A SIMPLIFIED EPSILON-SEPARATION METHOD - MULTICENTER INTEGRALS AND TEST CALCULATIONS OF SMALL MOLECULES

A new approximate method of evaluating multicenter molecular integrals with Slater functions has been developed with the help of a simplifie d version of the SIGMA-separation approach. With respect to calculatio nal effort and computation time the proposed method is approximately e quivalent to the Mulliken approximation and simple variants of the Rue denberg approximation. However, the method ensures greater accuracy co mpared to these earlier approaches. The advantage of the SIGMA-approxi mation is especially important in the evaluation of integrals with d a nd f functions. A new simplified nonempirical variant of the Hartree-F ock-Roothaan method has been developed within the general framework of the SIGMA-separation approach. The balance between certain types of o ne- and two-electron integrals is shown to exert a substantial influen ce on the overall accuracy of the method. The practical value of the a pproach is illustrated by calculations of total energies, dipole momen ts and force constants.