Showing papers by "Ernest R. Davidson published in 1968"
TL;DR: In this article, the LiH molecule was examined using limited configuration interaction and potential curves, dipole moments, and oscillator strengths were reported for the first 19 states of LiH.
Abstract: The low‐lying electronic states of the LiH molecule have been examined using limited configuration interaction. Potential curves, dipole moments, and oscillator strengths are reported for the first 19 states. The dipole moment of the ground state was examined in detail. It was found that the discrepancy between SCF and experiment was largely due to the effect of single excitations in second order of perturbation theory.
75 citations
TL;DR: In this article, an iterative procedure involving the use of an elliptical basis set, natural orbitals, pair equations, and consecutive use of the perturbation and variation methods, was used to calculate the ground-state energy of the BeH molecule.
Abstract: An iterative procedure, involving the use of an elliptical basis set, natural orbitals, “pair equations,” and consecutive use of the perturbation and variation methods, was used to calculate the ground‐state energy of the BeH molecule—the 2Σ state. The energy with the best 50‐configuration wavefunction, at the experimental internuclear separation of 2.5380 a.u., was found to be −15.221 a.u. (experimental = −15.254 a.u.), corresponding to 65% of the experimental dissociation energy. A dipole moment of −0.07 a.u. (−0.18 D) was obtained. Population analysis of the molecular wavefunctions was done to investigate the nature of the chemical bond involved. Less complex calculations involving configuration interaction were also done for the low‐lying excited states at nine different internuclear separations. Potential energy curves were plotted. The transition energy obtained (2Π→2Σ) was found to be within 1.3% of the experimental value. Transition probabilities were also considered.
40 citations
TL;DR: In this paper, the low-lying electronic states of the hydrogen fluoride molecule have been investigated using extensive configuration interaction with a limited basis set, and potential curves, dipole moments, and oscillator strengths are given for selected states.
Abstract: The low‐lying electronic states of the hydrogen fluoride molecule have been investigated using extensive configuration interaction with a limited basis set. Potential curves, dipole moments, and oscillator strengths are given for selected states. In agreement with experiment, the results indicate that it would be difficult to observe any discrete absorption spectrum for HF. The dipole moment of the ground state behaves very differently from the SCF result.
39 citations
31 citations
TL;DR: In this paper, a computationally convenient formulation of perturbation theory is developed, in which either the SCF wavefunction or the ScF function plus important corrections may be used as a trial function.
Abstract: A computationally convenient formulation of perturbation theory is developed. In this formulation either the SCF wavefunction or the SCF function plus important corrections may be used as a trial function. Corrections to the trial function are expanded in powers of the residual error. The reduced density matrices may be expanded in a series in this error. If the SCF function is used as the trial function, parts of the density matrix are needed to second order in the error to determine the natural orbitals and geminals to zeroth order.
29 citations
TL;DR: Using first-order perturbation theory, a general equation for the Fermi contact hyperfine splittings of any free radical was derived in this paper, which is essentially the same as that obtained previously by other workers, but restrictions concerning planarity of the radical, sigma-pi separability, choice of spin eigenfunctions, and choice of orbitals used to form excited configurations have been removed.
Abstract: Using first‐order perturbation theory, a general equation is derived for the Fermi contact hyperfine splittings of any free radical. This equation is essentially the same as that obtained previously by other workers, but restrictions concerning (1) planarity of the radical, (2) sigma–pi separability, (3) choice of spin eigenfunctions, and (4) choice of orbitals used to form excited configurations have been removed. The results of calculations of the proton hyperfine splitting (aH) of the prototype pi‐electron hydrocarbon radical, the ·CH fragment, are presented. A seven‐electron wavefunction with a minimum basis set of Slater atomic orbitals is employed. The CH bonding orbital incorporates sp2 hybridization and an electronegativity parameter which is optimized by variational minimization of the ground configuration energy. Admixture of three configurations is considered; two represent the single excitation from CH bonding to antibonding orbital and the third the corresponding double excitation. The splitt...
19 citations
TL;DR: In this paper, the energy of a molecule is expressed as a summation involving the expectations values of a two-electron Hamiltonian, and an approximation which has been used for atomic systems, involving the weight factors in this summation, was made to evaluate the energies for the lithium molecule and of an isolated lithium atom.
Abstract: The energy of a molecule is expressed as a summation involving the expectations values of a two-electron Hamiltonian. An approximation which has been used for atomic systems, involving the weight factors in this summation, was made to evaluate the energies for the lithium molecule and of an isolated lithium atom. It was found that at 4.0 a. u. the bonding energy was of the correct order of magnitude, but the correlation energy of the system was overestimated giving an error in the total energy of the molecule of 1.3%. An alternative approximation for the evaluation of the summation was proposed, and it was found to give rise to an error of 0.3% in the total energy and a bonding energy of the correct order of magnitude. Attention is given to the application of geminal methods to larger systems.
9 citations