[tdhf]

The [tdhf] section controls TDHF, TDDFT, spin-flip TDDFT, MRSF-TDDFT, and UMRSF-TDDFT response calculations. Use [input] method=tdhf to activate these workflows.

Background

MRSF-TDDFT is OpenQP's main multistate response method. It starts from an open-shell high-spin reference, builds spin-flip response spaces, and mixes the reference density information so target states are less affected by ordinary spin-flip spin contamination. In practice this makes the same response machinery useful for multiconfigurational ground-state surfaces, excited-state surfaces, conical-intersection work, gradients, NACME, SOC, and MRSF-EKT. See References for the original theory papers and recent overview articles.

Minimal MRSF-TDDFT Example

.oqp:

mrsf(nstate=5)/bhhlyp/6-31g*
geom="h2o.xyz"

Python:

from oqp.openqp import OpenQP

job = OpenQP("mrsf_keywords")
job.molecule(geometry="water", charge=0)
job.theory.mrsf(functional="bhhlyp", basis="6-31g*", nstate=5)

Legacy .inp:

[input]
method=tdhf

[scf]
type=rohf
multiplicity=3

[tdhf]
type=mrsf
nstate=5

Keywords

type

Field	Value
Type	string
Default	rpa
Values	rpa, tda, sf, mrsf, umrsf, qmrsf_dk, mrsf_ekt_ip, mrsf_ekt_ea
Used by	response model selection

Selects the response model. Use mrsf for production MRSF-TDDFT workflows. sf selects ordinary spin-flip TDDFT, and umrsf selects the unrestricted MRSF energy path. qmrsf_dk selects the quintet-reference dressed-kernel method described in QMRSF-DK. The legacy mrsf_ekt_ip and mrsf_ekt_ea values are energy-only; the current EKT workflow should use [input] runtype=ekt, [tdhf] type=mrsf, and the [ekt] section.

MRSF and SF workflows require an ROHF reference in the current code path. UMRSF-TDDFT requires a UHF reference. QMRSF-DK requires a quintet ([scf] multiplicity=5) ROHF/ROKS reference and [input] runtype=energy.

nstate

Field	Value
Type	integer
Default	1
Used by	number of response roots

Number of excited states to compute. It must be at least as large as the highest state requested by gradients, optimizations, NACME, SOC, Hessians, or EKT.

nstate_s, nstate_t

Field	Value
Type	integer
Defaults	0, 0
Used by	unequal singlet/triplet SOC spaces

Optional numbers of singlet and triplet roots for SOC. Zero means to use nstate for that common count. In a .oqp file, do not set these internal selectors directly; write soc(ns=3,nt=5). Both counts must be provided together, and that form must not be combined with route nstate.

target

Field	Value
Type	integer
Default	1
Used by	target-state workflows

Target response state for workflows that read a single TDHF/MRSF state.

multiplicity

Field	Value
Type	integer
Default	1
Used by	response-state spin selection

Requested response-state multiplicity. For MRSF-TDDFT, this is the target spin multiplicity after spin flip, not necessarily the same as the high-spin ROHF reference multiplicity. For SOC, do not set this as a single target multiplicity; the SOC workflow computes singlet and triplet response roots internally.

maxit

Field	Value
Type	integer
Default	50
Used by	Davidson/response solver

Maximum number of response-solver iterations.

conv

Field	Value
Type	float
Default	1.0e-6
Used by	response convergence

Convergence threshold for the response solver.

nvdav

Field	Value
Type	integer
Default	50
Used by	Davidson subspace

Maximum Davidson subspace dimension. The input checker warns when nvdav is smaller than nstate.

maxit_zv

Field	Value
Type	integer
Default	50
Used by	Z-vector solver

Maximum number of Z-vector iterations for gradient/property workflows.

zvconv

Field	Value
Type	float
Default	1.0e-6
Used by	Z-vector convergence

Convergence threshold for Z-vector equations.

z_solver

Field	Value
Type	integer
Default	0
Values	0, 1, 2, 3
Used by	Z-vector linear solver

Selects the Z-vector solver. In the source comments, 0 is CG, 1 is legacy GMRES, 2 is MINRES, and 3 is AUTO.

gmres_dim

Field	Value
Type	integer
Default	50
Used by	GMRES Z-vector solver

Subspace dimension for GMRES when that solver is selected.

