NONEMPIRICAL WAVE-FUNCTIONS FOR VERY LARGE MOLECULES .1. THE PRDDO M METHOD/

We describe enhancements to the method of partial retention of diatomi c differential overlap (PRDDO). The new method, denoted PRDDO/M, emplo ys a basis set of not quite orthogonal atomic orbitals (NQOAOs) and ut ilizes sparse matrix techniques to greatly increase the computational efficiency for large molecules. Other modifications, including a compl ete reparametrization of the method against ab initio STO-3G calculati ons and implementation of integral screening/damping algorithms, are d escribed. The method is an order of magnitude or more faster than are STO 3G single-point calculations using modem ab initio codes, with Lit tle loss in accuracy. (C) 1996 John Wiley & Sons, Inc.