Experimental gas-phase entropy

ThermoScreening is checked against the experimental entropy compilation published by Bains, Petkowski, Zhan, and Seager. The source combines 1,403 reported entropy values in eight source columns, including NIST, JANAF, and Yaws.

The benchmark downloads the original workbook from Zenodo record 4661783, verifies its checksum, and keeps it outside the repository.

Scope

The comparison covers 29 neutral gas-phase molecules containing H, C, N, O, F, P, S, and Cl. It includes linear and nonlinear molecules, triplet oxygen, doublet nitric oxide, high-symmetry species, and flexible molecules with low-frequency torsions.

For each molecule, the reference value is the median of the available experimental columns in the workbook. This avoids selecting a single transcription when the compilation contains conflicting entries, as it does for methane. The source publication states that radicals were excluded from the curated entropy set, although the workbook contains nitric oxide. It is included here only as an open-shell stress case. Its two entries differ by almost 10 J/(mol K), so the JANAF value is used. Values are converted from J/(mol K) to cal/(mol K).

Both engines start from the corresponding ASE G2 geometry and run a complete optimization, Hessian, symmetry analysis, and quasi-RRHO thermochemistry calculation at 298.15 K and 1 bar. GFN2-xTB uses a force threshold of 0.001 eV/A because methyl torsions remain sensitive at the normal screening tolerance.

Run the benchmark

DFTB+ and the 3ob parameter set are required for the default run:

python scripts/validate_experimental_entropy.py

Run both electronic-structure engines with:

python scripts/validate_experimental_entropy.py --engine both

The xTB comparison requires the optional tblite package. The script uses a separate working directory for every molecule and engine so fixed-name Hessian and vibration files cannot leak between cases.

Results

Engine	MAE	RMSD	Maximum absolute deviation
DFTB3/3ob	0.377	0.635	2.317 (ethanol)
GFN2-xTB	0.457	0.777	2.387 (ethanol)

All deviations are in cal/(mol K). The regression gate requires an MAE no greater than 0.75 and a maximum deviation no greater than 3.0 cal/(mol K). These limits preserve the behavior of this selected molecular set; they are not general accuracy criteria for other chemical spaces.

Interpretation

This is an end-to-end method benchmark: it exercises geometry optimization, frequencies, automatic rotational symmetry numbers, open-shell handling, quasi-RRHO, and entropy assembly. The remaining deviations include the accuracy limits of DFTB3/3ob and GFN2-xTB and are not numerical errors in the thermochemistry formulas. Those formulas are separately tested against ASE IdealGasThermo.

DFTB+ and GFN2-xTB independently generate the optimized geometries and frequencies, but both feed the same ThermoScreening thermochemistry implementation. Agreement between the engines therefore does not constitute an independent validation of those shared formulas.

The source workbook is not redistributed. See the dataset publication for provenance, experimental-source details, and limitations.