Jesús R. Flores

Tunneling Above the Crossover Temperature

Quantum mechanical tunneling of atoms plays a significant role in many chemical reactions. The crossover temperature between classical and quantum movement is a convenient preliminary indication of the importance of tunneling for a particular reaction. Here we show, using instanton theory, that quantum tunneling is possible significantly above this crossover temperature for specific forms of the potential energy surface. We demonstrate the effect on an analytic potential as well as a chemical system. While protons move asynchronously along a Grotthuss chain in the classical high-temperature range, the onset of tunneling results in a synchronization of their movement.

Accurately solving the electronic Schrödinger equation of small atoms and molecules using explicitly correlated (r12-)MR-CI. VIII. Valence excited states of methylene (CH2)

We compute the adiabatic transition energies of methylene (CH(2)) from the ground state to the lowest electronically excited valence states using the r(12)-MR-ACPF-2 method with a large basis set and an extended reference space. We recall that this method aims at reaching the basis-set and full configuration interaction (CI) limits simultaneously. Our best excitation energies, T(e) (T(0)), are 9.22 (8.87) (a (1)A(1), corrected for relativistic and adiabatic effects), 31.98 (31.86) (b (1)B(1)), and 57.62 (57.18) kcal mol(-1) (c (1)A(1)) (both uncorrected). We are able to reach the respective basis-set limits that closely that the remaining errors of our (uncorrected) calculations are clearly due to the MR-ACPF-2 method. While we are unable to assess the error of the latter method in a systematic way, we still believe that it is rather unlikely that the errors of our excitation energies exceed +/-0.10 kcal mol(-1). We finally observe that our (corrected) a state values deviate by only -0.10 (-0.10) kcal mol(-1) from the results of Csaszar et al. [J. Chem. Phys. 118, 10631 (2003)]--who did careful extrapolations to the valence full-CI and basis-set limits and added a correction for the core correlation--and that the deviation from experiment is only -0.13 (-0.13) kcal mol(-1). From these excellent agreements we conclude that our excitation energies to the b and c states are similarly accurate.

Towards benchmark second-order correlation energies for large atoms. II. Angular extrapolation problems

We have studied the use of the asymptotic expansions (AEs) for the angular momentum extrapolation (to l --> infinity) of atomic second-order Moller-Plesset (MP2) correlation energies of symmetry-adapted pairs (SAPs). The AEs have been defined in terms of partial wave (PW) increments to the SAP correlation energies obtained with the finite element MP2 method (FEM-MP2), as well as with the variational perturbation method in a Slater-type orbital basis. The method employed to obtain AEs from PW increments is general in the sense that it can be applied to methods other than MP2 and, if modified, to molecular systems. Optimal AEs have been determined for all types of SAPs possible in large atoms using very accurate FEM PW increments up to lmax = 45. The impact of the error of the PW increments on the coefficients of the AEs is computed and taken into account in our procedure. The first AE coefficient is determined to a very high accuracy, whereas the second involves much larger errors. The optimum l values (lopt) for starting the extrapolation procedures are determined and their properties, interesting from the practical point of view, are discussed. It is found that the values of the first AE coefficients obey expressions of the type derived by Kutzelnigg and Morgan [J. Chem. Phys. 96, 4484 (1992); 97, 8821(E) (1992)] for He-type systems in the bare-nucleus case provided they are modified by fractional factors in the case of triplet and unnatural singlet SAPs. These expressions give extremely accurate values for the first AE coefficient both for the STO and the FEM Hartree-Fock orbitals. We have compared the performance of our angular momentum extrapolations with those of some of the principal expansion extrapolations performed with correlation consistent basis sets employed in the literature and indicated the main sources of inaccuracy.

Hierarchic computation of atomic correlation energies using a p-version finite element method

Abstract A new method is presented, based on a p-version finite element formalism, for calculating atomic second-order correlation energies using the Moller—Plesset approach. The method permits the definition of a hierarchic sequence of computations; this feature greatly simplifies the calculations via a simple adaptive technique. Results for He, Be and Ne are given and compared with the most accurate values available.

Product energy distributions for the four-center HF elimination from 1,1-difluoroethylene. A direct dynamics study

Product energy distributions (PEDs) were computed on the four-center HF elimination from 1,1-difluoroethylene by using direct trajectory calculations. The vibrational and rotational populations of HF obtained with a quasi-classical normal mode/rigid rotor excitation model compare very well with the experimental results. Also, the translational energy distributions obtained with an efficient microcanonical sampling (EMS) at the barrier are in excellent accord with experiment and do not substantially change as the excitation energy increases.

Second-order picture of correlation effects in closed-shell atoms

Results of many calculations of the MP2 correlation energies for ground states of closed-shell atoms (referred to as MP2/CA energies) are presented and studied. Special attention is paid to laying down rules which govern the dependence on the nuclear charge (Z dependence) and on the number of electrons (N dependence) of (a) the partial wave (PW) increments to the second-order pair energies, (b) the second-order pair energies, and (c) the configurational pair energies. It has been found that these energy increments disclose many regularities useful from both the physical and computational points of view. Some of the MP2/CA increments are quasi-transferable between similar systems. The results obtained are used as a starting point for an extensive discussion of the methodological significance of MP2/CA studies, and for indicating various actual and potential areas of application of the MP2/CA approach in many-electron theory.

