Showing papers by "Ernest R. Davidson published in 1970"
TL;DR: In this article, a detailed study of the correlation energy of Ne has been made in order to analyze the nature of the configurationinteraction (CI) method in ab initio calculations, and both the Bethe-Goldstone method of Nesbet and the total-pair-excitation-block method (TPEB) were examined.
Abstract: A detailed study of the correlation energy of Ne has been made in order to analyze the nature of the configuration-interaction (CI) method in ab initio calculations. Both the Bethe-Goldstone method of Nesbet and the total-pair-excitation-block method (TPEB) were examined. A series of calculations was made using both "atom-centered" and "shell-centered" basis sets. The most detailed calculations gave - 0.333 a.u. (88%) for the correlation energy by the TPEB method, and - 0.396 a.u. (104%) by the sum-of-the-pairs technique. The difference between these two values is mainly the so-called pair-pair interactions, which have been considered too small to be important to other investigators. A series of perturbation calculations on the triple and quadruple excitations gives \ensuremath{\sim} 1.5% of the total correlation energy. A complete CI calculation with a very limited basis set was done on the $p$ block of Ne. The results of this calculation are in agreement with our total-atom calculation, except that now the TPEB calculation gave about 98.5%, and the sum of the pairs about 115% of the complete CI result. The effect on the pair correlation energy of a unitary transformation of the outer-shell occupied self-consistent-field orbitals was also studied. Only a small difference in the results was obtained.
134 citations
TL;DR: Chang et al. as mentioned in this paper performed nonempirical calculations of the ground-state electronic wavefunction, energy, and proton and carbon−13 contact hyperfine splittings of methyl radical in its planar configuration using the spin-restricted SCF method and configuration interaction including all spin configurations involving up to double replacement of space orbitals.
Abstract: Nonempirical calculations of the ground‐state electronic wavefunction, energy, and proton and carbon‐13 contact hyperfine splittings of methyl radical in its planar configuration are performed using the spin‐restricted SCF method and configuration interaction including all spin configurations involving up to double replacement of space orbitals. Two minimum basis sets of Slater‐type orbitals are employed, one in which orbital exponents are chosen according to Slater's rules (unoptimized) and the other in which they are optimized through minimization of the SCF energy. Some calculations are made which involve double‐zeta basis sets. Considerable progress toward the goal of accurate ab initio calculations of hyperfine splittings is reported [S. Y. Chang, E. R. Davidson, and G. Vincow, J. Chem. Phys. 49, 529 (1968), paper I in this series]. The proton splittings computed using the unoptimized (unopt.) (1) and optimized (opt.) (2) minimum basis sets and a double‐zeta set (3) (free atom exponents on carbon and...
58 citations
TL;DR: In this article, a large configuration-interaction calculation has been performed to determine the wave function, energy, and molecular properties of CO. The most important configurations were used to obtain the natural geminals and their occupation numbers.
Abstract: A large configuration-interaction calculation has been performed to determine the wave function, energy, and molecular properties of CO. The most important configurations were used to obtain the natural geminals and their occupation numbers. A pair-energy approach to the correlation energy was attempted with results which differ significantly from the configuration-interaction results.
49 citations
TL;DR: Within the Born-Oppenheimer framework, extended configuration interaction calculations, involving the use of natural orbitals, are carried out to study the ground-state energy, wave function, and selected molecular properties at the experimental equilibrium internuclear separation of the MgH molecule.
Abstract: Within the Born–Oppenheimer framework, extended configuration‐interaction calculations, involving the use of natural orbitals, are carried out to study the ground‐state energy, wavefunction, and selected molecular properties at the experimental equilibrium internuclear separation of the MgH molecule. Electronic population analysis of the elaborate molecular wavefunctions is done to investigate the nature of the chemical bond involved. More limited configuration‐interaction calculations are also carried out to study the low‐lying excited states of the MgH molecule. Potential‐energy curves are plotted, and transition probabilities are also considered. An estimated dissociation energy of 2.0 ± 0.2 eV has been deduced for the MgH molecule. A dipole moment of − 0.595 a.u. (− 1.511 D) was also obtained. The various results obtained confirmed the intuitive belief that the BeH and the MgH molecules are cogeners. Careful analysis of the accurate molecular wavefunctions reveals that the promotion of an s electron to the corresponding p orbital in bond formation is characteristic of the BeH and the MgH molecules. The calculated transition energy (A 2Π → X 2Σ+) for the MgH molecule was within 5% of the experimental value.
30 citations
26 citations