The analytic gradient for the equation-of-motion coupled-cluster energy with a reduced molecular orbital space: An application for the first excited state of formaldehyde

The analytic gradient method for the equation-of-motion coupled-cluster (EOM-CC) energy has been extended to employ a reduced molecular orbital (MO) space. The geometrical and the vibrational properties of the first excited singlet state of formaldehyde have been studied by using several different sizes of dropped MO space. It is shown that reliable results can be produced with high efficiency by employing the EOM-CC with a reduced MO space, which provides a new hierarchy in the study of photochemical processes in molecules.