Spin-Orbit Coupling

OpenQP supports MRSF-TDDFT spin-orbit coupling through runtype=soc. SOC mixes spin-free electronic states through relativistic one-electron terms and, when requested, a mean-field treatment of the two-electron SOC operator. This workflow is useful when intersystem crossing, phosphorescence, or singlet-triplet mixing is important. See the References page for OpenQP's relativistic MRSF-TDDFT SOC method and the mean-field SOC operator background.

Input style:

[input]
runtype=soc
method=tdhf
functional=bhhlyp
basis=6-31G(2df,p)
soc_2e=1

[scf]
type=rohf
multiplicity=3
scal_rel=2

[tdhf]
type=mrsf
nstate=12

Python style:

from oqp.openqp import OpenQP

job = OpenQP("h2o_soc", silent=1)
job.molecule(geometry="water", charge=0)
job.theory.mrsf(functional="bhhlyp", basis="6-31G(2df,p)", nstate=12)
job.workflow.soc(soc_2e=1, scal_rel=2)

mol = job.run()
soc = mol.get_soc()

Runnable inputs:

• examples/SOC/H2O_BHHLYP_SOC.inp
• examples/SOC/CH3Br-BHHLYP-SOC.inp

SOC Terms

[input] soc_2e controls the SOC Hamiltonian. The current MRSF-SOC driver forms one-electron Breit-Pauli SOC matrix elements and can add a mean-field two-electron SOC contribution:

Value	Meaning
0	One-electron SOC terms only.
1	One-electron plus mean-field two-electron SOC terms.

SOC workflows currently require MRSF-TDDFT: a triplet ROHF reference and [tdhf] type=mrsf. The SOC driver computes both singlet and triplet response roots internally, so Python scripts should use job.workflow.soc(...) after job.theory.mrsf(...) rather than setting job.tdhf.multiplicity. The Python helper sets [scf] scal_rel=2 by default for the SOC workflow; pass scal_rel=0, 1, or 2 to override it.

Scalar Relativistic Correction

Scalar relativistic correction is separate from SOC. It changes the spin-free one-electron core Hamiltonian and does not mix spin states by itself. In the current OpenQP input surface, the exposed scalar-relativistic keyword is [scf] scal_rel:

Value	Meaning
0	No scalar relativistic correction.
1	First-order Douglas-Kroll-Hess correction.
2	First- and second-order Douglas-Kroll-Hess correction.

Use scal_rel when the spin-free Hamiltonian should include DKH scalar relativistic effects. Use runtype=soc and soc_2e when the calculation needs spin-orbit coupling. X2C support is under implementation and should be documented as a user-facing option only after an OpenQP keyword is exposed.