MP2 Section

The MP2 section controls Møller–Plesset second-order correlation energy calculations.

Keywords

Keyword	Type	Default	Description
`Scaling`	`none` \| `scs` \| `sos`	`none`	Spin-component scaling
`OS`	float	scaling default	Opposite-spin correlation scale factor
`SS`	float	scaling default	Same-spin correlation scale factor
`DF`	boolean	false	Density-fitting / RI-MP2 request
`Aux`	string	—	Auxiliary basis set for DF-MP2
`KeepHalf`	boolean	true	Keep half-transformed AO integrals
`DiscardVV`	boolean	false	Discard virtual–virtual blocks (DF only)
`Skip`	boolean	false	Skip MP2 evaluation
`FreezeCore`	`false` \| `true` \| `auto` \| integer	`false`	Frozen-core policy

Use OS and SS to set custom scale factors.

FreezeCore false, Full, or NoFC correlates all occupied orbitals.
FreezeCore true or FreezeCore auto freezes a chemically standard core from atomic numbers.
FreezeCore N freezes exactly N lowest occupied orbitals.

Canonical all-electron MP2:

MP2
  Scaling none
  FreezeCore false
END

Spin-scaled MP2 (SCS-MP2):

MP2
  Scaling  scs
  FreezeCore false
END

SOS-MP2:

MP2
  Scaling  sos
END

DF-MP2 request with auxiliary basis:

MP2
  DF   true
  Aux  def2-TZVP-RI
END

Frozen-core MP2:

MP2
  FreezeCore auto
END

MP2 currently computes in-core TwoElectronIntegrals for the AO-to-MO transform, even when direct_scf is enabled for SCF.
The driver reports occupied, correlated occupied, virtual, and frozen-core counts.
DF and Aux are parsed for future RI-MP2 support; the current production path is canonical in-core MP2.