Analytic energy gradients for the two-determinant coupled cluster method with application to singlet excited states of butadiene and ozone

Recently, the coupled cluster method has been generalized to a two-determinant reference (TD-CC method) applicable to open-shell singlet states. In this paper, we report the formulas and an efficient algorithm to calculate analytic energy derivatives for this new method. The implementation has been carried out in the ACES II many-body ab initio program system. The new method was first applied to the 1Bu state of butadiene and several excited states of ozone. We report geometries and one-electron properties for those states.