Quasiclassical trajectories on a finite element density functional potential energy surface : The C++H2O reaction revisited

A new method for the representation of potential energy surfaces (PESs) based on the p version of the finite element method is presented and applied to the PES of the [COH2]+ system in order to study the C++H2O-->[COH]++H reaction through the quasiclassical trajectory method. Benchmark ab initio computations have been performed on the most relevant stationary points of the PES through a procedure that incorporates basis set extrapolations, the contribution of the core correlation energy, and scalar relativistic corrections. The electronic structure method employed to compute the many points needed to construct the PES is a hybrid density functional approach of the B3LYP type with geometry-dependent parameters, which improves dramatically the performance with respect of the B3LYP method. The trajectory computations shed light on the behavior of the COH2+ complex formed in the collision. At a fixed relative translational energy of 0.62 eV, which corresponds to the crossed beam experiments [D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985)], the complex dissociates significantly into the reactants (37%). However, the behavior for a thermal sampling at T=300 K is significantly different because only 9% of the trajectories where capture occurs lead to dissociation into the reactants. The latter kind of behavior is coherent with the view that simple ion-molecule reactions proceed quite often at the capture rate provided it is corrected by the fraction of the electronic states which, being nearly degenerate for the reactants, become attractive at short distances. For both T=300 K and crossed beam conditions, the trajectory computations indicate that COH2+ is the critical intermediate, in agreement with a recent work [Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003)] and in contrast with the interpretation of the crossed beam experiments. Besides, virtually all trajectories generate COH++H (>99%), but a significant proportion of the isoformyl cation is formed with enough vibrational energy as to surmount the COH+-HCO+ isomerization barrier, about 37% at T=300 K.

Theoretical Study of the Chemiluminescence of the Al + H2O Reaction

We performed surface hopping simulations of Al + H(2)O collisions by a direct semiempirical method, reproducing the conditions of previous beam-gas experiments. We observed the formation of the HAlOH species, that dissociates to AlOH + H after a lifetime of about 0.6 ps. This species undergoes nonadiabatic transitions to its first excited state and is responsible for chemiluminescence in the visible range, while the Al-H(2)O complex emits in the infrared. The computed emission band in the visible is red-shifted with respect to the experimental one, because of slight inaccuracies of the potential energy surfaces. However, collisions with more water molecules and exciplex formation with excited Al((2)S, (4)P) atoms may also contribute to the short wavelength emission, as we show by accurate ab initio calculations.

On the accuracy of second-order Møller-Plesset correlation energies

Abstract Accurate second-order Moller-Plesset correlation energies are computed and compared with several semi-empirical estimates of the total correlation energies including those provided by Clementi, Anno and Teruya, and the recent results of Davidson, Froese and co-workers, for atoms with ten, twelve and eighteen electrons. Somewhat surprisingly, the MP2 correlation energies present what is considered to be in good agreement with the newest estimates, especially when the behaviour with the nuclear charge is examined.

Size, Adsorption Site, and Spin Effects in the Reaction of Al Clusters with Water Molecules: Al17 and Al28 as Examples

The first step of the reaction of two relatively large Alm clusters (m = 17, 28) with a few water molecules has been studied by electronic structure methods. The complexes Alm·(H2O)n (n = 1-2) have been characterized, and the saddle points corresponding to the first step in the reaction, namely, formation of HAlmOH·(H2O)n-1 systems, have been located. The Al28 cluster is special in the sense it has two electronic states, singlet and triplet, which are very close in energy and also have quite similar equilibrium structures. The preferred adsorption and reaction sites have been determined. We find quite clear preferences toward some sites, the effect of cluster distortion being moderately significant in the stability of the complexes. The interaction with water does not appear, in general, to bring the triplet state of the Al28·(H2O)2 adducts below the singlet; not even the corresponding saddle points appear to be lower in energy. The rate coefficients, tunneling transmission factors, and activation free energies have been computed and compared with those of the Al13 and Al3 clusters, even with those of the Al atom. It turns out the rates are quite close to those of Al3 and much larger than those of Al and Al13. There is no dramatic difference between the reactivity of the singlet and triplet state of Al28; however, there are very significant differences between different sites. Finally, we studied the interaction between the lowest-lying singlet and triplet states of Al28 through multireference configuration interaction (MRCI) spin-orbit computations. The vertical excitation energies corresponding to a number of low-lying singlet and triplet states are also determined by MRCI computations. It turns out that the spin-orbit interaction is very weak, which suggests that both states, the lowest-lying singlet and triplet, could evolve somehow independently, at least when interacting with closed-shell molecules. It is suggested that the situation could be quite different in a reaction with molecular radicals or if external fields are applied.