Approximations in local density functional calculations for molecules and clusters: applications to C2and H2O

Approximate expressions for the Coulomb U and exchange-correlation E xc energies in LDF theory for molecules and atomic clusters are discussed. Evaluating U using a constrained fit to the charge density leads to terms in the potential, derived from U, not discussed by previous workers. A method is described of evaluating E xc which is fast and requires only a fit to the charge density. Results of self-consistent pseudopotential calculations are given for the molecules C 2 and H 2 O.