Non-adiabatic couplings in the EOM-CC framework

This talk will describe theory of non-adiabatic coupling (NAC) forces within the EOM-CCSD framework and present a recent implementation. Using illustrative examples, I will compare NAC forces computed using EOM-CC and multi-reference (MR) wave functions (for selected cases, we also consider configuration interaction singles). In addition to the magnitude of the NAC forces, I will also analyze their direction, which is important for the calculations of the rate of nonadiabatic transitions. The numeric results confirm that the accuracy of NAC forces depends critically on the quality of the underlying wave functions. Within their domain of applicability, EOM-CC provides a viable alternative to MR methods.