A. Ramírez-Solís

Calculation of electric dipole (hyper)polarizabilities by long-range-correction scheme in density functional theory: A systematic assessment for polydiacetylene and polybutatriene oligomers

The long-range correction (LC) for treating electron exchange in density functional theory, combined with the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional, was used to determine (hyper)polarizabilities of polydiacetylene/polybutatriene oligomers. In comparison with coupled-cluster calculations including single and double excitations as well as a perturbative treatment of triple excitations, our values indicate that the tendency of conventional functionals to result in a catastrophic overshoot for these properties is alleviated but not eliminated. No clear-cut preference for LC-BLYP over Hartree-Fock values is obtained. This analysis is consistent with the calculations of Sekino et al. [J. Chem. Phys. 126, 014107 (2007)] on polyacetylene and molecular hydrogen oligomers. Thus, the performance of LC-BLYP with regard to (hyper)polarizabilities of quasilinear conjugated systems is now well characterized.

Hydrogen interactions and catalytic properties of platinum-tin supported on zinc aluminate

Abstract Pt-Sn/ZnAl2O4 catalysts prepared by two-step impregnation (first tin), with a Sn/Pt atomic ratio ranging from zero to 6.72, were characterized by hydrogen chemisorption, temperature-programmed reduction and tested in isobutane dehydrogenation. Metal dispersion correlates linearly with reaction rate; both parameters reach a maximum when the Sn/Pt atomic ratio is about one. The activity of the sites capable of hydrogen chemisorption, as expressed in turnover frequency number, TOF, decrease as the tin concentration is increased. From theoretical ab-inito calculations, it is proposed that tin reduces the platinum-hydrogen charge transfer responsible for hydrogen dissociation through an orbital overlapping.

Theoretical study of the spectroscopy of low-lying electronic states of the CuF molecule

Abstract Ab initio multireference configuration-interaction (MRCI) calculations using localized molecular orbitals (LMO) were made to estimate adiabatic purely electronic transition energies of the six lowest excited states (a 3 Σ + , A 3 Π, B 1 Σ + , C 1 Π, E 1 Δ) to the X 1 Σ + ground state of the CuF molecule. Spectroscopic constants for these states arefound to be in very good agreement with experimental data and are also reported. Results show again the very strong ionic character of the wavefunctions for all the studied states and that the excited states arise from the Cu + 3 D(d 9 s 1 ) F − 1 S(s 2 p 6 ) and Cu + 1 D(d 9 s configurations.

Ab initio study of the three lowest‐lying (X 1Σ+, 3Σ+, and 1Σ+) electronic states of AgF

Ab‐initio pseudopotential two‐configuration self‐consistent field followed by extensive variational and perturbational second order Mo/ller–Plesset multireference configuration interaction calculations using localized molecular orbitals were performed to characterize the structure and adiabatic potential energy curves of the three lowest (X 1Σ+, 3Σ+, and 1Σ+) purely electronic states of the AgF molecule. Spin‐orbit interactions were introduced semiempirically in a second step. The very strong coupling of the neutral Ag(4d105s1)F(2s22p5) and ionic Ag+(4d95s1)F−(2s22p6) configurations at rather short internuclear distance for both excited 3,1Σ+ states is responsible for the appearance of very shallow minima, thus leading to a limited number of stable vibrational levels for these excited states as suggested previously for the AO+ state. In contrast with the CuF molecule, where only the ionic configuration Cu+(3,1D)F−(1S) is present in the 3,1Σ+ states, this coupling of ionic and neutral structures in AgF is ...

Theoretical calculation of transition energies of low-lying electronic states of the CuCl molecule

Ab initio multiconfiguration self-consistent field followed by multireference configuration interaction calculations using localised molecular orbitals were made to estimate adiabatic purely electronic transition energies of the six lowest excited states (3 Sigma +, 1 Sigma +, 3 Pi , 1 Pi , 3 Delta , and 1 Delta ) to the X 1 Sigma + ground state of the CuCl molecule. Results show the very strong ionic character of the wavefunctions for all the studied states, di-ionic species having a less important role in the excited states than in CuF, this being especially true for both Pi states. The unobserved X 1 Sigma + from 1 Delta transition is predicted to be found around 26 500 cm-1. Spectroscopic constants are reported and found to be in good agreement with experimental data for most of the states studied. A comparative analysis of ionicity with CuF is performed through valence, total and orbital electronic densities and a correlated first-order Moller-Plesset Mulliken population analysis for the ground state of both molecules.

