ACCURATE ELECTRON-DENSITIES FROM THE HILLER-SUCHER-FEINBERG IDENTITY APPLIED TO CONSTRAINED WAVE-FUNCTIONS

When applied to electronic wavefunctions calculated with Gaussian-type basis functions, the Hiller-Sucher- Feinberg (HSF) identity improves the accuracy of the electron density at non-hydrogen nuclei by more th an an order of magnitude, yielding approximate electron nuclear cusps. However, the HSF electron densities at hydrogen nuclei bound to heavy atoms are greatly overestimated. This phenomenon is associated with t he asymptotic behaviour of the HSF density, which incorrectly decrease s to a constant when the sum of Hellmann-Feynman forces acting on nucl ei is finite. A method for constraining variational wavefunctions to y ield vanishing Hellmann-Feynman forces is described. Hartree-Fock calc ulations of the constrained HSF (CHSF) electron densities with the 6-3 1G, 6-31G*, and 6-311++G** basis sets are reported at the nuclei of v arious diatomic molecules, and are compared with their corresponding c onventional, HSF, and Hartree-Fock limit values. These calculations sh ow that differences between HSF and CHSF densities are minor at non-hy drogen nuclei. Importantly, the calculated HF/6-311 + +G* CHSF densit ies are on average three times more accurate than the conventional den sities at hydrogen nuclei.