A new method for deriving force fields for molecular simulations has been developed. It is based on the derivation and parameterization of analytic representations of the ab initio potential energy surfaces. The general method is presented here and used to derive a quantum mechanical force field (QMFF) for alkanes. It is based on sampling the energy surfaces of 16 representative alkane species. For hydrocarbons, this force field contains 66 force constants and reference values. These were 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 this force field to reproduce the features of the ab initio energy surface 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 molecules. Also, although only two atom types are employed, the force field accounts for the properties of both highly strained species, such as cyclopropane and methylcyclopropanes, as well as unstrained systems. The information contained in the quantum energy surface indicates that it is significantly anharmonic and that important intramolecular coupling interactions exist between internals. The representation of the nature of these interactions, not present in diagonal, quadratic force fields (Class I force fields), is shown to be important in accounting accurately for molecular energy surfaces. The Class I1 force field derived from the quantum energy surface is characterized by accounting for these important intramolecular forces. The importance of each 'Author to whom all correspondence should be addressed. tl'resent address: Dept. of Chemistry, Ben Gurion Univ. of the Negev, Beersheva 84105, Israel. Journal of Computational DERIVATION OF CLASS II FORCE FIELDSof the interaction terms of the potential energy function has also been assessed. Bond anharmonicity, angle anharmonicity, and bond/angle, bond/ torsion, and angle/angle/ torsion cross-term interactions result in the most significant overall improvement in distorted structure energies and energy derivatives. The implications of each energy term for the development of advanced force fields is discussed. Finally, it is shown that the techniques introduced here for exploring the quantum energy surface 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.

A critical survey of previously reported van der Waals parameters for alkali metal cations and halide anions is presented. A new set of force field parameters is proposed, derived by fitting the experimental lattice constants and lattice energies of 20 ionic alkali halide crystals. These parameters are constrained to satisfy two relationships connecting the ions with the isoelectronic noble gasesthe relative van der Waals radii R* and the coefficients of the London dispersion energies C 6using the experimentally determined noble gas van der Waals parameters. In addition to reproducing physical trends in common with atoms of isoelectronic species, the present parameters predict more accurate crystal structures and energies and, when combined with a molecular force field for water, also quite accurate gas-phase ion−water interaction energies and aqueous solution structures compared to the computed results previously reported by other authors.

Ab initio computations indicate the existence of several stable and some unstable conformers in isolated Q and p glucose molecules. All of the lower-energy conformers exhibit a strikingly regular pattern of internal hydrogen bonding. Five such stable structures have been identified for each of the Q and p anomers, differing primarily in the orientation of the CHzOH group. In each conformer, the a anomer is predicted to be lower in energy than the corresponding conformers of p anomer. The difference is about 2 kcal/mol in the 4-31G basis but only 0.4 kcal/mol in the 6-31G* basis. It is found that the electronic contributions to the free energy difference stabilize the Q anomer while the nuclear motion contributions stabilize the / 3 anomer. The implications of these predictions and the future investigations required to understand the relative stabilities of the two anomers are pointed out. 0 1992 by John Wiley & Sons. Inc.

Experimental and ab initio theoretical vibrational Raman optical activity (VROA) spectra are presented for N-acetyl-N‘-methyl-l-alaninamide (NANMLA), in order to determine the predominant conformations of this molecule in the solution phase. The experimental spectra were obtained in three different solvents, CHCl3, H2O, and D2O. The ab initio VROA spectra were predicted for nine different conformations of NANMLA optimized with the 6-31G* basis set. The vibrational frequencies and normal modes for all nine conformers were also obtained with the 6-31G* basis set. From a comparision of the predicted and observed VROA spectral patterns, it is suggested that there is reasonable support for the presence of C7,eq−C5, C7,eq, and αR conformers in aqueous solution and for C7,eq−C5 and αR conformers in chloroform solutions.

A class II valence force field covering a broad range of organic molecules has been derived employing ab initio quantum mechanical "observables." The procedure includes selecting representative molecules and molecular structures, and systematically sampling their energy surfaces as described by energies and energy first and second derivatives with respect to molecular deformations. In this article the procedure for fitting the force field parameters to these energies and energy derivatives is briefly reviewed. The application of the methodology to the derivation of a class II quantum mechanical force field (QMFF) for 32 organic functional groups is then described. A training set of 400 molecules spanning the 32 functional groups was used to parameterize the force field. The molecular families comprising the functional groups and, within each family, the torsional angles used to sample different conformers, are described. The number of stationary points (equilibria and transition states) for these molecules is given for each functional group. This set contains 1324 stationary structures, with 718 minimum energy structures and 606 transition states. The quality of the fit to the quantum data is gauged based on the deviations between the ab initio and force field energies and energy derivatives. The accuracy with which the QMFF reproduces the ab initio molecular bond lengths, bond angles, torsional angles, vibrational frequencies, and conformational energies is then given for each functional group. Consistently good accuracy is found for these computed properties for the various types of molecules. This demonstrates that the methodology is broadly applicable for the derivation of force field parameters across widely differing types of molecular structures. Copyright 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1782-1800, 2001