Use of promolecular ASA density functions as a general algorithm to obtain starting MO in SCF calculations

Atomic shell approximation (ASA) constitutes a way to fit first-order density functions to a linear combination of spherical functions. The ASA fitting method makes use of positive definite expansion coefficients to ensure appropriate probability distribution features. The ASA electron density is sufficiently accurate for the practical implementation of quantum similarity measures, as was proved in previous published work. Here, a new application of the ASA density formalism is analyzed, and employed to obtain an initial guess of the density matrix for SCF procedures. The number of cycles needed to assess the convergence criterion in electronic energy calculations appears comparable to or less than those obtained by other means. Several molecular structures of different classes, including organic systems and metal complexes, were chosen as representative test cases. In addition, an ASA basis set for atoms Sc-Kr fitted to an ab initio 6-311G basis set is also presented. (C) 2002 John Wiley Sons, Inc.