Chemistry Reference
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Fig. 4 Saturated vapor pressure of ammonia on the basis of different force fields by Impey and
Klein (
open diamonds
)[
108
], Krist
´
f et al. (
open squares
)[
246
], Eckl et al. (
open circles
)[
97
], as
well as Zhang and Siepmann (
open inverted triangles
)[
247
]. The simulation results are compared
with a reference equation of state (
solid line
)[
249
]
vapor pressure data and the critical temperature is underestimated by 2.4% [
97
].
A further improvement was achieved by the model from Eckl et al. [
97
] with mean
deviations from the critical temperature, saturated liquid density, vapor pressure,
and enthalpy of vaporization of 0.8, 0.7, 1.6, and 2.7%, respectively. The recently
introduced force field by Zhang and Siepmann [
247
] reproduces the saturated liquid
densities up to 375 K with a similar accuracy to that of the model of Eckl et al. [
97
].
This force field predicts the critical density, critical pressure, and normal boiling
point with deviations of 0.9, 2, and 0.5%, respectively.
Figures
3
and
4
show the saturated densities and the vapor pressure on the basis of
the force fields by Impey and Klein [
108
], Krist´f et al. [
246
], Zhang and Siepmann
[
247
], and Eckl et al. [
97
] for the whole temperature range from triple point to
critical point together with a reference equation of state [
249
] for comparison.
6.3 Properties of the Homogeneous State
As discussed in Sect.
2
, force fields should not only be able to represent the
thermodynamic properties that were used for their parameterization, but should
also be capable of predicting other properties at different thermodynamic conditions.
The force field for ammonia by Eckl et al. [
97
] is an example of such a force field.
Eckl et al. [
97
] predicted the density and the enthalpy of liquid, gaseous, and
supercritical ammonia at 70 different state points, covering a wide range of states
for temperatures up to 700 K and pressures up to 700 MPa. They found typical
deviations from experimental data below 3 and 5% for the density and the residual
enthalpy, respectively. Figure
5
shows the density results on the basis of this force
field compared with a reference equation of state [
249
].
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