| Popis: |
This review summarizes the work in our laboratory dealing with the development of a new class of force fields for simulating molecular properties for a wide variety of practical applications. This new generation of force fields, termed Class II, is based on the derivation and parameterization of analytic representations of quantum mechanical energy surfaces. The energy surface is derived from a set of representative species by systematically sampling distorted molecular structures and computing the corresponding quantum mechanical energy, energy gradient and Hessian matrices, i.e. the energy and its first and second derivatives with respect to atomic positions. This development has a number of key implications for the development, refining and testing of molecular force fields. It has been found that molecular energy surfaces are characterized by significant anharmonic and coupling interactions. Analysis of the quantum mechanical results indicates that these effects must be included in analytical representations of the force fields if accurate structures, energies, and dynamic properties such as vibrational frequencies are to be obtained from molecular mechanics and dynamics calculations. In addition the inclusion of such terms greatly increases the transferability of the force field. Such transferability is critical in predicting the properties of new species not included in the derivation of the force field, as is invariably the requirement in practical research applications. In addition it has been shown that this methodology also has the pragmatic advantage of allowing for the derivation of a reasonable force field based on quantum mechanics for molecules where little or no experimental data exist. In this report we review the methodology of developing and testing the quantum mechanically based force fields, showing the derivation of the functional form of the energy expression, sampling the quantum energy surface, testing against explicit quantum results, scaling the force field to account for systematic errors, and testing against experiment. We focus simultaneously on criteria for establishing force field accuracy and transferability. Where possible we describe parallel comparisons with the earlier diagonal-quadratic or Class I force fields. Finally we discuss deficiencies remaining in the existing level of force field development and outline how they may be addressed in future work. |
