ENSEMBLE REPRESENTABLE DENSITIES FOR ATOMS AND MOLECULES .1. GENERAL-THEORY

A method to obtain ensemble representable densities from experimental diffraction data is proposed. The method uses ab initio molecular dens ities instead of the commonly employed one-electron orbital densities, and as a result, few parameters need to be optimized in the fitting p rocedure to the experimental structure factors. The optimized coeffici ents can provide information about intra- and intermolecular electroni c correlations, spin-orbit coupling, etc. This work also provides new explicit formulas to determine the rank of a fermionic wave function, i.e., the rank of the one-fermion density matrix. (C) 1995 John Wiley & Sons, Inc.