xnn.ffnn.models.opls#

OPLS: the all-atom (and united-atom) fixed-topology force field.

OPLS (optimized potentials for liquid simulations; Jorgensen, Maxwell & Tirado-Rives, J. Am. Chem. Soc. 118, 11225, 1996) writes the total energy as harmonic bond and angle terms, a three(four)-term Fourier series per proper dihedral, V2-only improper dihedrals at trigonal centers, and Coulomb plus Lennard-Jones nonbonded interactions between all pairs of atoms separated by three or more bonds:

E_ab = sum_ij [ q_i q_j e^2 / r_ij

  • 4 eps_ij (sigma_ij^12/r_ij^12 - sigma_ij^6/r_ij^6) ] f_ij

with geometric combining rules sigma_ij = (sigma_i sigma_j)^1/2, eps_ij = (eps_i eps_j)^1/2 and f_ij = 1 except for intramolecular 1,4 pairs, where f_ij = 1/2 (eqs 1-4 of the paper). The united-atom variant (OPLS-UA) and reparameterizations such as L-OPLS for long hydrocarbons (Siu, Pluhackova & Boeckmann, J. Chem. Theory Comput. 8, 1459, 2012) share this functional form and differ only in their parameter libraries, so all of them are served by this one model plus a library (see xnn.ffnn.models.oplslib).

Unlike ReaxFF, OPLS is not reactive: it needs a fixed molecular topology – per-atom OPLS types and the bond list, from which angles, dihedrals, exclusions and 1,4 pairs follow (xnn.ffnn.models.topology). The topology is bound to the model instance; every structure evaluated by the model must be a conformation of that same system (this is what lets batches of conformers flow through the standard dataset / trainer / ASE-calculator machinery unchanged). Bonded terms and 1,4 pairs use minimum-image displacements, so molecules may wrap across periodic boundaries.

Conventions and scope#

  • All parameters live in a OPLSForceField module as plain tensors (converted to eV / Angstrom / radians at assembly); any group can be made trainable (trainable=...), so classical parameters can be refit by gradient descent exactly like ReaxFF’s. Several OPLS instances (different molecules) can share one OPLSForceField to fit transferable parameters jointly.

  • Nonbonded interactions are evaluated on the model’s neighbor list within cutoff and truncated there (optionally smoothed over the last switch_width Angstrom with a quintic switching function); 1,4 pairs are evaluated exactly from the topology, independent of the cutoff, and scaled by the library’s fudge factors.

  • Excluded (1,2 / 1,3 / 1,4) pairs are excluded in every periodic image, the standard molecular-mechanics convention; keep the cutoff below half the box length, as usual.

Classes

OPLS(ffield, topology, *[, cutoff, ...])

The OPLS fixed-topology force field (all-atom or united-atom).

OPLSForceField(library[, trainable])

The trainable parameter tensors of an OPLS parameter library.
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