Analytical energy gradients for a unitary coupled-cluster theory

Our recently developed and tested unitary multiconfigurational coupled-cluster electronic wavefunction method is extended to permit, for the first time, the analytical evaluation of energy derivatives. The unitary nature of this method admits a variational energy functional whose stationary nature plays a key role in simplifying our derivation. Explicit expressions are given for the gradient (first energy derivative) for both the full unitary coupled cluster and its coupled electron pair approximation (CEPA).