chem-thermochemistry

Gas-Phase Thermochemistry Skill

Goal

Compute temperature-dependent thermodynamic quantities — enthalpy ($H$), entropy ($S$), and Gibbs free energy ($G$) — for gas-phase molecules, and reaction thermochemistry ($\Delta H$, $\Delta S$, $\Delta G$) for balanced chemical reactions, using Machine Learning Interatomic Potentials (MLIPs) with ASE's ideal-gas / rigid-rotor / harmonic-oscillator (IGRRHO) framework.

[!IMPORTANT] This skill extends chem-vibration. It reuses the same MLIP-powered Hessian calculation and adds thermodynamic post-processing via ase.thermochemistry.IdealGasThermo.

Background

The IGRRHO partition function decomposes into independent translational, rotational, vibrational, and electronic contributions:

$$q = q_\text{trans} \cdot q_\text{rot} \cdot q_\text{vib} \cdot q_\text{elec}$$

From the partition function, thermodynamic quantities at temperature $T$ and pressure $P$ are:

• Enthalpy: $H(T) = E_\text{pot} + E_\text{ZPE} + \sum_i \frac{h\nu_i}{e^{h\nu_i/k_BT}-1} + k_BT$ (+ rotational and translational terms)
• Entropy: $S(T,P) = S_\text{trans} + S_\text{rot} + S_\text{vib} + S_\text{elec}$
• Gibbs free energy: $G(T,P) = H(T) - TS(T,P)$

For a balanced reaction, $\Delta X = \sum_\text{products} n_i X_i - \sum_\text{reactants} n_j X_j$ where $X = H, S, G$.

1. Prerequisites

• An MLIP wrapper must be available (MACEWrapper, MatGLWrapper, or FAIRCHEMWrapper).
• ASE must be installed (provides ase.thermochemistry.IdealGasThermo).
• Structures must be molecules or gas-phase species (non-periodic).

2. Choosing a Foundation Potential

Refer to the foundation-potentials skill for model selection.

[!IMPORTANT] For molecular thermochemistry, use models with good force accuracy (e.g., MACE-OMAT-0-small, MACE-MH-1 with omol head). Accurate forces are critical for reliable vibrational frequencies and thus thermodynamic quantities.