MOLCAS manual: 6.6 MBPT2 — A Second-Order Many-Body PT RHF Program The MBPT2 program performs second-order Many Body Perturbation Theory calculations based on a RHF-type of wave function (MP2 method). The calculation is to some extent defined by the SCF calculation which must be performed before running the MBPT2 program. Therefore, there is no difficulty related to the input file unless an analysis of the correlation energies of specific electron pairs or contribution from external orbitals wants to be performed. In this case keywords SFROzen and SDELeted have to be used as described in section  MBPT2 section of the user's guide. To run the program the ORDINT integral file(s) generated by the SEWARD program and the RUNFILE file generated by the SCF program are needed. The program can be otherwise run in a direct manner. Therefore the SEWARD program can be run with the option DIREct included in its input. Only the ONEINT will then be generated and used by the SCF module. The input file used to run an MBPT2 calculation on the ground state of the water molecule is displayed in figure 6.6. For large molecules it is also possible to use the Cholesky decomposition technique to speed up the calculations. This will be described in another section of the tutorials.   &MBPT2 Title=  MP2  of  ground  state  of  C2v  Water Frozen=  1  0  0  0 Figure 6.6: Sample input requested by the MBPT2 module tocalculate the MP2 energy for the ground state of the water in C2v symmetry. The output of MBPT2 is self-explanatory.