Calculations with rimp2 and ricc2

1. RI-MP2 calculations require the specification of auxiliary basis sets ($cbas) and a converged SCF calculation with the one-electron density convergence threshold set to $denconv 1.d-7 or less. In addition, the options $freeze (frozen core approximation) and $maxcor (maximum core memory usage) should be set. All these settings can be done during the input generation with the program define under the entry mp2\cc2 of last main menu.
2. Alternatively, the interactive program Rimp2prep can be used: This program sets default values for auxiliary basis sets (data group $cbas), for frozen core orbitals (data group $freeze, all orbitals with energies below -3a.u. are suggested to be frozen) and for the amount of memory to be allocated ($maxcor. These defaults can be confirmed with return or modified, if desired. Note: the amount of memory to be allocated determines the number of multiple passes and thus the efficiency of rimp2.
   It is also possible to run Rimp2prep directly after define.
3. The ricc2 program requires the data group:
   $ricc2
mp2
geoopt model=mp2
   (Where the last line should only be included if the calculation of gradients is needed e.g. in geometry optimizations.) This can be prepared with define in the menu mp2\cc2.
4. For explicitly-correlated MP2-F12 calculations with ricc2 also the data group $rir12 is needed.
5. Start a single rimp2 calculation with the command ricc2 or rimp2.
6. For optimisation of structure parameters at the RI-MP2 level use the command jobex -level cc2 or jobex -ri -level mp2. For geometry optimizations with RI-JK-SCF as reference for RI-MP2 with the ridft and ricc2 binaries the additional option -rijk has to be given.
7. The combination of RI-MP2 with RI-JK-SCF can lead to significant computational savings in particular for geometry optimizations for small and medium sized molecules with large basis sets (quadruple-ζ and beyond) or basis sets with diffuse functions (e.g. the aug-cc-pVXZ basis set families). For large molecules with TZVPP or similar basis sets, conventional direct SCF calculations usually more efficient.
8. With ricc2 spin-component scaled SCS- or SOS-RI-MP2 calculations can be carried out by adding in the $ricc2 data group the line
   scs cos=1.2d0 css=0.3333d0
   where the two parameters are the scaling factors for, respectively, the opposite- and same-spin contribution. The specification of the scaling factors is optional; the default values are cos=6/5 and css=1/3 as recommended by S. Grimme in J. Chem. Phys. 118 (2003) 9095. The abbreviation sos can be used for SOS-MP2 calculations with cos=1.3 and css=0.0 (Y., Jung, R.C. Lochan, A.D. Dutoi, and M. Head-Gordon, J. Chem. Phys. 121 (2004) 9793.).
9. For technical recommendations and additional options for parallel RI-MP2 calculations with the ricc2 program see Secs. 3.2 and 9.5.