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force field were derived from rotational potentials obtained by molecular mechanics
(MM) simulations, while in the most recent versions of the OPLS-AA force field, the
torsional potentials were fitted to ab initio calculations at the MP2 level of theory with
the 6-31G
*
or even the correlation consistent polarized triple zeta (cc-pVTZ) basis
set. The parameterization of the intermolecular interactions was performed to repro-
duce saturated liquid density and enthalpy of vaporization. In some versions of the
OPLS-AA force field [
137
,
138
], the partial charges were fitted to the ab initio ESP
with the CHELPG [
79
] procedure. Since the OPLS-UA force field for hydrocarbons
was parameterized considering primarily short alkane chains, the deviations to
experimental vapor-liquid equilibrium data become more significant for larger
chain lengths [
141
].
4.1.2 TraPPE Force Field
The transferable potential for phase equilibria (TraPPE) was originally developed
by devising a united-atom representation for the alkyl segments (TraPPE-UA).
However, a TraPPE force field with explicitly considered hydrogens (TraPPE-EH)
[
119
] was also formulated. The TraPPE force field is available for a large number of
compound families, including linear and branched alkanes [
117
,
118
], alcohols
[
114
], ethers [
120
], ketones [
120
], glycols [
120
], amines [
122
], amides [
122
], thiols
[
115
], aromatics [
119
,
121
], and acrylates [
116
], among others. The TraPPE force
field takes the intermolecular interactions into account by the LJ 12-6 potential (6)
and coulombic terms (14). For the unlike LJ interactions, the standard Lorentz-
Berthelot combining rule (12) is assumed. The intramolecular interactions covered
by this force field are: angle bending on the basis of a harmonic potential (22) and
torsional motion expressed as a set of cosine series (25) and a harmonic improper
dihedral potential (27). The bond lengths are fixed and thus bond stretching is
not taken into account. Usually, the intramolecular parameters for angle bending
were transferred from the AMBER94 [
66
] force field and the dihedral parameters
were taken from the OPLS-UA force field. The LJ and point charge parameters
were fitted to reproduce experimental vapor-liquid coexistence data. The TraPPE
force field reproduces the saturated liquid density of linear alkanes with a mean
accuracy of approximately 1%, which has to be seen in the light of traditional
predictive methods like Lee-Kesler that have an accuracy of 2-3% [
9
]. The TraPPE
force field reproduces vapor pressure, saturated vapor density, and critical point
with more significant deviations to the experiment [
9
]. It does not reproduce the
second virial coefficient well and consistently underpredicts the shear viscosity of
short chain paraffins [
106
].
4.1.3 OPPE-AUA4 Force Field
The anisotropic united-atom optimized potential model for phase equilibria
(OPPE) force field is an elaboration of the anisotropic united-atom (AUA) force
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