DERIVATION OF CLASS-II FORCE-FIELDS .1. METHODOLOGY AND QUANTUM FORCE-FIELD FOR THE ALKYL FUNCTIONAL-GROUP AND ALKANE MOLECULES

A new method for deriving force fields for molecular simulations has b een developed. It is based on the derivation and parameterization of a nalytic representations of the ab initio potential energy surfaces. Th e general method is presented here and used to derive a quantum mechan ical force field (QMFF) for alkanes. It is based on sampling the energ y surfaces of 16 representative alkane species. For hydrocarbons, this force field contains 66 force constants and reference values. These w ere fit to 128,376 quantum mechanical energies and energy derivatives describing the energy surface. The detailed form of the analytic force field expression and the values of all resulting parameters are given . A series of computations is then performed to test the ability of th is force field to reproduce the features of the ab initio energy surfa ce in terms of energies as well as the first and second derivatives of the energies with respect to molecular deformations. The fit is shown to be good, with rms energy deviations of less than 7% for all molecu les. Also, although only two atom types are employed, the force field accounts for the properties of both highly strained species, such as c yclopropane and methylcyclopropanes, as well as unstrained systems. Th e information contained in the quantum energy surface indicates that i t is significantly anharmonic and that important intramolecular coupli ng interactions exist between internals. The representation of the nat ure of these interactions, not present in diagonal, quadratic force fi elds (Class I force fields), is shown to be important in accounting ac curately for molecular energy surfaces. The Class II force field deriv ed from the quantum energy surface is characterized by accounting for these important intramolecular forces. The importance of each of the i nteraction terms of the potential energy function has also been assess ed. Bond anharmonicity, angle anharmonicity, and bond/angle, bond/ tor sion, and angle/angle/torsion cross-term interactions result in the mo st significant overall improvement in distorted structure energies and energy derivatives. The implications of each energy term for the deve lopment of advanced force fields is discussed. Finally, it is shown th at the techniques introduced here for exploring the quantum energy sur face can be used to determine the extent of transferability and range of validity of the force field. The latter is of crucial importance in meeting the objective of deriving a force field for use in molecular mechanics and dynamics calculations of a wide range of molecules often containing functional groups in novel environments. (C) 1994 by John Wiley and Sons, Inc.