NAMD Coupling Schemes

.oqp control reference

Use canonical routes and controls in input files. Raw names identify implementation storage. A control without an accepted canonical spelling is not available in .oqp files.

A named scheme fixes the four coupled choices that define one surface-hopping treatment. This avoids combinations that do not correspond to the intended comparison.

scheme	tdc	rescale	thrshe	frustrated
BaeckAn	baeck_an	isotropic	0.015936 Ha	none
Overlap	npi	isotropic	0.015936 Ha	none
TDC_NAC	npi	hop_analytic_nac	0.367493 Ha	reflect
NAC	analytic	analytic_nac	0.367493 Ha	reflect

The two gap limits correspond to 10 kcal mol⁻¹ and 10 eV, respectively. TDC_NAC is the default recommendation on a supported MRSF route.

scheme=custom

Use custom only when none of the four named methods represents the intended calculation. All four low-level choices are then required:

namd(S1,scheme=custom,tdc=npi,rescale=isotropic,
     thrshe=0.367493,frustrated=reflect)

Do not combine a named scheme with tdc, rescale, thrshe, or frustrated; OpenQP rejects the ambiguous input.

tdc

Value	Electronic-amplitude propagation
fd	Antisymmetric finite-difference overlap expression.
npi	Norm-preserving interpolation of the phase-aligned state-overlap matrix.
analytic	Analytic MRSF derivative-coupling vectors contracted with nuclear velocity.
baeck_an	Lagged time-dependent Baeck--An estimate from three consecutive energy gaps.

npi is the principal overlap-based choice. analytic requires same-spin singlet MRSF states from a two-SOMO ROHF/ROKS triplet reference and SCF and response convergence thresholds no larger than 1e-8.

Exact determinant-factorized state-overlap evaluation is a fixed preceding operation: it constructs the many-electron overlap matrix from consecutive molecular orbitals. tdc=npi then converts that matrix into a norm-preserving time-derivative coupling. NPI is therefore not TLF, and there is no separate NAMD option for choosing an approximate TLF order.

The Baeck--An coupling at the central point is based on

[ |\tau_{ij}^{\mathrm{BA}}|=\frac12\sqrt{ \frac{\mathrm d^2\Delta E_{ij}/\mathrm dt^2}{\Delta E_{ij}}}. ]

It is applied one nuclear step after the centred curvature becomes available. The magnitude uses the sign of the phase-tracked overlap TDC. A nonpositive radicand, zero gap, or gap above ba_gap_max gives zero for that pair. The first interval and an interval after discontinuous history use overlap NPI as a warm-up value.

rescale

Value	Velocity adjustment after an accepted hop
isotropic	Scale all velocity components.
analytic_nac	Use the analytic derivative-coupling direction already evaluated for all pairs.
hop_analytic_nac	Evaluate the active--candidate analytic vector only after a hop candidate is selected.
auto	Use hop-triggered analytic NAC on a supported route and isotropic adjustment otherwise.

rescale concerns a physical surface hop. The numerical energy correction in cases B and D is separately documented under NAMD Numerical Continuity.

thrshe

Positive maximum state-energy gap in Hartree for a hop attempt. Default: 0.367493 Ha (10 eV). A named scheme supplies its own value.

frustrated

For a directional hop with insufficient kinetic energy, none leaves the velocity unchanged and reflect reverses its component along the analytic derivative-coupling vector. Default low-level value: reflect.

Decoherence

Keyword	Default	Meaning
decoherence	edc	Energy-based decoherence; off disables it.
edc_c	0.1 Ha	Constant in the energy-based decoherence rate.
substep	50000	Electronic-amplitude integration substeps per nuclear step.

Optional Trivial-Crossing Following

trivial=False is the default. trivial=True enables overlap-triggered root following with trivial_thresh=0.5. This is a method-specific approximation, not part of standard FSSH and not one of the four numerical-continuity cases.

QM/MM Scope

QM/MM NAMD supports TDC_NAC and NAC for a QM region without link atoms under the frozen-embedding-field QM-region NAC approximation. The analytic vector omits embedding-operator and MM-coordinate derivatives, and hop rescaling changes QM velocities only. See NAC before using this approximation in a production calculation.

Scientific references

Nonadiabatic Dynamics; Software libraries and data formats.