Chemistry Reference
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Fig. 3 Coexistence densities,
coexistence pressure, and
interface tension of hydrogen
sulfide. The
dashed lines
are
experimental results [
130
].
Symbols
are MC results of a
coarse grained model (see
Sect.
2.3
) using two different
values for the dipole moment
m
,
m
1
:
5 D and
m
1
:
1D,
respectively, and results of
various GEMC simulations of
atomistic models [
159 163
].
Model 1
depicts results
tabulated in [
159
] for a three
atom model of H
2
S[
160
].
Model 2
shows results where
H
2
S is represented by a model
with four “atoms” [
161
].
Model 3
is a
reparametrization of model
2in[
159
] and
pol
is a model
[
162
] that includes a
polarizable site (for a total
of five interacting “atoms”
per molecule). Finally,
3
unit
is a recent [
163
]
reparametrization of a three
site model, which also
includes a three center LJ
interaction. From Mognetti
et al. [
55
]
However, the slow decay of the dipole dipole interaction in (
6
) (proportional
to
r
3
) makes the use of Ewald summation techniques necessary when one uses (
6
)
in a simulation. Thus, the computational effort of working with the full Stockmayer
potential (
6
) is comparable to the effort needed for all-atom models with partial
charges at the atoms.
As an example for the problems in obtaining an accurate description of the
thermodynamic properties of dipolar fluids from the computer simulation of atom-
istic models, Fig.
3
presents a comparison [
55
] of experimental data for H
2
S[
130
]
with a coarse-grained model [
55
] that we shall explain in Sect.
2.3
and various
simulations of atomistic models [
159 163
]. Again, one concludes that simple
versions of atomistic models do a rather poor job, whereas the more complicated
recent versions can provide a reasonable description of the coexistence data,
although (due to the use of Gibbs ensemble MC, GEMC, data) they neither yield
results close to criticality nor provide any information on the surface tension. The
coarse-grained model proposed in [
55
], based on a mapping of the SM (
6
)onan
