UFF and UFF4MOF

xtalforge-ff implements the Universal Force Field natively. The parameter table is data/uff.json: 218 rows ported from Crystal-Builder xtal/ff/uff/params.py (Rappé 1992, Addicoat 2014, Coupry 2016), with the reference units (Angstrom, degrees, kcal/mol). The public boundary converts to eV and eV/Angstrom.

Typing

An atom type is (element, geometry, oxidation state). The typer combines the element table, the perceived bond graph and the local geometry:

• explicit rules for H, C, N, O, F, Cl, Br, I, S, P and the common metals;
• aromatic ring bonds use the resonant carbon/nitrogen types;
• terminal atoms refine their geometry from the number of neighbours;
• framework metals prefer the UFF4MOF rows when the set is Uff4Mof;
• a dummy X atom has no parameters and never contributes.

Typing records the chosen AtomType and a Confidence, so a caller can see why an atom got a type. UffModel exposes the prepared topology.

Energy terms

• Bond: 0.5 k (r - r0)^2, k = 664.12 z1 z2 / r0^3.
• Angle: the reference cosine expansion whose curvature at theta0 is the UFF K_IJK.
• Torsion and inversion: the reference Fourier terms.
• van der Waals: Lennard-Jones with the UFF x and D parameters, 1-2 and 1-3 exclusions, a cutoff and a shifted potential.

Every term has an analytic gradient. The finite-difference test moves each coordinate by a small Cartesian step and compares forces within 1e-4 eV/A.

Validation

• Ethylene energy 0.1113230794 eV, matching the reference term-by-term sum.
• Diamond periodic energy 1.7200457169 eV.
• Argon dimer shifted well -0.0080043731 eV.
• Methane at its UFF bond length, water at its angle, a benzene resonant ring and a Zn4O UFF4MOF node.

The Crystal-Builder golden comparison over the full corpus is not run in CI; the values above are the regression anchors.