Equation Of States Utility

Equation of state (EOS) utilities.

This module centralizes common equation-of-state formulas used across ReaxKit. It is responsible for reusable EOS computations and is not tied to a single workflow implementation.

Usage context

• fitting energy-volume relationships from parsed simulation outputs
• computing legacy trainset-style energies for translated generator logic

Notes

• The two Vinet implementations use different parameterizations/units, so they are conceptually related but not drop-in replacements for each other.
• Explanation for the Rose-Vinet equation of state can be found here: doi:10.1029/JB092iB09p09319

Function: vinet_energy_ev

Compute Vinet EOS energy from volume in eV units.

This evaluates the energy-volume form used for fitting E(V) data in ReaxKit workflows, with bulk modulus expressed in eV/Angstrom^3.

Parameters:

Name	Type	Description	Default
V	ndarray	Volume(s) (Angstrom^3).	required
E0	float	Equilibrium energy (eV).	required
K0_eV_A3	float	Bulk modulus at equilibrium (eV/Angstrom^3).	required
V0	float	Equilibrium volume (Angstrom^3).	required
C	float	Vinet shape parameter (dimensionless).	required

Returns:

Type	Description
ndarray	Energy at each volume V (eV).

Examples:

>>> import numpy as np
>>> V = np.array([15.0, 16.0, 17.0])
>>> E = vinet_energy_ev(V, E0=-10.2, K0_eV_A3=0.75, V0=16.0, C=4.0)
>>> E.shape
(3,)

Function: vinet_energy_trainset

Compute legacy trainset-style EOS energy from scalar volume input.

This matches the translated Fortran elastic_energy_v2 bulk-block logic. It returns energies in the generator's legacy energy units and preserves the existing conversion convention through energy_conversion_factor.

Parameters:

Name	Type	Description	Default
volume	float	Current volume V (Angstrom^3).	required
reference_volume	float	Reference volume V0 (Angstrom^3).	required
bulk_modulus_gpa	float	Bulk modulus B0 (GPa).	required
bulk_modulus_pressure_derivative	float	Pressure derivative B0' (dimensionless).	required
reference_energy	float	Reference energy offset E0 (legacy units).	0.0
energy_conversion_factor	float	Generator conversion factor currently used in trainset workflows.	required

Returns:

Type	Description
float	EOS energy in the generator's legacy energy units.

Examples:

>>> e = vinet_energy_trainset(
...     volume=16.0,
...     reference_volume=16.2,
...     bulk_modulus_gpa=180.0,
...     bulk_modulus_pressure_derivative=4.2,
...     reference_energy=0.0,
...     energy_conversion_factor=160.21766208,
... )
>>> isinstance(e, float)
True