Extension of Gaussian‐2 (G2) theory to bromine‐ and iodine‐containing molecules: Use of effective core potentials (original) (raw)
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Journal of Molecular Structure: THEOCHEM, 1995
In order to prepare basis sets particularly well suited for the calculation of electron-pair correlation energies III molecular systems. two-electron functions are constructed from components which lead to integrals of adequate simplicity: i.e., from Gaussian functions representing the one-electron components, and with the aid of faCtOr5 exp(-yv:*) and of two parameters (C,. C,). The one-electron functions are generated from four parameters (A,,, A,. A2, A3). With a transition from He to Li+, Be'+, B3+, C4+. N"+. 06'. F7+. Ne8+, or H-, the reoptimization of a single parameter (A") is sufficient to obtain an electronic energy of the same high precision (between 10m6 and lo-' a.u. for a Cl dimension of 416) which results from a reoptimization of all six parameters.
Re-examination of atomization energies for the Gaussian-2 set of molecules
The Journal of Chemical Physics, 1999
Atomization energies were computed for 73 molecules, many of them chosen from the GAUSSIAN-2 and G2/97 test sets. A composite theoretical approach was adopted which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory with a quasiperturbative treatment of triple excitations and three corrections: (1) a coupled cluster core/valence correction; (2) a configuration interaction scalar relativistic correction; and (3) an atomic spin-orbital correction. A fourth correction, corresponding to more extensive correlation recovery via coupled cluster theory with an approximate treatment of quadruple excitations, was examined in a limited number of cases. For the molecules and basis sets considered in this study, failure to consider any of these contributions to the atomization energy can introduce errors on the order of 1-2 kcal/mol. Although some cancellation of error is common, it is by no means universal and cannot be relied upon for high accuracy. With the largest available basis sets (including, in some cases, up through aug-cc-pV6Z), the mean absolute deviation with respect to experiment was found to lie in the 0.7-0.8 kcal/mol range, neglecting the effects of higher order excitations. Worst case errors were 2-3 kcal/mol. Several complete basis set extrapolations were tested with regard to their effectiveness at improving agreement with experiment, but the statistical difference among the various approaches was small.
Highly accurate calculations of molecular electronic structure
1999
The highly accurate calculation of molecular electronic structure requires the expansion of the molecular electronic wavefunction to be as nearly complete as possible both in one-and nelectron space. In this review, we consider the convergence behaviour of computed electronic energies, in particular electronic enthalpies of reaction, as a function of the one-electron space. Based on the convergence behaviour, extrapolations to the limit of a complete one-electron basis are possible and such extrapolations are compared with the direct computation of electronic energies near the basis-set limit by means of explicitly correlated methods. The most elaborate and accurate computations are put into perspective with respect to standard and-from a computational point of view-inexpensive density functional, complete basis set (CBS) and Gaussian-2 calculations. Using the explicitly correlated coupled-cluster method including singles, doubles and non-iterative triples replacements, it is possible to compute (the electronic part of) enthalpies of reaction accurate to within 1 kJ mol −1 . To achieve this level of accuracy with standard coupled-cluster methods, large basis sets or extrapolations to the basis-set limit are necessary to exploit fully the intrinsic accuracy of the coupled-cluster methods.
The Journal of Chemical Physics, 1998
The Gaussian-2 ͑G2͒ collection of atoms and molecules has been studied with Hartree-Fock and correlated levels of theory, ranging from second-order perturbation theory to coupled cluster theory with noniterative inclusion of triple excitations. By exploiting the systematic convergence properties of the correlation consistent family of basis sets, complete basis set limits were estimated for a large number of the G2 energetic properties. Deviations with respect to experimentally derived energy differences corresponding to rigid molecules were obtained for 15 basis set/method combinations, as well as the estimated complete basis set limit. The latter values are necessary for establishing the intrinsic error for each method. In order to perform this analysis, the information generated in the present study was combined with the results of many previous benchmark studies in an electronic database, where it is available for use by other software tools. Such tools can assist users of electronic structure codes in making appropriate basis set and method choices that will increase the likelihood of achieving their accuracy goals without wasteful expenditures of computer resources.
