- 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. - 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`. - 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 menus`mp2`

or`cc2`

. - For explicitly-correlated MP2-F12 calculations with
`ricc2`also the data groups`$rir12`and`$lcg`is needed. - Start a single
`rimp2`calculation with the command`ricc2`or`rimp2`. - 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. - 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 are usually more efficient. - 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`

*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.). SOS-MP2 is in`ricc2`implemented with (^{4}) scaling costs. For such calculations the data group`$laplace`has to be added. - For technical recommendations and additional options for
parallel RI-MP2 calculations
with the
`ricc2`program see Secs. 3.2 and 9.6.