MMFF VI. MMFF94s option for energy minimization studies

This article describes the derivation of MMFF94s, which is the "s" (static) variant of MMFF94. MMFF94s incorporates altered out of plane bending parameters that yield planar (or nearly planar) energy-minimized geometries at unstrained delocalized trigonal nitrogen centers. Some experimental and most theoretical structures show appreciable puckering at nitrogen in isolated structures. However, condensed-phase effects or even strong intermolecular hydrogen bonding tends to reduce, but need not eliminate, such puckering; in contravention to results reported on the lower level Hartree-Fock surface, we show in the correlated LMP2/6-31G** calculations for the Watson-Crick guanine-cytosine base pair that one of the hydrogen-bonded NH₂ groups remains appreciably puckered. The resultant MMFF94s geometries emulate the "time-averaged" structures typically observed in crystallographic and most other experimental structure determinations. MMFF94s also employs modified torsion parameters for interactions that involve such centers, but is identical to MMFF94 for other systems. Isolated instances are found in which MMFF94s fails to locate a (probably shallow) local minimum found on the MMFF94 and reference MP2/6-31G* surfaces. In general, however, MMFF94s describes conformation energies for delocalized trigonal nitrogen systems nearly as well as MMFF94 does. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 720-729, 1999.