Add Grimme's quasi-RRHO vibrational entropy option - #67
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The rigid-rotor-harmonic-oscillator entropy diverges as a vibrational mode's frequency goes to zero, so low-frequency modes (floppy torsions, weak rotors) get a spuriously large entropy. Add Grimme's quasi-RRHO treatment (Chem. Eur. J. 2012, 18, 9955): each mode's entropy is interpolated between the harmonic-oscillator and free-rotor values with weight 1/(1+(100/nu)^4), so high-frequency modes are unchanged while low-frequency modes approach the finite free-rotor limit. - thermo/thermo.py: `Thermo(..., quasi_rrho=False)`; `_compute_vibrational_entropy` uses the new `_quasi_rrho_entropy` interpolation when enabled. The harmonic path is numerically unchanged, so the default preserves existing results. - thermo/api.py, thermo/screening.py, cli/thermo.py: thread `quasi_rrho` through `run_thermo`, `dftbplus_thermo`, `screen`, and `screen --quasi-rrho`. Applies to every engine (it acts on the frequencies) and pairs with implicit solvation, where low-mode entropy is the main RRHO weakness. Validated: modes above ~100 cm^-1 are unchanged (real DFTB+ ethane matches the harmonic result), while a molecule with 25/40 cm^-1 modes has its entropy reduced by ~3 cal/mol/K.
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Adds a
quasi_rrhooption (CLI--quasi-rrho) for the vibrational entropy.Why
The rigid-rotor-harmonic-oscillator entropy of a mode diverges as its frequency → 0, so low-frequency modes (floppy torsions, weak/hindered rotors, non-covalent complexes) get a spuriously large entropy — a well-known RRHO weakness. This is also the dominant error left in implicit-solvent free energies (#66).
What
Grimme's quasi-RRHO (Chem. Eur. J. 2012, 18, 9955): each mode's entropy is interpolated between the harmonic-oscillator and free-rotor values, weighted by
w = 1 / (1 + (100/ν)^4). High-frequency modes stay harmonic; low-frequency modes approach the finite free-rotor limit.thermo/thermo.py—Thermo(..., quasi_rrho=False);_compute_vibrational_entropyuses the new_quasi_rrho_entropyinterpolation when enabled. The harmonic path is numerically unchanged, so the default preserves existing results.thermo/api.py,thermo/screening.py,cli/thermo.py— threadquasi_rrhothroughrun_thermo,dftbplus_thermo,screen, andscreen --quasi-rrho.Engine-independent (it acts on the frequencies), so it also applies to a future xTB engine.
Usage
Validation