Mesoscale Coarse Graining

Fraunhofer Institute for Algorithms and Scientific Computing SCAI

An important step to efficient soft matter development

By the method of Mesoscale Coarse Graining it is possible to significantly increase the simulated time and the size of chemical systems  in computer simulations. This improvement results in more reliable predictions for physical quantities and properties which are indispensable for the development of new materials.

For this purpose, the atomistic, fully detailed system ist mapped to a coarser "mesoscopic" model. It is important to represent the omitted degrees of freedom in a suitably averaged way in the coarsened system such that the chemical identity of the substance under consideration is maintained. This can be guaranteed by reproducing key properties of the system.

Background

Many macroscopic polymer properties can only be understood after a deep study of microscopic details. In general, computer simulations are an appropriate tool to  turn microscopic basic knowledge in macroscopic information. For this purpose, quantum chemical computations or experimental data are used to construct a model which takes all involved atoms into account.

Unfortunately, in simulations based on this approach, problems may arise by the detailed treatment of the fast degrees of freedom (e.g. bond vibrations), which require that the time propagation of the system can proceed only in tiny little steps. The full simulated time is thus also very short (typically some nanoseconds) - even if high performance computers are used for weeks. This may even result in applications in which the slow degrees of freedom (e.g. the radius of gyration) do not reach the state of equilibrium at all which means that some properties remain unpredictable.

Very large atomistic systems cannot be simulated due to too large computing times either. Coarsened approaches  ("coarse graining") offer one way of overcoming these dilemmas: with this method the fast degrees of freedom can be eliminated from the system, the time steps in the computation can be enlarged and the slow degrees of freedom can thus be relaxed. Using greater time and length scales are reached, which makes it possible to gain more information on a system and to  find explanations for macroscopic physical effects. This is a decisive progress for the generation of new materials.

Polyacrylic acid

Fig. 1

In the case of polyacrylic acid (PAA) in solution, data for of atomistic simulations were available for an oligomer. This system was coarsened in the following way: a full structural component, i.e.-CH2-CH1(-COOH)-ä was replaced by a single sphere around its center of mass (see. Fig. 1, where H-atoms are omitted). If this procedure is carried out for all structural components, a new coarsened chain is obtained (see Fig. 2). If it is possible, to reproduce the behavior of the atomistic chain, this model can be used to study longer PAA chains.

On the coarser level static effects such as adsorption at organic surfaces can be investigated much better because the free energy is influenced substantially by entropic contributions. Moreover, it is possible to re-introduce the atomistic details and to relax the fast degrees of freedom of the system if this is needed to understand certain effects.

Software

A software package which will make the benefits of Mesoscale Coarse Graining available, is ESPResSo++.

Publications

Coarse Graining of Nonbonded Interparticle Potentials Using Automatic Simplex Optimization to Fit Structural Properties, 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).