Ab initio compact group model potentials for describing environment effects in cluster calculations

A method for the determination of ab initio group model potentials within t he Hartree-Fock framework is reported. Following the theory of separability of many electron systems, a new way to incorporate the effect of complete chemical entities by means of polycenter compact model potentials is presen ted. The interaction between active and frozen electrons is partitioned as a sum of long- and short-range terms. The long-range term is described as t he effect of -2e charges placed in the center of the charge of the frozen g roup molecular orbitals; the short-range one, the exchange and Pauli repuls ion, is developed as a spectral representation in a nonorthogonal basis set . An algorithm to solve the problem associated with the rotation of the pol ycenter model potential is presented and implemented in an all-purpose quan tum chemical program. In order to check the method, a group model potential for H2O was obtained and tested. (C) 1999 John Wiley & Sons, Inc.