Force Fields for Molecular Dynamics

Fraunhofer Institute for Algorithms and Scientific Computing SCAI

Automated and optimized computation of force field parameters

Generally molecular dynamics simulations require the adaptation of different atom or molecular models to desired physcial or chemical properties. In this procedure the  appropriate choice of force fields and of corresponding parameters is a crucial issue.

SCAI develops a modular program package, which can be simply tuned to the different occurring optimization problems. Once started, the program runs without user interaction until the force field ist generated. In this way force field parameters can be determined for complicated atomistic and coarse grained models of any chemical matter.

Background

Before molecular dynamics computer simulations can be executed, appropriate models have to be developed. This process consists of two fundamental steps: First, a general model of the occurring force fields is designed, then the free parameters of this model have to be determined. Within a model, different matters are distinguished only by their parameters. These parameters define thus the shape of the force field. The choice of these parameters is often more important for the quality of the chosen model than its analytical form.

The problem

It turns out that the parameter opimization for concrete systems is very time consuming. Therefore, often so-called standard force fields are used, which are known from the literature or contained in existing simulation software (e.g. GROMOS, CHARM or AMBER). However, there are many applications for which these common force fields are not well suited. Either the force fields are not optimized at all or only for certain matter classes (e.g. proteins) or adapted empirically under certain conditions (e.g. aqueous solution). The results of computations which have been based on such force fields may not be very reliable in applications with different conditions.

The idea and its fruits

To use the program being developed by SCAI, the user decides on a suitable generic model. The corresponding free parameters are set to reasonable initial values. Using this first force field a simulation is run and analyzed comparing sensitive indicators (the target function) with available reference values. Thus an impression of the quality of the current force field is obtained and a relation to physical quantities is established. The algorithm optimizes now the current parameters automatically in further iterative simulation runs.

The automatic optimization of model parameters is going to become an indispensable tool of our research. In the atomistic level it allows for the fast adaptation of force fields, which are tailored for the system of interest, its surrounding conditions and the particular question of interest. The use of general »standard force fields« can thus be avoided.

In the field of coarse grained polymer models the model development is simplified significantly. We can concentrate more on the general model development and leave the optimization of each model to the computer. This accelerates, for example in the field of soft matter research, the development of new materials.

Publications

Automatic Parameterization of Force Fields for Liquids by Simplex Optimization, R. Faller, H. Schmitz, O. Biermann und F. Müller-Plathe, J. Comput. Chem. 20, 1009-1017 (1999).

Coarse Graining of Nonbonded Inter-particle Potentials Using Automatic Simplex Optimization to Fit Structural Parameters, H. Meyer, O. Biermann, R. Faller, D. Reith und F. Müller-Plathe, J. Chem. Phys. 113, 6265-75 (2000).

Mapping Atomistic to Coarse-grained Polymer Models, D. Reith, H. Meyer, und F. Müller-Plathe, Macromolecules 34, 2335-45 (2001).