9.4 General Comments on MP2 Calculations, Practical Hints

Recommendations

Fast RI-MP2 calculations with the ricc2 program: As pointed out above, the ricc2 program includes (almost) all functionalities of the rimp2 program. Because of slightly refined batching algorithms, screening and symmetry treatment the ricc2 program is usually somewhat faster than rimp2. This is in particular the case when the molecular point group is D2h or a subgroups thereof and a significant number of atoms is positioned on symmetry elements (e.g. planar molecules) and when, because of memory restrictions, the rimp2 program needs many passes for the integral evaluation.

All what is needed for a RI-MP2 gradient calculation with the ricc2 program is a $ricc2 data group with the entry geoopt model=mp2. If you want only the RI-MP2 energy for a single point use as option just mp2. To activate in MP2 energy calculations the evaluation of the D1 diagnostic (for details see Sec. 10.1). use instead mp2 d1diag. (Note that the calculation of the D1 diagnostic increases the costs compared to a MP2 energy evaluation by about a factor of three.)

Comments on the Output

Further output contains indications of the suitability of the (HF+MP2) treatment.