9.3 How to Prepare and Perform MP2 Calculations

Prerequisites

Calculations with mpgrad, rimp2 or ricc2 require

Calculations with rimp2 and ricc2 moreover require

This is not needed for mpgrad , but here one needs

For explicitly-correlated MP2-F12 calculations one needs–depending the details of the applied approximations–additionally a so-called complementary auxiliary basis set (CABS, defined in $cabs) and a RI-SCF auxiliary basis set defined in $jkbas.

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 menus mp2 or cc2.
4.
For explicitly-correlated MP2-F12 calculations with ricc2 also the data groups $rir12 and $lcg 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 are 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.). SOS-MP2 is in ricc2 implemented with O(N4) scaling costs. For such calculations the data group $laplace has to be added.

9.
For technical recommendations and additional options for parallel RI-MP2 calculations with the ricc2 program see Secs. 3.2 and 10.6.

MP2 calculations with a ROHF reference state

With the program ricc2 it is possible to compute MP2 (and spin-component scaled) MP2 energies with single-determinant restricted open-shell reference wavefunctions. No additional input is required apart from the usual ROHF input for the dscf and ridft programs and a standard MPn or CC input for ricc2.

TURBOMOLEs Hartree-Fock codes can handle within the ROHF framework many cases, which include beside common high- and low-spin configuration state functions also weighted averages of high-spin CSFs (see Sec. 6.3 for further details). The Møller-Plesset perturbation theory and coupled cluster functionalities implemented in ricc2 require a single-determinant reference state and can thus only deal with high-spin open-shell cases (not averaged):

For non-abelian points groups this implies that shells with degenerate orbitals (as e.g. t1 in point group I) must be half-filled. An average over the different (symmetry-equivalent or inequivalent) high-spin determinants that are obtained when a shell of degenerate orbitals is less than or more than half-filled is not possible with single-point ricc2 calculations.

For states with less than or more than half-filled shells of degenerate orbitals the calculations half to be done in a point group that lifts the degeneracy such that it becomes possible to assign integer occupation numbers. A symmetry-breaking of the orbitals can be avoided by doing the Hartree-Fock calculation in the full point group. The input (and MO coefficients) can then be transformed to a lower point group using define only for the ricc2 calculation.

Calculations with mpgrad

1.
Add $denconv 1.d-7 to the control file and perform a dscf run.
2.
If any orbitals are decided to be excluded from MP2 treatment, add data group $freeze manually to the control file, see also Section 20.2.16.
3.
For preparation of an mpgrad run use the script Mp2prep:
mp2prep -e/g -m memory -p discspace [scratch file directory]
As an example, with the command
mp2prep -e -m 100 -p 1000 /work
an MP2-energy calculation is prepared, the amount of available core memory is restricted to 100 MB, the MOs are blocked, so that integral scratch files—located in the directory /work—do not need more than 1000Mb. The number of blocks, i.e. the number of passes with repeated integral evaluations, is written to the control file ($traloop) as well as the specification of scratch files ($mointunit, see Section 20.2.16). Note: less disc space means more passes and thus lower efficiency of mpgrad, but due the technical limitations discspace should be limited to values < 16Gb to avoid integer overflow errors. Settings obtained by mp2prep may be changed manually. You may change the number of passes in $traloop by editing the control file (e.g. if the originally intended disc space is not available). To adapt the size of scratch files add $statistics mpgrad to control file and start an mpgrad statistics run with the command mpgrad.
4.
Start a single mpgrad calculation with the command mpgrad.
5.
For optimisation of structure parameters at the (non-RI-) MP2 level use the command jobex -level mp2. Note, that the frozen core approximation is ignored in this case.