A theoretical study on the ionization of OCS with an analysis of vibrational structures of the photoelectron spectrum

 Ab initio calculations have been performed to study the molecular structures and vibrational levels of the four low-lying ionic states (1, 2²Π, and 1, 2²Σ⁺) of carbonyl sulfide. The global regions of the potential-energy surfaces have been obtained by multireference single and double excitation configuration interaction calculations. Vibrational calculations using explicit vibrational Hamiltonians have been used for vibrational analysis. The equilibrium molecular structures and a vibrational analysis of the four ionic states are presented. The theoretical ionization intensity curves including the vibrational structures of the ionic states are also presented and are compared with the photoelectron spectrum. Received: 20 January 2001 / Accepted: 22 August 2001 / Published online: 30 October 2001     

Model studies of the chemisorption of hydrogen and oxygen on the Au (1 0 0) surface

 The convergence of chemisorption energy for hydrogen and oxygen on gold clusters is studied. Two theoretical approaches have been employed; wavefunction methods at the self-consistent-field second–order M?ller–Plesset level and density functional theory and the two methods are compared. Relativistic effective core potentials exploited in the former approach were developed in this work. Received: 25 October 1999 / Accepted: 21 February 2001 / Published online: 11 October 2001     

An overlap criterion for selection of core orbitals

 An overlap criterion is defined that connects the identification of core orbitals in a molecular system, which can be problematic, to that in isolated atoms, which is well defined. This approach has been tested on a variety of troublesome systems that have been identified in the literature, including molecules containing third-row main-group elements, and is shown to remove errors of up to 100 kcal/mol arising from an inconsistent treatment of core orbitals at different locations on a potential-energy surface. For some systems and choices of core orbitals, errors as large as 19 kcal/mol can be introduced even when consistent sets of orbitals are frozen, and the new method is shown to identify these cases of substantial core–valence mixing. Finally, even when there is limited core–valence mixing, the frozen-core approximation can introduce errors of more than 5 kcal/mol, which is much larger than the presumed accuracy of models such as G2 and CBS-QB3. The source of these errors includes interatomic core–core and core–valence dispersion forces. Received: 31 August 2001 / Accepted: 11 October 2001 / Published online: 9 January 2002     

A new look at the reduced-gradient-following path

 The mathematical structure of the reduced-gradient-following (RGF) path introduced by Quapp et al. (1988 J. Comput. Chem. 19:1087) is reviewed and analyzed. We report two new algorithms to evaluate the RGF path. The RGF path is also compared mathematically and computationally with the gradient extremals path. An example of the evaluation of the RGF path is also reported. Received: 21 May 2001 / Accepted: 27 September 2001 / Published online: 9 January 2002     

The performance of the rapid estimation of basis set error and correlation energy from partial charges method on new molecules of the G3/99 test set

Experimental enthalpies of formation have been approximated using single-point Hartree–Fock (HF)–self-consistent-field (SCF) total energies plus the rapid estimation of basis set error and correlation energy from partial charges (REBECEP) energy corrections. The energy corrections are calculated from the HF–SCF partial atomic charges and optimized atomic energy parameters. The performance of the method was tested on 51 closed-shell neutral molecules (50 molecules from the G3/99 thermochemistry database plus urea, composed of H, C, N, O, and F atoms). The predictive force of the method is demonstrated, because these larger molecules were not used for the optimization of the atomic parameters. We used the earlier RECEP-3 [HF/6-311+G(2d,p)] and REBECEP [HF/6-31G(d)] atomic parameter sets obtained from the G2/97 thermochemistry database (containing small molecules) together with natural population analysis and Mulliken partial charges. The best results were obtained using the natural population analysis charges, although the Mulliken charges also provide useful results. The root-mean-square deviations from the experimental enthalpies of formation for the selected 51 molecules are 1.15, 3.96, and 2.92 kcal/mol for Gaussian-3, B3LYP/6-11+G(3df,2p), and REBECEP (natural population analysis) enthalpies of formation, respectively (the corresponding average absolute deviations are 0.94, 7.09, and 2.27 kcal/mol, respectively). The REBECEP method performs considerably better for the 51 test molecules with a moderate 6-31G(d) basis set than the B3LYP method with a large 6-311+G(3df,2p) basis set. Received: 10 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001     

