Analytical gradient of the CIS ( D ) perturbative correction to single‐excitation configuration interaction excited states

The complete spin–orbital formulation of the analytical first‐derivative expression of the CIS ( D ) energy correction (doubles correction to the excitation energy from configuration interaction with single substitution, CIS ) is presented. The CIS ( D ) method gives a perturbative correction to the CIS energy (which is roughly analogous to the Hartree–Fock energy for ground states) in a way that resembles second–order Møller–Plesset ( MP 2) theory for ground states. © 1995 John Wiley & Sons, Inc.