectroscopic and reaction dynamics studies.

A potential energy surface is reported at the complete basis set limit for the ground electronic state of HO 2 + via a four dimensional interpolation in relaxed hyperspherical coordinates. The dissociated products OH + and O 2 + have also been studied at the complete basis set level and subsequently analytically modelled. In the case of O 2 + , the vibrational states predicted from the model potential have also been directly included in the fit. The potential form so obtained matches the RKR levels with a root mean square deviation below 1 cm - 1 . Spin orbit + relativistic effects have also been taken into account for the O 2 + ion.

Despite its paradigmatic role in early molecular beam experiments and theoretical molecular reaction dynamics, an attempt is made in the present work to elucidate basic aspects of the title reaction. Both thermal rate constants and its rotation specificity are studied using two recently reported ab initio potential energy surfaces by running quasiclassical trajectories. Some inconsistencies in previous studies are pinpointed, but the results show that they do not alter significantly the agreement between experiment and theory, thus suggesting that further work at both levels is necessary for a more complete understanding.

We report quantum dynamics calculations of the O + OH → H + O2 reaction on the CHIPR (Varandas, 2013) and DMBE IV (Pastrana et al., 1990) potential energy surfaces (PESs) for ground-state HO2 using the 3D time-dependent wavepacket formalism based on hyperspherical coordinates. Reaction probabilities for J = 0 are calculated for several initial rovibrational states of the OH radical (v = 0; j = 0–5). The J-shifting approximation is used to obtain initial state selected rate-coefficients in the range 0–400 K. Total and state-to-state rate-coefficients are predicted in good agreement with recent experimental studies and theoretical calculations.

A recent generalization of the Jahn-Teller coordinate proposed for modeling cusps on single-sheeted adiabatic potential energy surfaces for triatomics is extended to any type of crossing seam. The model is applied to the 2 A ″ state of NO 2 and the more complicated HN 2 ( 2 A ′ ) system, which shows permutationally equivalent and strongly curved seams.

Sub-femtosecond nuclear dynamics and high-order harmonic generation (HHG) studies are reported for the X 2 B 1 and A 2 A 1 states of Mu 2 O + . The photoelectron spectra and autocorrelation functions are calculated by solving the time-dependent Schrodinger equation, and the HHG signals from the autocorrelation functions for the two cationic states. Good agreement is observed with our earlier studies, with the autocorrelation function ratios revealing maxima as a function of time. Expectation values of bond lengths and bond angle show quasiperiodic oscillations that reflect repeated passages of the wavepacket at minima of the potential surfaces, thence being responsible for the HHG peaks.

The performance of various hierarchized basis sets has been assessed by extrapolating the correlation energy to the estimated one-electron complete basis set limit. New hierarchical numbers are obtained which are found to lie close to previously reported ones for the Dunning correlation-consistent basis. Universal hierarchical numbers are also suggested, with correlation extrapolations showing a significantly better performance than when based on traditional protocols. It is observed that the accuracy of the extrapolation is enhanced when the underlying plot approaches linearity.

Time-independent quantum reactive scattering studies are performed for the X + O2 → OX + O reactions, where X = Mu, H, D and T are atomic hydrogen isotopes, using the accurate CHIPR potential energy surface (Varandas, 2013) and the popular ABC code. J-shifting thermal rate coefficients have been computed, thence extending our recently reported (Teixidor and Varandas, 2015) values for the reaction H ( 2 S ) + O 2 ( 3 Σ g − ) → OH ( 2 Π ) + O ( 3 P ) . The results are compared with each other and the best experimental and theoretical data available in the literature, with the effect of isotopic substitution on state-specific rate constants being highlighted.

A simple scheme is suggested to extrapolate the valence electron correlation to the one-electron complete-basis-set limit in species formed by atoms of hydrogen through neon. The performance of the novel scheme is benchmarked for the correlation energy with a data set of 106 systems and, for the polarizability, with 8 molecules. For the former, the results are excellent, most often better than when extrapolated with the best available two-parameter protocols. For the polarizability, they show an enhancement relative to the raw ab initio values, and good agreement with the experimental ones.

We report a theoretical investigation of the minimum energy path for isomerization of HO3 with density functional theory (DFT). Specifically, we search for a functional that can reproduce the energy difference between the cis- and trans-isomers of HO3 which has been accurately determined in previous work. By envisaging a full-dimensional map of the isomerization path, the calculations are restricted to a cost-effective model chemistry with a medium-sized cc-pVTZ basis, with the fraction of exact exchange in one-parameter hybrids used to minimize the differences between the ab initio and DFT calculations.

