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Parameterization

Force field parameters can be divided into three categories: valence, electrostatic and van der Waals (VDW) terms. Electrostatic and VDW terms are usually referred to as nonbond terms. Valence terms are useful in predicting localized molecular properties such as structural parameters and vibrational frequencies. Once the valence parameters are fixed, it is the nonbond terms that determine extended properties such as chain conformations, inter-molecular interactions, liquid and solid structures, and thermophysical properties.

In principle, all parameters can be derived from ab initio data. However, deriving VDW parameters from ab initio data calculated for molecular clusters does not work very well for modeling molecules in condensed phases. Direct Force Field TM utilizes several techniques to derive force field parameters.

Generally speaking, a parameterization procedure consists of the following sequential steps:
Step 1. Prepare quantum mechanics data.
Step 2. Derive charge parameters by fitting ab initio electrostatic potentials only.
Step 3. Derive valence parameters by fitting ab initio energetic data, with charge and VDW parameters fixed at optimized and estimated values, respectively.
Step 4. Optimize VDW parameters by fitting liquid thermophysical data, with charge and valence parameters fixed at their optimized values.

The above steps may need to be iterated, depending on how strongly the three categories of force field terms are correlated. In most force fields, after VDW parameters are optimized, only very minimal changes are required for a few (torsion) valence terms.

An artificial intelligence process has been implemented in DFF. This technique enables fitting of complex potential energy surfaces with minimum human operations. For most common molecular systems, it takes only a few simple steps to generate high-quality parameters representing ab initio energy surfaces. With the parameterization tools, new parameters can be readily generated and added into the TEAM database.

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Aeon Technology Inc.
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Tel: 858-964-0869 Fax: 858-677-0240
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