tlf

Field	Value
Type	integer
Default	2
Used by	MRSF state-overlap minor determinants

Selects how the MRSF state overlap between consecutive geometries, <Psi_I(t-dt)|Psi_J(t)>, is evaluated. The reference determinant is shared, so the overlap factorizes into a contraction of the response amplitudes with three classes of minor determinants of the MO overlap matrix: s_ij one-hole occupied minors, s_ab particle minors, and s_ia mixed minors. tlf selects the treatment of s_ij and s_ab; s_ia is always exact.

tlf	Minors	Notes
0 (notlf, exact)	Exact Gaussian-elimination minors, no truncation	Invariant to orbital rotations between steps. This is not the zeroth-order TLF(0) of the paper, which is not implemented.
1	First-order truncated Leibniz formula, TLF(1)	JCTC 15, 882 (2019)
2	Second-order truncated Leibniz formula, TLF(2)	Most accurate TLF approximation; KNU-GAMESS ndtlf=2

The truncated Leibniz formula assumes the MOs of consecutive steps are nearly orthonormal, i.e. that the MO overlap matrix is close to diagonal. When near-degenerate doubly occupied orbitals rotate into each other within one nuclear step -- a 45-degree mixing of two occupied orbitals has been observed in hot uracil trajectories -- the diagonal MO overlaps fall to about 0.7 and TLF(2) returns a collapsed state overlap (diagonal elements around 0.3-0.4) even though the SCF solution and the MRSF surfaces are continuous. Norm-preserving interpolation then turns that collapse into a large spurious time-derivative coupling.

The exact minors are invariant to such rotations, and for molecules the size of uracil (30 occupied alpha orbitals, 6-31G*) they cost the same wall time as TLF(2). For large systems where the nvir^2 particle minors dominate, the recommended route is Jacobi's complementary-minor identity -- all one- and two-hole minors from one LU factorization of the occupied block -- rather than truncation.

The state-overlap section of the log states which evaluation was used (state-overlap minors: exact minor determinants (tlf=0, default; ...) or TLF(n) truncated-Leibniz minors; ...). NAMD additionally warns when every column norm of the retained state overlap falls below 0.5.

hfscale, cam_alpha, cam_beta, cam_mu

Field	Value
Type	float
Defaults	-1.0, -1.0, -1.0, -1.0
Used by	response functional parameter overrides

Override exact-exchange and CAM/range-separated parameters for response calculations. Negative values mean use the selected functional defaults.

For type=qmrsf_dk these set the exchange carried by the dressed kernel, independently of the reference. With a global hybrid the kernel uses hfscale; with a range-separated reference it uses cam_alpha*K + cam_beta*K(erf(cam_mu*r)/r), and any of the three that is left negative is inherited from [dftgrid].

spc_coco, spc_ovov, spc_coov

Field	Value
Type	float
Default	-1.0
Used by	spin-purification correction parameters

Advanced MRSF/SF response parameters. Leave negative unless following a specific validated protocol.

conf_threshold

Field	Value
Type	float
Default	5.0e-2
Used by	configuration analysis/output

Threshold for reporting or using response configurations.

ixcore

Field	Value
Type	string
Default	-1
Used by	core-level response workflows

Core-orbital selector for core-level response calculations such as X-ray absorption workflows.

resp_cutoff

Field	Value
Type	float
Default	auto (follows perf; 1e-8 baseline)
Used by	MRSF-TDDFT — response 2e-integral cutoff

2e-integral cutoff for the MRSF response build. 1e-8 is exact to ≪ µeV; looser values (e.g. 1e-6) trade a few µeV for speed. Never tighter than the SCF integral cutoff. See Performance.

fp32

Field	Value
Type	string
Default	auto (follows perf; no preset enables it)
Values	on, off, auto
Used by	MRSF-TDDFT — single-precision response digestion

Single-precision MRSF response Fock digestion. Non-reproducible and can flip near-degenerate states; net-slower than FP64 on CPU. Opt-in only.

zv_warmstart

Field	Value
Type	string
Default	auto (follows perf; on at perf ≥ 1)
Values	on, off, auto
Used by	MRSF-TDDFT gradients — z-vector (CPHF) warm-start

Reuse the previous geometry step's z-vector as the CG/GMRES initial guess (exact; the solve still converges to the same tolerance). Most effective across many nearby geometries (optimization, MD).