Analytic Molecular Hessian Calculations for CC2 and MP2 Combined with the Resolution of Identity Approximation

An implementation of analytic second derivatives for the approximate coupled cluster singles and doubles model CC2 and for second-order Møller-Plesset perturbation theory (MP2) will be presented. The RI approximation for the two-electron repulsion integrals is used to reduce memory demands, operation count, and I/O requirements. During the calculation, the storage of [Formula: see text] quantities (where [Formula: see text] is a measure for the system size) can completely be avoided. It is shown that with the MP2 method and an appropriate scaling of the harmonic frequencies, especially C-F stretch frequencies are reproduced much better in comparison to experiments than with the B3LYP density functional. Similar advantages are observed for molecules with strong, internal van der Waals interactions. Spin scaling offers additional improvements in these cases. The implementation has been tested for molecules with up to 81 atoms and 684 basis functions.