The significance of double substitutions in well‐correlated electronic wave functions

The enhancement of the weighting of double substitutions in correlated wave functions is a very significant effect from including into a wave function the higher‐level substitutions. The coupled cluster model seems quite effective at bringing about this enhancement and because of this it yields correlation energies in line with those obtained by variational configuration interaction explicitly involving higher‐level substitutions. The enhancement of double substitutions as a prominent higher‐order effect is demonstrated by calculations of the HF dipole moment curve, where this enhancement provides an important adjustment to the curve.