The stability of dipositronium is discussed. Our purpose is not to compete with highly accurate calculations that are available but point out that its dissociation energy can be understood on the basis of simplified models if satisfying the important requirement of consistency in the description of the ‘atoms’ and molecules of positronium. The importance of correlations in positronium and dipositronium is analyzed. Also studied is the replacement of positrons by heavier Fermions in positronium and dipositronium. It is found that the Born–Oppenheimer approximation is valid provided the mass of the heavier Fermions exceeds 200 times the mass of the electron.

Full geometry optimizations have been carried out in order to determine the structure of the global minimum for the quintet state of the isolated α -center [FeO] 2+ using the neutral cluster OFe(OH) 2 as a model. The intrinsic reaction coordinate and a potential energy cut were calculated aiming at the examination of other low-energy structures. The absolute minimum here reported for the title species differs from the structures reported in other molecular orbital-based studies. Such a structure has been analyzed in order to determine the oxidation state of the Fe atom in the ferryl catalyst.

Rate coefficients for the N ( 2 D ) + N 2 collisions were calculated employing quasiclassical trajectories and the first available set of potential energy surfaces for such excited nitrogen interactions. The details of the vibrational energy transfer are discussed, such as the contributions from reactive and non-reactive trajectories as well as the contribution of each electronic symmetry. The calculated state-to-state and state-to-all rate coefficients show that deactivation is far more probable than excitation, and multi-quanta deactivation play an important role.

The single-sheeted double many-body expansion potential energy surface for the ground-state of NH 2 is refined to attain near spectroscopic accuracy from a multiproperty fit to accurate ab initio energies and experimental vibrational levels, including the Renner–Teller effect for total angular momentum (excluding spin) of N = 0. Quasiclassical trajectory calculations on both the original and newly reported potential energy surfaces suggest that the dynamical properties of the original form remain essentially unaltered as aimed.

We present quantum mechanical calculations for the title reaction employing an adiabatic time-dependent wavepacket method. The initial state specified rate constants calculated from the fitted integral cross sections (ICSs) cover temperatures between 10 and 4000 K. ICSs obtained from the capture model method are also given. This approximation should give reliable estimates of the exact ICSs if denser partial waves are used for the low-energy region.

Using a highly accurate potential energy surface reported by Poveda and Varandas (2005) [12] for the quartet state of NH 2 , we have applied the quasi-classical trajectory and quantum mechanical methods to calculate reaction probabilities for the title reaction at zero total angular momentum. The total reaction cross section and product rotational alignment have also been calculated for the ground vibrational–rotational state of the reactants. The specific vibrational–rotational rate constant shows an Arrhenius-type behavior over the investigated temperature range of 200–2500 K, in unprecedentedly good agreement with the experimental data available.

The question of how many states are subject to geometrical phase in a N -fold electronic degeneracy is examined for Jahn–Teller systems. It is shown that it can be diagnosed from the sign change of the adiabatic electronic vector states when a rotation operator is applied to them in functional space defined in N -dimensional spherical polar coordinates, rather than by adiabatic transportation along a closed path in the higher-dimensional configuration space. It is also demonstrated that the Longuet-Higgins theorem for twofold conical intersections (namely that the electronic wave function changes sign on completing a closed path around a doubly-degenerate intersection) can be used to rationalize the results, irrespectively of N .

We report a simple, yet reliable, scheme for the direct diabatization of two adiabatic states showing an avoided intersection. The method requires only discrete ab initio points for the adiabatic curves, but can provide also a robust tool for analytic modeling purposes. The approach is illustrated for the dicarbon molecule.

using time-dependent nonadiabatic and adiabatic wave packet methods, we investigate nonadiabatic effects in collisions of d+(h+) with h-2(d-2) for collision energies from 1.7 to 2.5 ev. a new accurate diabatic potential energy surface [l.p. viegas, a. alijah, a.j.c. varandas, j.chem. phys. 126 (2007) 74309] of the h-3(+) system fitted from full multireference configuration interaction calculations with a cc-pv5z basis set is incorporated into the quantum calculations of initial state resolved integral cross sections. comparison with previous calculations and experimental measurements are used to give insight on the nonadiabatic charge transfer process and isotope effect. (c) 2009 elsevier b. v. all rights reserved.

