xnn.ffnn.models.dreiding#

DREIDING: the rule-generated generic force field.

DREIDING (Mayo, Olafson & Goddard, J. Phys. Chem. 94, 8897, 1990) writes the total energy as valence terms generated from per-atom parameters by hybridization rules plus nonbonded terms:

E = E_bond + E_angle + E_torsion + E_inversion

  • E_vdw + E_coulomb + E_hbond

  • Bonds (eqs 4-9): harmonic 1/2 k_n (R - R0)^2 (or the Morse form, the DREIDING/M variant) with R0 = R0_I + R0_J - 0.01 A from per-type bond radii and k_n = n * 700 (kcal/mol)/A^2 for bond order n.

  • Angles (eqs 10-12): the harmonic-cosine form 1/2 K/sin^2(t0) (cos t - cos t0)^2 with t0 set by the central atom and K = 100 (kcal/mol)/rad^2; linear centers use K (1 + cos t). The plain harmonic-theta form (eq 11) is available as an option.

  • Torsions (eqs 13-23): 1/2 V {1 - cos[n (phi - phi0)]} with (V, n, phi0) set by the rule engine (torsion_rule()) from the central atoms’ hybridizations and bond order; V is the total barrier of the central bond, split evenly over its dihedrals.

  • Inversions (eq 28): at every 3-coordinate center of a listed type, the spectroscopic umbrella term K (1 - cos psi) (planar centers) or 1/2 K/sin^2(psi0) (cos psi - cos psi0)^2, averaged over the three axis choices with weight 1/3.

  • van der Waals: Lennard-Jones D0 [rho^-12 - 2 rho^-6] with rho = R/R0 (eq 31’), or the exponential-6 form (eq 32’, the DREIDING/X6 variant); well depths combine geometrically, LJ radii arithmetically by default (eq 36c). 1,2 and 1,3 pairs are excluded; 1,4 pairs count in full (the DREIDING default).

  • Electrostatics (eq 37): 322.0637 Q_i Q_j / R kcal/mol over the same pairs, when partial charges are supplied (DREIDING itself prescribes none; the paper uses Gasteiger charges where needed).

  • Hydrogen bonds (eq 38): D_hb [5 (R_hb/R)^12 - 6 (R_hb/R)^10] cos^4(t_DHA) over donor-hydrogen…acceptor triplets involving the explicit H__HB hydrogen type, with the donor-acceptor distance R and the angle restricted beyond 90 degrees.

Like OPLS, DREIDING is a fixed-topology force field: an instance binds the parameter library to one molecular topology (with bond orders), and every structure it evaluates must be a conformation of that system. Unlike OPLS, all bonded parameters are generated, so what is trainable are the generators themselves – per-type radii, angles and vdW parameters, and the global force constants and rule barriers – which stays true to the DREIDING philosophy while letting the force field be refit to new systems by gradient descent (trainable=...), exactly like ReaxFF’s and OPLS’s parameters. Several Dreiding models can share one DreidingForceField to fit transferable parameters jointly.

Classes

Dreiding([ffield, topology, cutoff, ...])

The DREIDING rule-generated force field, bound to one topology.

DreidingForceField([library, trainable])

The trainable generator tensors of a DREIDING parameter set.
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