Ensembles and SOC-NAMD

Nuclear Ensemble

ensemble

Value	Current support
nve	Default; no thermostat after the initial velocity is prepared.
nvt	Langevin thermostat at temperature with damping friction.
npt	Reserved but currently rejected.

NPT requires a QM/MM energy evaluation for every barostat trial box. The current ESPF-QM/MM driver does not provide that trial-box energy, so OpenQP rejects ensemble=npt rather than performing an invalid NPT simulation.

friction is in ps⁻¹ and defaults to 1.0; it must be positive for NVT. The legacy thermostat keyword accepts off or langevin and maps to ensemble=nve or ensemble=nvt. In legacy [md] input, the corresponding scalar names are init_temp, thermostat_temperature, and thermostat_friction (default 1.0 ps⁻¹).

The NVE energy criteria are documented separately under NAMD Diagnostics. They do not apply to thermostat energy exchange in NVT.

Adaptive Nuclear Timestep

Keyword	Default	Meaning
dt_adaptive	False	Reduce the timestep when the displacement criterion requires it.
dt_min	0.05 fs	Smallest allowed adaptive timestep.
dx_max	0.02 bohr	Largest accepted per-step atomic displacement.

Adaptive stepping is a separate integration option. Case-D energy-triggered substepping repeats one nominal interval from the same phase point and is documented under NAMD Numerical Continuity.

SOC-NAMD

Development status

SOC-NAMD, including SOC-NAMD-QMMM, is still under active development and must be treated as experimental. The inputs and restart records are available for method development and numerical verification, but their availability does not establish that the present force and coupling approximations are ready for production scientific conclusions. Report the selected basis and all approximations with any result.

A complete input with a reference trajectory is in the formaldehyde SOC-NAMD worked example.

soc

Default: False. soc=True enables surface hopping with spin--orbit coupling among ns singlets and nt triplets. Each triplet contributes three (M_S) sublevels, giving ns + 3*nt MCH basis functions before spin-adiabatic transformation.

soc_basis

Value	Meaning
adiabatic	Propagate spin-adiabatic SOC eigenstates; the force uses the weighted-MCH approximation.
mch	Propagate the spin-pure MCH basis with the active-root MCH gradient.

Default: adiabatic. For current development and numerical verification, the mch form is preferred because it avoids the approximate weighted force of the spin-adiabatic implementation. This preference does not change the experimental status of SOC-NAMD.

init_state

Optional zero-based MCH character label such as S0, S1, T0, or T1. It selects the initial spin-adiabatic state with the largest matching MCH character and overrides active in an SOC calculation.

Spin-adiabatic force diagnostics

Keyword	Default	Meaning
grad_wthr	0.001	Minimum MCH weight included in the weighted diagonal gradient.
soc_du_dt_corr	False	Add a finite-difference eigenvector-change correction.
soc_tdc_grad_corr	False	Add an approximate TDC-projected gradient correction.
econs	False	Rescale every step as a temporary energy-drift stabilizer.

These controls examine the approximate spin-adiabatic force and are not needed for soc_basis=mch. econs is not a substitute for a physically consistent force and should remain off unless a documented diagnostic requires it.

QM/MM SOC-NAMD

Adding qmmm(...) selects the corresponding SOC-QM/MM driver. See SOC-NAMD-QMMM for the dispatch table and the current force and coupling approximations.