Molecular Schrödinger–Riccati calculations. Test for the hydrogen-ion molecule

 The hydrogen-ion molecule has been used as a test system for the application of the Schr?dinger–Riccati formulation to molecular calculations. Some of the points discussed are the characteristics (quasiconstancy of the local energies, size, number of points) of the sampling region to be chosen, the dependence on the starting function, the precision of the calculations, and the excellent behaviour of the predicted function (by comparison to an accurate function). Received: 1 May 2001 / Accepted: 25 July 2001 / Published online: 11 October 2001     

Fully relativistic correlated benchmark results for uranyl and a critical look at relativistic effective core potentials for uranium

 A complete set of fully-relativistic benchmark results for the bond lengths and vibrational frequencies of uranyl at various levels of correlation treatment are presented. It is shown that the relativistic and correlation effects are of the same magnitude and should be treated on an equal footing. Results of uranyl calculations using various relativistic effective core potentials (RECP) are presented. Appropriate choices for RECPs for use in density functional theory (DFT) calculations in the local density approximation (LDA) and with the gradient-corrected B3LYP exchange-correlation functional are discussed. The conclusion is reached that small-core RECPs need to be used and that the best results as compared to the benchmark values are obtained by using a DFT functional that includes some fraction of Hartree-Fock Exchange. Received: 18 May 2001 / Accepted: 25 July 2001 / Published online: 30 October 2001     

Implementation of solvent reaction fields for electronic structure

 Methods are described to incorporate solvent reaction field effects into solute electronic structure calculations. Included are several old and new approaches based on approximate solutions of Poisson's equation through boundary element methods, wherein the solutions are represented in terms of certain apparent surface charge or apparent surface dipole distributions. Practical algorithms to set up and solve the requisite equations are described and implemented in a new general reaction field computer program. Illustrative computational results are presented to show the performance of the program. Received: 2 July 2001 / Accepted: 11 September 2001 / Published online: 19 December 2001     

Comparison of solvent reaction field representations

 Alternative ways are examined for representing a reaction field to treat the important effects of long-range electrostatic interaction with a solvent in electronic structure calculations on the properties of a solute. Several extant boundary element methods for approximate representation of the solvent reaction field in terms of surface charge distributions are considered, and analogous new methods for approximate representation in terms of surface dipole distributions are introduced. Illustrative computational results are presented on representative small neutral and ionic solutes to evaluate the relative accuracy of various methods. Received: 2 July 2001 / Accepted: 10 September 2001 / Published online: 19 December 2001     

Dual-level direct dynamics calculations of deuterium kinetic isotope effects for the Cl(2P)+C2H6 abstraction reaction

 Kinetic isotope effects, KIEs, for hydrogen abstraction from C₂H₆ and C₂D₆ by chlorine atom have been studied by the dual-level direct dynamics approach. A low-level potential energy surface is obtained with the MNDO-SRP method. High-level structural properties of the reactants, transition state, and products were obtained at the MP2 level with the cc-pVDZ, aug-cc-pVDZ, and the cc-pVTZ basis sets. Using the variational transition state theory with microcanonical optimized multidimensional tunneling, the values of deuterium KIE, at 300 K, range from 2.28 to 3.27, in good agreement with the experimental values (2.69–5.88). Received: 6 June 2001 / Accepted: 12 July 2001 / Published online: 19 November 2001     

Electron-pairing analysis from localization and delocalization indices in the framework of the atoms-in-molecules theory

This article presents an overview of recent advances in the study of electron pairing through the use of localization and delocalization indices obtained from double integration over atomic basins of the exchange–correlation density in the framework of the atoms-in-molecules theory. These localization and delocalization indices describe the intra- and interatomic distribution of the electron pairs in a molecule. The main results of the application of these second-order indices to the analysis of molecular structure and chemical reactivity are briefly reviewed. It is shown that localization and delocalization indices represent a powerful tool to describe the electron-pair structure of molecules, which, in turn, provides deeper insight into relevant chemical phenomena such as electron correlation effects and the formation of localized α, β electron pairs. Received: 8 April 2002 / Accepted: 26 June 2002 / Published online: 6 September 2002 Acknowledgements. Financial help was furnished by the Spanish DGES projects no. PB98-0457-C02-01 and BQU2002-04112-C02-02. J.P. thanks the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for benefiting from a doctoral fellowship, no. 2000FI-00582. M.S. is indebted to the Departament d'Universitats, Recerca i Societat de la Informació of the Generalitat de Catalunya for financial support through the Distinguished University Research Promotion, 2001. We also thank the Centre de Supercomputació de Catalunya for providing us with computing facilities. Correspondence to: M. Solà e-mail: miquel.sola@udg.es     