The potential energy surface of exohedral HeC 36 is analyzed in detail and extrapolated to the complete basis set limit using Moller-Plesset perturbation calculations. A limited number of coupled cluster singles and doubles calculations has also been performed. Exohedral complexes above the hexagonal faces toward the major axis are found to be the most stable, but the helium atom can move almost freely from the equator to the poles of C 36 without dissociating the complex due to the disparity of masses. The effective 1D interaction potential has van der Waals attributes in good agreement with those estimated from He 2 and a Girifalco function for the C 36 dimer, supporting three bound states.

A recently proposed scheme that enables a potential energy surface corresponding to a calculation with a large target basis set to be obtained from small basis set calculations via scaling of the electron correlation at a single-pivotal geometry is generalized to include an arbitrary number of such geometries. If the correlation is extrapolated to the complete basis set limit at the pivotal geometries and used for the scaling, the method can yield accurate potentials at costs up to factors nearly fifty times cheaper than required otherwise. The approach, free from parameters alien to the ab initio methods, is tested on N2, O2, and F2.

Accurate time-dependent wave packet calculations have been performed for the isotopic N( 2 D) + D 2 reaction on the recent reported 1 2 A″ DMBE (double many body expansion) potential energy surface of the NH 2 system. The total reaction probabilities for total angular momentum J up to 75 have been calculated to get the converged integral cross-section over a collision energy range of 0.0–1.0 eV. Then, the ground-state thermal rate constant has been derived from the integral cross-section and the intermolecular isotope effect is further discussed.

By expressing an unknown state in terms of a complete set, a simple scheme for approximate quantization of the continuous vibrational-rotational energy distributions that are obtained from quasi-classical trajectory calculations is suggested. The problem of zero-point energy leakage is also revisited, and the new method tested on the prototype O + OH and H + D2 reactions. http://www.sciencedirect.com/science/article/B6TFN-4NCJCX1-3/1/cba1ca88eec1685b1502de957a8cc175

We report a quasi-classical trajectory study of the title reaction using a double many-body expansion potential energy surface for ground state HSO 2 . Calculations for specific low ro-vibrational states of the reactants, as well as thermalized reactants over the range of temperatures 1400 ⩽ T /K ⩽ 2200, have been performed. The calculated cross sections indicate a barrier-type mechanism, and nearly thermalized product distributions. Both HOSO and HSO 2 isomers are formed in quite different instances during the collisional process. The rate constant is found to be enhanced by adding internal energy to the reactants, with the thermalized result being in good agreement with existing experimental data.

We report a quasiclassical trajectory study of the title reaction using a recently reported double many-body expansion potential energy surface for ground state HSO2. Two methods that aim to account for zero point energy leakage are used. A capture-type behavior is predicted, with recrossing effects manifesting mainly at high translational energies. Reaction cross sections and a comparison of the calculated rate constant with available literature data are also reported.

It is shown that the molecular potential energy surface corresponding to a high level of ab initio theory can be accurately predicted by performing calculations with smaller basis sets and then scaling the electron correlation at a single point calculated with the larger target basis set.

By direct-fitting the parameters in the physically motivated extended Hartree–Fock approximate correlation energy model to spectroscopic data, potential curves have been obtained for a variety of diatomic systems that mimic the ro-vibrational data often within much less than 1 cm−1. The approach offers a simple, yet accurate, scheme for modeling the two-body terms that are the leading contributions in a cluster expansion of potential energy surfaces of larger dimensionality.

We report accurate close coupling quantum wave packet calculations of the rate constant for the reaction N( 2 D) + H2 ! NH + H using a recently reported single-sheeted double many-body expansion potential energy surface for NH2(1 2 A 00 ) that has been calibrated from high level ab initio MRCI energies. Results from quasi-classical trajectories run on the same potential energy surface are also given, and used to rationalize the quantum ones. The calculated rotationally averaged rate constant is found to be in good agreement with experimental data. A comparison with other theoretical results is also presented. � 2006 Elsevier B.V. All rights reserved.

The mechanism of the H + ClONO2 reaction is examined by performing QCISD calculations at geometries optimized at the MP2 level. Each of the six reaction channels involves stereoisomeric transition states that have identical energy barriers. The lowest energy barrier is 24.2 kcal mol(-1) for the indirect metathetical pathway leading to OH + cis-CIONO, being the corresponding rate constant calculated employing TST theory. The NO2-elimination channel and the indirect metathetical pathway leading to OH + trans-ClONO should compete with each other as they have barriers of 24.8 and 25.1 kcal mol(-1). For Cl-substitution, Cl-abstraction, and N-attack, the barriers are 27.4, 35.1, and 41.3 kcal mol(-1). (c) 2005 Elsevier B.V. All rights reserved.

