B. Roos

Gaussian basis sets for the first and second row atoms

Gaussian basis sets consisting for first row atoms of 7 s-type and 3 p-type and for second row atoms of 10 s-type and 6 p-type functions with optimized exponents are reported. These basis sets consists of at least two functions per atomic orbital.ZusammenfassungEs werden für die Atome der ersten und zweiten Reihe Basissätze aus Gaußfunktionen mitgeteilt, die aus 7 Funktionen vom s-Typ und 3 Funktionen vom p-Typ für die Elemente der ersten Reihe und 10 Funktionen vom s-Typ und 6 Funktionen vom p-Typ für die Elemente der zweiten Reihe mit optimierten Exponenten bestehen. Diese Basissätze bestehen aus wenigstens zwei Funktionen pro Atomorbital.RésuméUne base de 7 gaussiens du type s et 3 du type p est presenté pour les éléments du premier rang et de 10 gaussiens du type s et 6 du type p pour des éléments du deuxième rang; les exposants sont optimisés. Les bases consistent au moins en deux fonctions par orbital atomique.

Density matrix averaged atomic natural orbital (ANO) basis sets for correlated molecular wave functions: IV. Medium size basis sets for the atoms H-Kr

SummaryGenerally contracted Basis sets for the atoms H-Kr have been constructed using the atomic natural orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over the most significant electronic states, the ground state of the cation, the ground state of the anion for some atoms and the homonuclear diatomic molecule at equilibrium distance for some atoms. The contracted basis sets yield excellent results for properties of molecules such as bond-strengths and-lengths, vibrational frequencies, and good results for valence spectra, ionization potentials and electron affinities of the atoms, considering the small size of these sets. The basis sets presented in this article constitute a balanced sequence of basis sets suitable for larger systems, where economy in basis set size is of importance.

A critical study of basis set effects and the use of approximate natural orbitals in SCF-CI calculations of molecular geometries and heats of reaction

SCF-CI calculations have been performed on a number of chemical reactions between closed shell molecules in order to determine the heats of reaction. Contracted Gaussian type atomic basis sets of three different qualities were used and the CI calculations were performed in a truncated approximate natural orbital space. The conclusions to be drawn from these calculations are rather pessimistic. For heats of reaction, errors up to 6 kcal/mole are obtained on the SCF-level with a double zeta plus polarization atomic basis. A further improvement is only possible if extended basis sets are used. Correlation effects on heats of reaction are of the same size and CI calculations are therefore only meaningful with large atomic basis sets.For the CI calculations a one-electron space of approximate natural orbitals, obtained from second order RS perturbation theory, was used. Different truncations, using the occupation number as criterion, were tested. The general conclusion is that errors in energy differences obtained with a truncated basis set are of the same magnitude as the error in the total correlation energy. In practice this means that not more than 20–30% of the approximate natural orbitals can be deleted if the error is to be kept less than a few kcal/mole.Finally the truncation error in calculations of bond distances was tested for a few cases. Errors of around 10% of the total change due to correlation were found when 30% of the lowest occupied natural orbitals were deleted.

MC–SCF and CI calculations for the ammonia molecule

Multiconfiguration self‐consistent field calculations are reported for the ammonia molecule, both with and without symmetry restrictions. Correlation energies of 0.039 and 0.080 a.u. have been obtained, respectively, for the core and the valence shell (without the symmetry restriction). It is found that the MC–SCF method is not stable with respect to the symmetry constraint, as the result of a better description of the left‐right correlation. A comparison is made of configuration interaction calculations based either on canonical SCF or MC–SCF or approximate natural orbitals. Comparable values of the correlation energy are obtained from CI calculations which are based on the same number of MC–SCF orbitals or approximate natural orbitals. Analysis in terms of pair correlation energies shows that the approximate natural orbitals achieve less of the intrapair and more of the interpair correlation energy than the MC–SCF orbitals. Strongly occupied natural orbitals are close to the corresponding SCF orbitals, ...