Ensemble representable densities for atoms and molecules. I. General theory

A method to obtain ensemble representable densities from experimental diffraction data is proposed. The method uses ab initio molecular densities instead of the commonly employed one-electron orbital densities, and as a result, few parameters need to be optimized in the fitting procedure to the experimental structure factors. The optimized coefficients can provide information about intra- and intermolecular electronic 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. © 1995 John Wiley & Sons, Inc.