A simplified E-separation method: multicenter integrals and test calculations of small molecules

Abstract

A new approximate method of evaluating multicenter molecular integrals with Slater functions has been developed with the help of a simplified version of the Σ-separation approach. With respect to calculational effort and computation time the proposed method is approximately equivalent to the Mulliken approximation and simple variants of the Ruedenberg approximation. However, the method ensures greater accuracy compared to these earlier approaches. The advantage of the Σ-approximation is especially important in the evaluation of integrals with d and f functions. A new simplified nonempirical variant of the Hartree-Fock-Roothaan method has been developed within the general framework of the Σ-separation approach. The balance between certain types of one- and two-electron integrals is shown to exert a substantial influence on the overall accuracy of the method. The practical value of the approach is illustrated by calculations of total energies, dipole moments and force constants.