First-order nonadiabatic coupling matrix elements using coupled cluster methods. I. Theory

It is shown how first-order nonadiabatic coupling matrix elements can be ca lculated using coupled cluster electronic structure methods. The formalism is consistent with the coupled cluster response theory approach for calcula tion of excitation energies and adiabatic transition properties. Expression s are derived that are in the limit of a complete coupled cluster expansion give results equivalent to the full configuration interaction results. Com putational tractable expressions are given for the first-order nonadiabatic coupling matrix in coupled cluster theory. The final expressions are quite similar to those employed in the implementation of the analytical calculat ion of molecular gradients. (C) 1999 American Institute of Physics. [S0021- 9606(99)30601-2].