Efficient and accurate expansion methods for molecules in local density models

The effects of truncation of the effective Hamiltonian of local density theory through shape approximations to the molecular charge density are examined. A self-consistent multicenter-multipolar (SCM) representation of the density is introduced, which allows calculation of the Coulomb and exchange-correlation potentials to any desired level of precision. The related question of quality of wave function expansion bases required to reproduce spectra and densities with chemical accuracy is explored. The efficiency and accuracy of the SCM approach are verified by applications to O2 and CO; results are given for the metal cluster compound Ru3(CO)12.