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Add equilibration_control=qtb option that routes all thermostat call sites in the solid free-energy path through LAMMPS fix qtb (paired with fix nph for NPT and fix nve for NVT). - input.py: QuantumThermalBath model (damping, f_max, N_f, seed) - phase.py: fix_qtb/unfix_qtb helpers; qtb branches in zero-pressure and finite-pressure equilibration, (un)constrained pressure convergence, and __init__ damping wiring - solid.py: qtb branches in spring-constant convergence and the Einstein-crystal TI switching block (Langevin -> qtb) - liquid.py / alchemy.py: refuse QTB at run_integration with a clear NotImplementedError. Liquid Uhlenbeck-Ford reference is intrinsically classical so pairing with QTB sampling would be inconsistent. - phase.py: refuse QTB at reversible_scaling / temperature_scaling (LAMMPS fix qtb does not support ramped temperature). Layer 1 is plumbing only; the Einstein-crystal reference free energy remains the classical formula. Layer 2 will add the quantum harmonic reference required for self-consistent QTB free energies. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Add quantum=True branch to get_einstein_crystal_fe that returns the
quantum harmonic-oscillator free energy:
F_e/N = sum_i (1/2) hbar omega_i + k_B T ln(1 - exp(-hbar omega_i/k_BT))
with omega_i = sqrt(k_i/m_i). Wired in solid.thermodynamic_integration
to activate automatically when equilibration_control == "qtb".
This is the reference required for self-consistent free energies under
QTB sampling: the QTB injects quantum-statistical fluctuations into
classical MD, so the reference free energy at lambda=0 must be the
quantum (not classical) harmonic oscillator. Pairing classical
sampling with the quantum reference, or QTB sampling with the
classical reference, gives an inconsistent thermodynamic path that
breaks down below the Debye temperature.
The CM correction is retained in its classical form: it is a
phase-space integration correction whose leading form is unchanged
by QTB sampling.
Verified analytically: F_qm - F_cl -> 0 above ~3 Theta_D, and
F_qm(T=0) = 1.5 hbar omega per atom = ZPE.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Spell out the reason mode=ts cannot use QTB sampling in the NotImplementedError message: the Frenkel-Smit reversible-scaling identity relies on classical Boltzmann factorisation exp(-beta * lambda * U) = exp(-(lambda*beta) * U). Under QTB, the sampled distribution is quantum-statistical and does not factorise that way: when potential is scaled by lambda, mode frequencies become omega * sqrt(lambda), so the QTB spectrum theta(omega, T_start) does not match the target system's theta(omega*sqrt(lambda), T_start/lambda) spectrum. The classical scaling trick therefore breaks down. This documents why QTB+ts is *methodologically* refused, not just an implementation gap. mode=fe at discrete T remains the supported path for QTB free energies. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The previous integration overloaded equilibration_control, a field whose documented role is "thermostat for the initial equilibration cycle only", to also drive QTB sampling throughout the workflow and to select the quantum Einstein reference. That's semantic creep that bites future readers. Replace with a proper mode-level surface. User-facing: - mode: fe-qtb is the new way to ask for QTB sampling + quantum reference - equilibration_control reverts to its original narrow meaning (nose-hoover or berendsen for the equilibration thermostat only) - mode=fe-qtb with reference_phase=liquid raises ValidationError at input-parsing time (UF reference is intrinsically classical) Internal: - Calculation gains a private _qtb: bool flag, set by the model validator when mode == "fe-qtb"; mode is then normalised to "fe" so Phase-class dispatch is unchanged. - All call sites in phase.py / solid.py that previously checked equilibration_control == "qtb" now check self.calc._qtb. The if/elif/else chains in phase.py are reordered so _qtb takes precedence over equilibration_control, with a warning if the user set equilibration_control=berendsen alongside mode=fe-qtb. - NotImplementedError guards in phase.reversible_scaling / phase.temperature_scaling / liquid.run_integration / alchemy.run_integration are removed. They were defensive in nature but the new mode dispatch makes them unreachable: mode=fe-qtb only enters solid Frenkel-Ladd; nothing else can flip _qtb on. Smoke-tested at 300 K Cu FCC: classical F = -3.5628, QTB F = -3.5611 (dF = +1.7 meV), matching the previous laptop benchmark within stochastic noise. Quantum harmonic Einstein reference is selected automatically, confirmed in the log. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Issue #247 review pass — code already implemented and verified, but docs and tests were out of date. Docs: - inputfile.md: add mode: fe-qtb to the mode field description with reference-phase restriction note, add a quantum_thermal_bath block with thermostat_damping/barostat_damping/f_max/n_f/seed entries, add cross-reference from equilibration_control noting it is ignored when mode: fe-qtb is set. Add quantum_thermal_bath to the page-level navigation grid. - gettingstarted.md: add PKG_QTB=ON to the list of optional LAMMPS cmake flags with a one-liner explaining when it is needed. Code: - input.py: update QuantumThermalBath docstring to point at the mode: fe-qtb surface instead of the obsolete equilibration_control: qtb surface, and note the NPH/NVE pairing. Tests: - tests/test_qtb_input.py: 4 schema-level tests verifying that mode: fe-qtb sets the internal _qtb flag and normalises mode to 'fe' for downstream dispatch; that mode: fe leaves _qtb=False; that user-set quantum_thermal_bath parameters round-trip through the schema; and that mode: fe-qtb with reference_phase: liquid is rejected at input-parsing time (Uhlenbeck-Ford liquid reference is classical). Issue #247 sub-tasks addressed: 1. Implement Dammak's QTB in MD phase → done 2. NVE / NPH ensemble support → done (fix nve / fix nph paired with fix qtb in phase.py fix_qtb) 3. Configure via input.yaml, not init_commands → done (mode: fe-qtb + quantum_thermal_bath: block) Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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