Davidson Correction - Size-consistency and Size-extensivity Problem

Size-consistency and Size-extensivity Problem

Davidson correction improves both size consistency and size extensivity of CISD energies. Therefore, Davidson correction is frequently referred to in literature as size-consistency correction or size-extensivity correction.

However, neither Davidson correction itself nor the corrected energies are size-consistent or size-extensive. This is especially the case in larger molecules, where contribution from higher than quadruple excitations becomes more significant.