Analytical nuclear gradients for the MP2-R12 method

Analytical energy gradients have been implemented in the DALTON program at the level of explicitly correlated second-order Moller-Plesset perturbation theory (MP2-R12). The implementation has been accomplished for standard approximation A using an auxiliary basis which is the same as the orbital basis for the resolution-of-the-identity, with and without frozen core. As a test calculation, the geometries of nine stationary points on the potential energy surface of the water dimer have been optimized and the results are compared with earlier studies.