The search for stationary points on a quantum mechanical/molecular mechanical potential-energy surface

 We propose a methodology to locate stationary points on a quantum mechanical/molecular mechanical potential-energy surface. This algorithm is based on a suitable approximation of an initial full Hessian matrix, either a modified Broyden–Fletcher–Goldfarg–Shanno or a Powell update formula for the location of, respectively, a minimum or a transition state, and the so-called rational function optimization. The latter avoids the Hessian matrix inversion required by a quasi-Newton–Raphson method. Some examples are presented and analyzed. Received: 16 July 2001 / Accepted: 9 October 2001 / Published online: 9 January 2002     

Second-order Møller–Plesset perturbation theory with terms linear in the interelectronic coordinates and exact evaluation of three-electron integrals

 The second-order correlation energy of M?ller–Plesset perturbation theory is computed for the neon atom using a wave function that depends explicitly on the interelectronic coordinates (MP2-R12). The resolution-of-identity (RI) approximation, which is invoked in the standard formulation of MP2-R12 theory, is largely avoided by rigorously computing the necessary three-electron integrals. The basis-set limit for the second-order correlation energy is reached to within 0.1 mE _h. A comparison with the conventional RI-based MP2-R12 method shows that only three-electron integrals over s and p orbitals need to be computed exactly, indicating that the RI approximation can be safely used for integrals involving orbitals of higher angular momentum. Received: 9 May 2001 / Accepted: 31 October 2001 / Published online: 9 January 2002     

Local-orbital-based correlated ab initio band structure calculations in insulating solids: LiF

 A local-orbital-based ab initio approach to calculate correlation effects on quasi-particle energies in insulating solids is presented. The use of localized Wannier-type Hartree–Fock orbitals allows correlation effects to be efficiently assessed. First a Green's function approach based on exact diagonalization is introduced and this is combined with an incremental scheme, while subsequently different levels of perturbative approximations are derived from the general procedure. With these methods the band structure of LiF is calculated and good agreement with experiment is found. By comparing the different approximations proposed, including the exact diagonalization procedure, their relative quality is established. Received: 25 June 2001 / Accepted: 31 August 2001 / Published online: 19 December 2001     

Modeling the H5+ potential-energy surface: a first attempt

 Ab initio molecular electronic structure calculations are performed for H₅ ⁺ at the QCISD(T) level of theory, using a correlation-consistent quadruple-zeta basis set. Structures, vibrational frequencies and thermochemical properties are evaluated for ten stationary points of the H₅ ⁺ hypersurface and are compared with previous calculations. The features of the H₃ ⁺…H₂ interaction at intermediate and large intermolecular distances are also investigated. Furthermore, an analytical functional form for the potential-energy surface of H₅ ⁺ is derived using a first-order diatomics-in-molecule perturbation theory approach. Its topology is found to be qualitatively correct for the short-range interaction region. Received: 15 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001     

Accurate thermochemistry from corrected Hartree–Fock results: rapid estimation of nearly experimental quality total energy using the small 6-31G(d) basis set

Gaussian-3 ground-state total electronic energies have been approximated using single point 6-31G(d) basis set Harteee–Fock self-consistent-field (HF-SCF) total energies and partial charges based on our earlier rapid estimation of correlation energy from partial charges method. Sixty-five closed-shell neutral molecules (composed of H, C, N, O, and F atoms) of the G2/97 thermochemistry database were selected for the present study. The main feature in this work is that the␣basis set error has been treated by the least squares fit of rapid estimation of basis set error and correlation energy from partial charges (REBECEP) atomic parameters. With these parameters a rather accurate closed-shell ground-state electronic total energy can be obtained from a small basis set HF-SCF calculation in the vicinity of stationary points. The average absolute deviation of the best REBECEP enthalpies of formation from the experimental enthalpies of formation is 1.39 kcal/mol for the test set of 65 enthalpies of neutral molecules. Received: 11 December 2000 / Accepted: 6 February 2001/Published online: 11 October 2001     

