Purpose:
To direct MP2 calculation. Note that MP2 energies as well as properties also are available through the coupled cluster module, see Chapter 27.
Comments:
The MP2 module expects canonical Hartree-Fock orbitals. The MP2 module will check the orbitals and exits if the Fock matrix has off-diagonal non-negligible elements. If starting from saved canonical Hartree-Fock orbitals from a previous calculations, although no Hartree-Fock calculation will be done the number of occupied Hartree-Fock orbitals in each symmetry must anyway be specified with the ".DOUBLY OCCUPIED" under "*SCF INPUT".
The MP2 calculation will produce the MP2 energy and the natural orbitals for the density matrix through second order. The primary purpose of this option is to generate good starting orbitals for CI or MCSCF wave functions, but it may of course also be used to obtain the MP2 energy, perhaps with frozen core orbitals. For valence MCSCF calculations it is recommended that the ".MP2 FROZEN" option is used in order to obtain the appropriate correlating orbitals as start for an MCSCF calculation. As the commonly used basis sets do not contain correlating orbitals for the core orbitals and as the core correlation energy therefore becomes arbitrary, the ".MP2 FROZEN" option can also be of benefit in MP2 energy calculations.