`[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 requires an HF reference, so [input] functional must be empty. RHF references support analytic nuclear gradients and gradient-driven geometry calculations; UHF and ROHF references remain energy-only. In concise input, select the reference with mp2(reference=rhf|rohf|uhf)/BASIS; the value lowers to [scf] type and is not a keyword in this section.

Minimal Examples¶

Conventional MP2 in .oqp:

mp2/6-31g
geom="h2o.xyz"

Python:

from oqp.openqp import OpenQP

job = OpenQP("h2o_mp2")
job.molecule(geometry="water")
job.theory.mp2(basis="6-31g")
mol = job.run()

Analytic RHF-MP2 gradient in .oqp:

mp2/6-31g grad(S0)
geom="h2o.xyz"

The spin-scaling preset is applied to both the energy and its analytic gradient. For example, replace the first line with mp2(variant=scs-mp2)/6-31g for SCS-MP2.

Legacy .inp:

[input]
method=mp2
runtype=energy
functional=

For spin-component-scaled MP2, add an exact .oqp section call:

mp2/6-31g mp2(variant=scs-mp2)
geom="h2o.xyz"

The legacy section is:

[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 and RHF analytic gradient

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¶

RHF MP2 accepts runtype=energy, grad, optimize, ts, mep, and irc.
UHF and ROHF MP2 accept runtype=energy only; derivative requests stop before the electronic-structure calculation.
method=mp2 rejects non-empty [input] functional values.
The reference is selected through [scf] type; RHF, UHF, and ROHF references are supported for energies.
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]¶