We investigate the relaxation of highly vibrationally excited HO 2 in collisions with vibrationally cold O 2 . The calculations employ the quasiclassical trajectory method and a global double many-body expansion potential energy surface for HO 4 ( 2 A). Both deactivation and activation processes are observed, with the rate constants being found to be strongly influenced by the donor internal energy content. It is also found that the title process may not be ignorable when modelling the ozone budget in the middle atmosphere.

We report a dynamics study of both reactive and non-reactive O + OH( v ′, j ′) collisions under local thermodynamic disequilibrium. Besides the total rate constant for temperatures over the range 110 ⩽ T /K ⩽ 450, vibrationally specific ones are reported for quantum numbers v ′ ⩽ 9. Unknown thus far, such kinetic parameters may be of relevance in modelling the OH nightglow in the middle atmosphere.

The title reaction is studied for the first time using a global double many-body expansion potential energy surface. The major differences between the calculated and experimental results are: (a) the calculated activation energy is larger than the experimental one; (b) the reaction is predicted to occur exclusively via oxygen abstraction from ozone while it has been suggested from experimental work that hydrogen abstraction accounts for 88–95% of the reactive encounters. This discrepancy in mechanism is tentatively attributed to scrambling reactions, with further theoretical and experimental work being required to clarify pending issues.

We discuss the title atmospheric reaction for vibrational excited states of OH over the range 0⩽v⩽9. All calculations have employed the quasi-classical trajectory (QCT) method and a realistic double many-body expansion (DMBE) potential energy surface for HO 4 ( 2 A ) . For v=9, the calculated thermal rate coefficient is found to agree with measurements of OH quenching reported by others, which has been attributed to both chemical reaction and non-reactive collisions. The temperature dependence of the rate constant is described by a simple model for all studied vibrational states of OH.

We report a comparative dynamics study of the title reaction using a realistic double many-body expansion potential energy surface (DMBE I) for ground-state HO3 [Mol. Phys. 91 (1997) 301], and a recently reported DMBE II [Chem. Phys. Lett. 334 (2001) 173] surface obtained by modifying DMBE I to fit extensive ab initio calculations corrected for the complete basis set limit. The comparison provides strong evidence that the title reaction can be a significant source of ozone under stratospheric conditions.

We report a single-valued potential energy surface for HO4(2A) from the double many-body expansion method. All n-body (n=2-4) energy terms are taken from published studies on the relevant fragments, with a five-body energy term of Gaussian form added to mimic the experimental activation energy for the OH(v=0)+O3 reaction. A detailed dynamics study of this reaction is also reported using classical trajectories. Good agreement with existing experimental data is obtained. http://www.sciencedirect.com/science/article/B6TFN-41WBCGK-M/1/f846c63bce20f7a4c43e3cdd2338c852

We present a quantum mechanical, three-dimensional infinite-order-sudden-approximation study of the O+HO 2 reaction using a recently reported HO 3 double many-body expansion potential energy surface. The reaction is treated as coplanar, for which a 3D treatment represents still a reduced dimensionality analysis. The results are compared with experimental data and previously reported quasiclassical trajectory calculations which employed the same potential. Novel adiabatic approaches have also been developed. In comparison with the trajectory calculations and experiment, the agreement of the adiabatic results is good. Striking deficiencies are noted for the sudden approximation near the reaction threshold.

We have carried out a test dynamics study of the prototype H+H 2 exchange reaction by running quasi-classical trajectories `on-the-fly' using ab initio correlated energies which have been previously corrected semiempirically by the double many-body expansion plus scaling of the external correlation method to account for size-limitations of the one-electron basis set and configuration interaction expansion. The method is general and gives results in agreement with conventional trajectory calculations carried out on the accurate double many-body expansion potential energy surface for H 3 .

A realistic single-valued energy-switching potential energy surface recently proposed for the water molecule has been used to perform quasiclassical trajectory calculations for the title reaction. The calculated cross sections and rate constants are compared with theoretical and experimental data, particularly with a view to assess the temperature dependence of the rate constant.

The method of classical canonical transformations is used to determine the action associated with the tunneling mode in anharmonic potentials. A simple analog of the surface hopping method is suggested to incorporate tunneling effects in classical trajectories. The novel approach is illustrated using a 1D cubic potential.

