CCSD-PCM Excited State Energy Gradients with the Linear Response Singles Approximation to Study the Photochemistry of Molecules in Solution

We present the theory and the implementation for the excited state energy gradients with respect to nuclear displacements for the linear response coupled cluster with single and double excitations (LR-CCSD) combined with the polarizable continuum model of solvation within the LR formalism and the singles approximation (LR-PCM-PTES). This approach allows the exploration of the excited state potential energy surface of solvated molecules for the evaluation of minima and other stationary points, at a computational cost virtually identical to that of gas-phase LR-CCSD. The method is applied to the evaluation of vertical absorption and emission energies of 4-(N,N)-dimethyl-aminobenzonitrile (DMABN) in gas phase, cyclohexane, and acetonitrile solutions. The method is able to find the minima for the locally excited and charge transfer states, which are responsible for the dual-fluorescence of this compound. At the same time, some limitations of the LR and state specific (SS) formalisms for excited state solvation, which suffer from under or overestimation of the solvent effect in excited states, are discussed.