Theoretical study of the activation of methane by photoexcited zinc atoms

The Zn + CH4 reaction has been studied through self-consistent field followed by extensive variational and perturbational second-order Moller-Plesset multi-reference configuration interaction (CIPSI) calculations using extended Gaussian basis sets. Results indicate that for the formation of the HZnCH3 intermediate a previous excitation of Zn(1S) to its (1P) excited state is necessary. The following mechanism is proposed: the Zn(1P) + CH4 reactants follow the 2A′ attractive surface until they reach the originally repulsive 1A′ surface originating from the Zn(1S) + CH4 reactants. An avoided crossing is generated implying a non-adiabatic transition. When the 1A′ surface reaches the avoided crossing it becomes attractive leading to the C3v HZnCH3 intermediate which is stable and needs 63·9 kcal mol-1 to yield, without an activation barrier, the HZn + CH3 products or 69·3 kcal mol-1 to yield the H + ZnCH3 products. The Zn(3P) + CH4 reaction pathway presents a transition state 18 kcal mol-1 above the initial re...

Nonadditivity and the stability of Ag3. A multireference configuration interaction study

Variational (≃30 000 determinants) and perturbational (≃3.5 million determinants) Localized Multireference Configuration Interaction (LMRCI) calculations includingf polarization functions are made to study the role played by the three-body terms in the stabilization energy of three selected geometries of the silver trimer: linear, equilateral and a Jahn-Teller obtuse triangle conformation. A comparative analysis of the relative stability of these geometries is done through a many-body decomposition of the interaction energy. Like in Cu3, the most symmetrical arrangement (i.e. an equilateral triangle) is found to be less stable than the obtuse triangle because it has the highest three-body repulsion energy. The absolute minimum is the obtuse triangle having a Jahn-Teller stabilization energy of 328 cm−1. Unlike Cu3, the linear geometry is found to be less stable than the equilateral by 1282cm−1. Results show again the importance of three-body terms in the total interaction energy of these trimers.

Periodic density functional theory calculations for 3-dimensional polyacetylene with empirical dispersion terms

We report periodic B3LYP density functional theory calculations for three-dimensional (3D) trans-polyacetylene (t-PA) fibers. Empirical dispersion terms, as proposed by Grimme, are included with an appropriate re-scaling to yield the B3LYP+D* method implemented in CRYSTAL06. The dispersion corrections are critical for obtaining correct unit cell parameters. In our calculations the out-of-phase P2(1)/n structure turns out to be a transition state for the interchain relative translational motion, which lies about 0.35 kcal mol(-1) above the two symmetrically located in-phase P2(1)/a minima. These results provide a possible new explanation for the observed XRD intensities. Our calculations should also be useful for comparison with more costly non-empirical treatments of 3D PA and other pi-conjugated polymers.

Aqueous solvation of As(OH)3: a Monte Carlo study with flexible polarizable classical interaction potentials.

A theoretical study of the hydration of arsenious acid is presented. This study included ab initio calculations and Monte Carlo simulations. The model potentials used for the simulations were ab initio derived and they include polarizability, nonadditivity, and molecular relaxation. It is shown that with these refined potentials it is possible to reproduce the available experimental evidence and therefore permit the study of clusters, as well as of the hydration process in solution. From the study of stepwise hydration and the Monte Carlo simulation of the condensed phase it is concluded that As(OH)(3) presents a hydration scheme similar to an amphipathic molecule. This phenomenon is explained as due to the existence of both a positive electrostatic potential and a localized lone pair in the vicinity of As. These results are used to rationalize the known passage of As(OH)(3) through aqua-glyceroporines.

Towards accurate all-electron quantum Monte Carlo calculations of transition-metal systems: spectroscopy of the copper atom.

In this work we present all-electron fixed-node diffusion Monte Carlo (FN-DMC) calculations of the low-lying electronic states of the copper atom and its cation. The states considered are those which are the most relevant for the organometallic chemistry of copper-containing systems, namely, the (2)S, (2)D, and (2)P electronic states of Cu and the (1)S ground state of Cu(+). We systematically compare our FN-DMC results to CCSD(T) calculations using very large atomic-natural-orbital-type all-electron basis sets. The FN-DMC results presented in this work provide, to the best of our knowledge, the most accurate nonrelativistic all-electron correlation energies for the lowest-lying states of copper and its cation. To compare our results to experimental data we include the relativistic contributions for all states through numerical Dirac-Fock calculations, which for copper (Z=29) provide almost the entire relativistic effects. It is found that the fixed-node errors using Hartree-Fock nodes for the lowest transition energies of copper and the first ionization potential of the atom cancel out within statistical fluctuations. The overall accuracy achieved with quantum Monte Carlo for the nonrelativistic correlation energy (statistical fluctuations of about 1600 cm(-1) and near cancelation of fixed-node errors) is good enough to reproduce the experimental spectrum when relativistic effects are included. These results illustrate that, despite the presence of the large statistical fluctuations associated with core electrons, accurate all-electron FN-DMC calculations for transition metals are nowadays feasible using extensive but accessible computer resources.