A weighted spin density approximation for chemistry: importance of shell partitioning

We have previously developed an efficient implementation of the weighted density approximation for use in quantum chemistry. While the ground state atomic energies were accurate, the results for bond lengths, vibrations and dissociation energies of small homo-nuclear dimers were disappointing. This formulation, however, did not correct for the effects of inter-shell interactions. When the density is partitioned into core and valence contributions, the results are in good agreement with experimental measurements, and improve upon the predictions from the local spin-density approximation and a representative generalized gradient approximation.