Endohedral chemical shifts in higher fullerenes with 72–86 carbon atoms

For all isolated pentagon isomers of the fullerenes C₆₀–C₈₆ with nonzero HOMO–LUMO gap and for one nonclassical C₇₂ isomer (C_{2 v} ), endohedral chemical shifts have been computed at the GIAO-SCF/3-21G level using B3LYP/6-31G* optimized structures. The experimental ³He NMR signals are reproduced reasonably well in cases where assignments are unambiguous (e.g. C₆₀, C₇₀ and C₇₆). On the basis of the calculated thermodynamic stability order and the comparison between the computed and experimental ³He chemical shifts, the assignments of the observed ³He NMR spectra are discussed for all higher fullerenes, and new assignments are proposed for one C₈₂ and one C₈₆ isomer (C₈₂:3 and C₈₆:17). The calculated helium chemical shifts also suggest the reassignment of the δ(³He) resonances of two C₇₈ isomers. Received: 26 March 2001 / Accepted: 10 May 2001 / Published online: 11 October 2001     

Molecular dynamics, density functional and second-order Møller–Plesset theory study of the structure and conformation of acetylcholine in vacuo and in solution

 Molecular mechanics minimizations based on the CVFF force field and molecular dynamics simulation for a time of 2.5 ns were performed to examine the conformational behaviour and the molecular motion of acetylcholine in vacuo and in aqueous solution. Five low-lying conformations, namely the TT, TG, GG, G*G and GT, were obtained from molecular mechanics computations with the GT structure as the absolute minimum. Molecular dynamics trajectories in vacuo and in water show that only four (GT, GG, G*G and TG) and three (TG, TT and GT) conformations are present in the simulation time, respectively. Density functional B3LYP and second-order M?ller–Plesset (MP2) methods were then used to study all the five lowest-lying conformers of acetylcholine neurotransmitter in vacuo and in water by the polarizable continuum model approach. The B3LYP and MP2 computations show that in the gas phase all minima lie in a narrow range of energy with the G*G conformer as the most stable one. The relative minima GG, GT, TG and TT are located at 1.1 (3.3), 1.8 (4.2), 2.1 (4.5) and 4.3 (7.3) kcal/mol above the absolute one at the B3LYP (MP2) level. The preferred conformation in water is the TG. Solvation reduces the relative energy differences between the five minima in both computations. Received: 4 April 2001 / Accepted: 5 July 2001 / Published online: 30 October 2001     

Reparameterization of hybrid functionals based on energy differences of states of different multiplicity

 Low-spin/high-spin energy splittings for Fe(II) transition-metal complexes – particularly in weak ligand fields – cannot be well described by density functional methods. Different density functionals yield results which differ by up to 1 eV in transition-metal complexes with sulfur-rich first coordination spheres. We attribute this failure to the fact that the high-spin state is systematically favoured in Hartree–Fock-type theories, because Fermi correlation is included in the exact exchange, while Coulomb correlation is not. We thus expect that the admixture of exact exchange to a given density functional will heavily influence the energy splitting between states of different multiplicity. We demonstrate that the energy splitting depends linearly on the coefficient of exact exchange admixture. This remarkable result is found for all the Fe(II)–S complexes studied. From this observation we conclude in connection with experimental results that Becke's 20% admixture should be reduced to about 15% if meaningful energetics are sought for transition-metal compounds. We rationalize that this reduction by 5% will not affect the quality of the hybrid functional since we arrive at a slightly modified functional, which lies between the pure density functional and the hybrid density functional, which both give good results for “standard” systems. Received: 13 July 2001 / Accepted: 31 August 2001 /  Published online: 16 November 2001     

Monte Carlo simulation of 2-ethoxyethanol in continuum configurational biased procedure: conformational analysis and association in aqueous and non-aqueous media

 Monte Carlo simulations have been carried out for 2-ethoxyethanol (C₂E₁) in isothermal-isobaric ensemble (NPT) at different temperatures and 1 atm pressure with a continuum configurational biased procedure in water and chloroform media. Hydrogen bond bridges were formed between adjacent oxygen atoms in C₂E₁ (CH₃CH₂OCH₂CH₂OH) through water molecules. We also found that the stable conformers of C₂E₁ in water and CHCl₃ are different and the effect of temperature on solute-solvent interaction energies is considerable. The self-association of C₂E₁ in aqueous and nonaqueous media has been studied by statistical perturbation theory, and the relative free energy has been obtained at different reaction coordinates by double-wide sampling method. Two minima were found in water solvent in the potential of mean force (PMF), corresponding to the contact and solvent-separated pairs, but only one minimum was found in CHCl₃ solvent corresponding to a contact pair complex. Received: 18 January 2001 / Accepted: 22 October 2001 / Published online: 21 January 2002