`[mp2]`¶

The [mp2] section controls spin-component scaling for standalone [input] method=mp2 calculations. Ordinary MP2 does not require this section; the defaults are conventional unscaled MP2.

MP2 is a post-SCF energy workflow. It requires an HF reference, so [input] functional must be empty and [input] runtype must be energy.

Minimal Examples¶

Conventional MP2:

[input]
method=mp2
runtype=energy
functional=

[mp2]
variant=mp2

Spin-component-scaled MP2:

[mp2]
variant=scs-mp2

Custom scaling:

[mp2]
variant=custom
same_spin_scale=0.50
opposite_spin_scale=1.10

Keywords¶

`variant`¶

Field	Value
Type	string
Default	`mp2`
Values	`mp2`, `conventional`, `scs-mp2`, `scs`, `sos-mp2`, `sos`, `os-mp2`, `os`, `opposite-spin`, `ss-mp2`, `ss`, `same-spin`, `sss-mp2`, `sss`, `scs-mi-mp2`, `scs-mi`, `custom`
Used by	standalone MP2 energy

Selects the MP2 spin-scaling preset. The scales multiply same-spin and opposite-spin correlation components:

E(MP2) = c_ss * (E_aa + E_bb) + c_os * E_ab

Variant	Same-spin scale `c_ss`	Opposite-spin scale `c_os`
`mp2`, `conventional`	`1.0`	`1.0`
`scs-mp2`, `scs`	`1.0 / 3.0`	`1.2`
`sos-mp2`, `sos`	`0.0`	`1.3`
`os-mp2`, `os`, `opposite-spin`	`0.0`	`1.0`
`ss-mp2`, `ss`, `same-spin`, `sss-mp2`, `sss`	`1.0`	`0.0`
`scs-mi-mp2`, `scs-mi`	`1.29`	`0.40`
`custom`	`same_spin_scale`	`opposite_spin_scale`

Named presets set both scale factors internally. If you provide same_spin_scale or opposite_spin_scale with a named preset, those explicit numbers are not used; choose variant=custom instead.

`same_spin_scale`¶

Field	Value
Type	float
Default	`1.0`
Used by	`variant=custom`

Scale factor for the same-spin MP2 contribution, E_aa + E_bb, when variant=custom. The input checker requires finite numeric values for custom scales.

`opposite_spin_scale`¶

Field	Value
Type	float
Default	`1.0`
Used by	`variant=custom`

Scale factor for the opposite-spin MP2 contribution, E_ab, when variant=custom. The input checker requires finite numeric values for custom scales.

Notes¶

method=mp2 accepts runtype=energy only.
method=mp2 rejects non-empty [input] functional values.
The reference is selected through [scf] type; RHF, UHF, and ROHF references are supported.
SOS and OS-only variants set the same-spin scale to zero and skip same-spin work.

See the MP2 workflow for complete input and Python examples.

[mp2]¶