Automatic derivation and evaluation of vibrational coupled cluster theory equations.

A scheme for automatic derivation and evaluation of the expressions occurring in vibrational coupled cluster theory is introduced. The method is based on a Baker-Campbell-Hausdorff expansion of the similarity transformed Hamiltonian and is general both with respect to the excitation level in the parameter space and the mode coupling level in the Hamiltonian. In addition to deriving general expressions, intermediates that lower the computational scaling are automatically detected. The final equations are then evaluated. Due to the commutator based nature of the algorithm, it is also applicable to the evaluation of quantities needed for response theory. Different aspects of the theory and implementation are illustrated by calculations on model systems. Furthermore, all fundamental excitation energies of ethylene oxide are calculated.