[mp2]

The [mp2] section controls spin-component scaling and energy integral transformation 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.

transform

Field	Value
Type	string
Default	auto
Values	auto, direct, n5
Used by	standalone MP2 correlation energy

Selects the AO-to-MO transformation used for the correlation energy:

• auto uses direct per-occupied-virtual-pair Coulomb builds when their count is within OQP_MP2_MAX_JBUILDS and the basis has at most 120 functions. Otherwise it tries the packed transformation. If that fails, direct builds remain available only when their count satisfies the guard.
• direct requests direct Coulomb builds and retains the same pair-count guard. It stops when the guard is exceeded.
• n5 requires the packed transformation, including for small bases. A full transformation can fall back to occupied-virtual storage. If the packed transformation cannot fit or allocate, the calculation stops; it does not change to direct Coulomb builds.

The two packed forms use the same MP2 expression. UHF and ROHF opposite-spin components use the occupied-virtual form with both spin-orbital coefficient sets. Memory checks include scratch and the ranks on each node. This setting controls the energy transformation only; the analytic-gradient assembly has its own storage requirements.

Concise input and Python:

mp2(reference=uhf,transform=n5)/6-31g
geom="h2o.xyz"

job.theory.mp2(reference="uhf", basis="6-31g")
job.settings.mp2(transform="n5")

The exact section form mp2(reference=uhf)/6-31g mp2(transform=n5) is equivalent. In legacy input, use [mp2] transform=n5. The log reports MP2 path: direct-J, MP2 path: n5 full, or MP2 path: n5 occupied-virtual for each selected or attempted form. A full attempt can fail before the occupied-virtual fallback succeeds; a path message alone does not establish completion. The correlated total energy is reported only after a successful transformation. These messages do not constitute a performance comparison.

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.