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field, initially proposed by Toxvaerd [
142
,
143
] that was further developed by
Ungerer and coworkers [
130
]. This force field is currently available for
n
-alkanes
[
130
], olefins [
59
], alcohols [
68
], polyalcohols [
144
], amines [
123
], amides [
123
],
nitriles [
127
], sulfides [
126
], thiols [
126
], ketones [
128
], aromatic hydrocarbons
[
124
,
129
], or polycyclic aromatics [
125
]. The major novelty of AUA force fields
was that the force center is spatially located between the carbon and the hydrogen
atoms of the represented molecular group. The intermolecular interactions were
described by the LJ 12-6 potential and point charges [(6) and (14)]. The Lor-
entz-Berthelot combining rule (12) was used for the unlike LJ parameters. As in
the TraPPE force field, the bond lengths were kept fixed. Angle bending was
modeled by a trigonometric potential (23) and the torsional potential following
Ryckaert and Belleman (24). Some angle parameters were taken from the
AMBER94 [
66
] force field and the torsional potential parameters were taken from
the OPLS-UA [
96
] force field. In other cases, molecular geometry and electrostatic
charges were determined from ab initio calculations. Usually, geometries were
optimized with the B3LYP functional and the 6-311G
**
basis set. The partial
charges were parameterized according to the procedure of L´vy and Enescu [
145
]
to reproduce the ESP around the isolated molecule for several representative con-
formations using RESP [
80
]. The ab initio ESP of the molecules was determined at
the MP2 level of theory with a 6-31G
*
or a 6-311G
**
basis set. The LJ parameters
were optimized to reproduce experimental values of saturated liquid density,
enthalpy of vaporization, and vapor pressure. The OPPE force field provides a
good representation of the vapor pressure and a very accurate representation of the
liquid density over a wide temperature range for
n
-alkanes, branched alkanes, and
cycloalkanes [
56
]. The vapor pressure of alkanes is predicted with an average
deviation to experimental data of 15%, compared to 30% for the TraPPE force
field and 35% for classical prediction methods based on boiling temperature and heat
of vaporization [
9
]. Since transport properties are not well predicted by this force
field, Nieto-Draghi et al. [
107
] proposed a modification of the OPPE model by
adjusting the parameters of the torsional potential to reproduce experimental reori-
entation dynamics and shear viscosity.
4.1.4 NERD Force Field
The Nath, Escobedo, and de Pablo (NERD) force field [
100
,
131
-
133
] was developed
to provide accurate predictions of thermodynamic properties. It is currently available
for linear [
100
] and branched alkanes [
131
,
133
] as well as for alkenes [
132
]. It has a
similar functional form as the TraPPE-UA force field, but bond stretching is included.
This interaction and angle bending are represented by harmonic potentials [(20) and
(22)]. The torsional potential is of the form of (25), neglecting cross terms. The LJ
12-6 potential (6) is used to describe the intermolecular and intramolecular interac-
tions between sites that are separated by more than three bonds. The LJ parameters
were obtained from fits to experimental values of liquid density and second virial
coefficient. Saturated liquid densities from the NERD force field are in good
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