Total energy assessment of the quality of localized basis sets: O2 molecule

The quality of localized basis sets in molecular calculations is assessed using the total energy as a measure of variational freedom. Basis sets which also include a set of on-site functions obtained from positive ions are found to be far better variationally than those which include virtual (unoccupied) atomic levels: In particular, the inclusion of higher virtual states in the standard LCAO (linear combination of atomic orbitals) basis does not prove convergence. These efficient and accurate basis set results for O2 are in very good agreement with previously published full-potential linearized augmented plane-wave results and LCAO results using off-site functions.