A 3D time-dependent wavepacket propagation method has been used to study the transition state resonances on the two coupled diabatic states of the H 3 DMBE potential energy surface. We report 14 zero-bend resonances which are found to be in good agreement with the corresponding results calculated using only the lowest adiabatic sheet of the DMBE potential energy surface.

We report rate constants for the formation of HO2 and O3 using the chaperon mechanism, and with Ar as scattering partner. The ArHO2 and ArO3 potential energy surfaces have been written as a pairwise summation of realistic EHFACE2-type potentials for interactions involving Ar and reliable global DMBE potential surfaces for chemically stable triatomic species. The calculations have been carried out using the quasiclassical trajectory approach.

Trajectory calculations of the total reactivity for the H + O 2 reaction, out of the vibrational-rotational states ( v , j ) = (0, 1) and (3, 1), with the total angular momentum kept at J = 0, are shown to agree well with quantum mechanical reactivities calculated on the same (DMBE IV) potential energy surface. Trajectory calculations of the total reactive cross-sections are also reported. A simple hybrid model (QCT/QM-UJ) to calculate the total cross-section from the (0, 1) quantum mechanical reactivity, using the assumption of a uniform- J distribution in which the cut-off value of the total angular momentum is obtained quasiclassically, is also suggested and tested. Agreement with recent cross-section measurements is reasonable but, unlike experiment, no sharp maximum is observed on the reaction cross-section as a function of translational energy, leaving its origin unexplained.

By judicious neglect of some of the off-diagonal elements in a diatomics-in-molecules Hamiltonian matrix for the ionic states of MX 2 systems (M = alkali metal atom, X 2 =halogen diatomic), we are able to reduce the matrix to three 2×2 blocks, yielding analytical expressions for six diabatic potential energy surfaces. The resulting are exact for C 2v and linear geometries. All diatomic potentials include the appropriate two-body long-range electrostatic, induction, and dispersion forces, while their short-range part has been modelled using spectroscopic data or ab initio calculations. Specific results are presented for the LiF 2 system, and compared with existing experimental and ab initio data. The agreement is good. The potentials are well suited for modelling accurate ab initio energies for these triatomic systems.

A novel non-active model to correct for the leak of zero-point energy in quasi-classical trajectory calculations is proposed. It consists of eliminating every trajectory that fails to satisfy the zero-point energy requirement of quantum mechanics at the end of the trajectory, and then correct the results using a unified statistical approach which takes into account the relative probabilities of the reactive and non-reactive events. The correction factor assumes a simple analytic form, adding no extra cost of the traditional quasiclassical trajectory approach. Test calculations are presented for the total reactivity of the H + O 2 reaction out of the initial vibrational—rotational state ( v,j ) = (0, 0), keeping the total angular momentum J = 0. Comparison of the results with quantum mechanical reactivities calculated on the same (DMBE IV) potential energy surface shows good agreement. A possible generalization of the model to require a local zero-point energy along the trajectory is pointed out.

The virial theorem for the n -body energy terms in a cluster expansion of potential energy surfaces has been investigated. Integral conditions on the uniform scaling cuts of the kinetic and potential components of the n -body energy term are reported. The corresponding relations for n -body energy terms of molecular fragments are also examined. A use of the integral relations as constraints on the model n -body functions is suggested. An example is presented for the ground state surface of Li 3 .

A new formulation of the three-body dynamical correlation energy is presented which allows a correct description of the anisotropic long-range forces for all atom—diatom dissociation channels of a triatomic system, including the known dependence on the diatomic intramolecular coordinate. Illustrative results are reported for H 3 .

SCF and SCF Cl calculations were performed to obtain the isotiopic V 0 and leading anisotiopic V 2 terms of the H-H 2 potential surface. An analytic estimate of V 0 is obtained by adding to the near Hartree-Fock curve a function which approximately fits the correlation energy at small R and reduces at large R to the multipolar expansion of the dispersion energy. The behavior of V 2 is also discussed.

The dependence of the leading atom-diatom dispersion energy coefficient on the intramolecular coordinate R is discussed. New values of C 6 for the A-H 2 (A=H, rare gas) interactions are reported for R=1.28, 1.449 and 1.618 a 0 using a Pade technique. Based on a generalized London formula, a procedure is suggested for the analytical continuation of these C 6 coefficients, as well as those available for the united-atom and separated-atom limits of the diatomic, to the whole intramolecular coordinate space.

Using arguments based on permutational symmetry it is shown that it is possible to XXX a minimum potential energy function from high-symmetry to low-symmetry parts of the surface. The method has been applied in X 3 , X 4 and AX 4 systems.