ATOMIC SHELL APPROXIMATION - ELECTRON-DENSITY FITTING ALGORITHM RESTRICTING COEFFICIENTS TO POSITIVE VALUES

Expressing molecular electron densities as linear combinations of Guas sian 1S spherical functions extends accurate computations of nb initio quantum molecular similarity measures to large systems. These approxi mate least squares fitted functions lose the density of probability st atistical meaning since some coefficients might be negative. A new alg orithm using positive coefficients only and thus providing an electron density picture as a superposition of atomic shells is presented.