Chemistry Reference
In-Depth Information
series.
126
The united-atom ionic liquid force fields
30,39,56,127
are examples of
coarse-grained models. The basic idea behind this approach is to group
atoms together into single interaction sites. Usually, methyl and methylene
groups are each treated as a single site. The accuracy of a coarse-grained
model depends critically on how the model is developed. Typically, the
functional form of the model is preselected, and parameters are fit to match
some property of the fully atomistic model. If a poor choice is made for the
preselected analytical function form, then the model will fail to give accurate
results.
Wang, Izekov, Yan and Voth
128
recently proposed
multiscale coarse
graining
(MSCG) for modeling of an ionic liquid. Their method is based on
the concept that the forces on coarse-grained sites should ''match'' as closely
as possible the forces in the atomistic sites that make up the coarse-grained
sites. To do this, an atomistic force field of the type described by Eq. [5] is
set up for an ionic liquid, and individual atoms are then ''lumped'' into
coarse-grained sites, as shown in Figure 22. An atomistic simulation is per-
formed and the forces on all the atoms associated with each coarse-grained
site are recorded. The parameters in an empirical potential function represent-
ing the coarse-grained model are then adjusted until the forces present in the
coarse-grained sites match, in a least-squares sense, the forces on the atoms
constituting the coarse-grained sites. The authors show that structural proper-
ties such as radial distribution functions agree well with the fully atomistic cal-
culations. Densities and compressibilities are also captured well, although the
Figure 22
Coarse-graining strategy used to reduce the number of interaction sites
required to model an ionic liquid. In (a) the cation is represented with four sites A, B, C,
and E, while the anion is represented with a single site D. In (b) the simplified coarse-
grained model is depicted with associated site charges. (From Ref. 128 and used with
permission.)