Gaussian-3 (G3) theory for molecules containing first and second-row atoms
The Journal of Chemical Physics, 1998
Gaussian-3 theory ͑G3 theory͒ for the calculation of molecular energies of compounds containing first ͑Li-F͒ and second row ͑Na-Cl͒ atoms is presented. This new theoretical procedure, which is based on ab initio molecular-orbital theory, modifies G2 theory ͓J. Chem. Phys. 94, 7221 ͑1991͔͒ in several ways including a new sequence of single point energy calculations using different basis sets, a new formulation of the higher level correction, a spin-orbit correction for atoms, and a correction for core correlation. G3 theory is assessed using 299 energies from the G2/97 test set including enthalpies of formation, ionization potentials, electron affinities, and proton affinities. This new procedure corrects many of the deficiencies of G2 theory. There is a large improvement for nonhydrogen systems such as SiF 4 and CF 4 , substituted hydrocarbons, and unsaturated cyclic species. Core-related correlation is found to be a significant factor, especially for species with unsaturated rings. The average absolute deviation from experiment for the 148 calculated enthalpies of formation is reduced to under one kcal/mol, from 1.56 kcal/mol for G2 theory to 0.94 kcal/mol for G3 theory. Significant improvement is also found for ionization potentials and electron affinities. The overall average absolute deviation of G3 theory from experiment for the 299 energies is 1.02 kcal/mol compared to 1.48 kcal/mol for G2 theory.
Journal of Structural Chemistry, 2008
A systematic study was performed to examine the possibilities of the B3LYP DFT method in a dgdzvp fullelectron basis and of the method including a pseudopotential for iodine compounds. The full-electron basis generally gives better agreement for X-I bond lengths and reaction enthalpies of iodination of organic compounds and equally good agreement in calculations of the IR vibrations of the X-I bond length compared with the studies using the pseudopotential. The full-electron basis also allows adequate calculations of the quadrupole coupling constants of iodine atoms and is generally characterized by smaller computing times.
The Journal of Chemical Physics, 2002
Correlation consistent basis sets for accurately describing core-core and core-valence correlation effects in atoms and molecules have been developed for the second row atoms Al-Ar. Two different optimization strategies were investigated, which led to two families of core-valence basis sets when the optimized functions were added to the standard correlation consistent basis sets (cc-pVnZ). In the first case, the exponents of the augmenting primitive Gaussian functions were optimized with respect to the difference between all-electron and valence-electron correlated calculations, i.e., for the core-core plus core-valence correlation energy. This yielded the cc-pCVnZ family of basis sets, which are analogous to the sets developed previously for the first row atoms ͓D. E. Woon and T. H. Dunning, Jr., J. Chem. Phys. 103, 4572 ͑1995͔͒. Although the cc-pCVnZ sets exhibit systematic convergence to the all-electron correlation energy at the complete basis set limit, the intershell ͑core-valence͒ correlation energy converges more slowly than the intrashell ͑core-core͒ correlation energy. Since the effect of including the core electrons on the calculation of molecular properties tends to be dominated by core-valence correlation effects, a second scheme for determining the augmenting functions was investigated. In this approach, the exponents of the functions to be added to the cc-pVnZ sets were optimized with respect to just the core-valence ͑intershell͒ correlation energy, except that a small amount of core-core correlation energy was included in order to ensure systematic convergence to the complete basis set limit. These new sets, denoted weighted corevalence basis sets (cc-pwCVnZ), significantly improve the convergence of many molecular properties with n. Optimum cc-pwCVnZ sets for the first-row atoms were also developed and show similar advantages. Both the cc-pCVnZ and cc-pwCVnZ basis sets were benchmarked in coupled cluster ͓CCSD͑T͔͒ calculations on a series of second row homonuclear diatomic molecules (Al 2 , Si 2 , P 2 , S 2 , and Cl 2 ), as well as on selected diatomic molecules involving first row atoms ͑CO, SiO, PN, and BCl͒. For the calculation of core correlation effects on energetic and spectroscopic properties, the cc-pwCVnZ basis sets are recommended over the cc-pCVnZ ones.