Weighted density approximation applied to diatomic molecules

Early investigations into the weighted spin-density approximation (WSDA) report atomic energies which err by more than those in the much simpler local-spin-density approximation (LSDA). More recent work has refined the WSDA to obtain substantial improvement over the LSDA in atomic energies. These results suggest that a WSDA may improve quantum-chemistry calculations. The extension of the method from atoms to molecules is nontrivial, however, and we know of no published work applying a WSDA to molecules. We develop an energy functional which treats correlation locally, and exchange in the WSDA. The functional achieves a substantial improvement over the LSDA in atomic energies. We calculate bond lengths, dissociation energies, and vibrational frequencies for some first-row dimers. In particular, the hydrogen dimer is described well with this functional. Dissociation energies are also relatively accurate for the heavier dimers. Except for H-2, however, bond lengths tend to be longer than experiments indicate. We discuss features of the WSDA necessary to improve these findings.