Gradients for the partitioned equation-of-motion coupled-cluster method

Gradients for the partitioned equation-of-motion coupled-cluster method based on a second-order many-body perturbation theory ground state have been derived. The accuracy of this new method has been compared to the accuracy of other single reference methods for excited states, namely configuration interaction limited to single excitations (CIS), CIS with a perturbative doubles correction, and equation-of-motion coupled-cluster theory based on a coupled-cluster singles and doubles ground state. These have been compared based on calculations of low lying singlet excited states of H-2, BH, BF, C-2, CO, and N-2, as well as the lowest singlet excited states of ammonia, formaldehyde, acetaldehyde, and acetone. Also, a question about the assignments of the vibrational spectrum for the first excited state of acetone is resolved. (C) 1999 American Institute of Physics. [S0021-9606(99)01901-7].