Electron Pairs Method Research Articles

Ab initio study of the pair potentials and electric dipole moments of the ArHe and NeHe diatoms

Large basis set Ab Initio calculations are reported for the variation of energy and electric dipole moment with internuclear separation for the diatoms ArHe and NeHe. Electron correlation effects are treated explicitly by the self consistent electron pairs method, and are found to be large. The effect of basis set superposition is examined, and found to be significant. Calculated potential well depths De and equilibrium internuclear separation Re compare with the em and Rm values found by fitting various experimental data. The electric dipole moment pe for the ArHe diatom at distances ≈ RIe is larger than that for the NeHe diatom, in accordance with the fact that the full collision induced absorption spectrum has been observed experimentally only for the latter. The variation of pe with R shows fine structure at R ≈ Re not previously reported.

Recent computational developments with the self‐consistent electron pairs method and application to the stability of glycine conformers

AbstractSignificant improvements in calculational efficiency and capability with the self‐consistent electron pairs (SCEP) method has resulted from several new computational developments. A new procedure for constructing the important internal Coulomb and exchange operators has substantially reduced the preiteration time. A general scheme for utilizing molecular symmetry has been used to advantage in reducing the number of pair functions and external operators that must be found explicitly at each iteration. A projection operator tool has been implemented and found to be quite effective at minimizing the number of iterations required at some point on a potential energy surface when an SCEP wavefunction exists for some nearby point. These and other improvements in the program construction have yielded sizable reductions in time for some representative test cases, including water and a potential energy curve for formaldehyde. The new SCEP program also performs low‐order perturbation theory treatments and coupled electron pair approximation (CEPA) calculations using the same operator approach. The usefulness of the approach is demonstrated by very large scale calculations on the stability of the two interstellar glycine conformers. These calculations involve the variational treatment of 82,205 symmetry‐adapted singly and doubly substituted configurations involving 225 internal electron pairs.

Inclusion of some higher order effects with the self-consistent electron pairs method for correlated molecular wavefunctions

An efficient technique is presented for the inclusion of some higher order effects in correlated wavefunctions. Diagonal quadruple substitutions (e.g., i2j2→ a2b2) are variationally added to a self-consistent electron pairs (SCEP) wavefunction that includes single and double substitutions relative to a closed shell reference configuration. Through the use of approximate natural orbitals, these configurations include those which are important for the size-consistency correction at the limit of separated systems. While the number of the diagonal quadruple configurations is of the order of the number of doubles, the computational effort for including the quadruples is less than for the doubles, because of the nature of the required matrix elements and the flexibility of SCEP. Variationally additive quadruple corrections to SCEP pair energies are easily found with this new procedure.

The Theory of the Structure of Hydrogen Peroxide and Hydrazine

The method of electron pairs has been applied to determine the most stable configuration for the atoms in the molecules HO–OH and H2N–NH2. The calculations show that free rotation cannot occur in either of these molecules at ordinary temperatures and that the only stable forms are those in which the azimuth of one-half of the molecule with respect to the other is approximately 90°. The essential feature of the argument is that the electrons of the two central atoms arrange themselves to form the strongest possible bonds and in so doing render the charge density on these atoms unsymmetrical about the axis of the molecule. The interaction of the two electronic clouds is the dominant factor in determining the azimuth of one group relative to the other; all other types of interaction have been estimated to shift the equilibrium position only slightly, i.e., probably from 90° to 100°.