Analytic energy gradients for the equation-of-motion coupled-cluster method: Implementation and application to the HCN/HNC system

A recently developed theory for calculating analytic first derivatives of the energy for excited states treated by the equation-of-motion coupled-cluster (EOM-CC) method has been implemented and applied to study potential energy surfaces in the HCN/HNC system. The EOM-CC singles and doubles (EOM-CCSD) approximation is used to predict equilibrium structures, energies, dipole moments, harmonic vibrational frequencies and infrared intensities of HCN and HNC isomers in both the à 1A″ and B̃ 1A′ excited states. In addition, the activated complex for HCN→HNC isomerization in these states is investigated theoretically for the first time.