Thermodynamic Properties for Applications in Chemical Industry via Classical Force Fields - Multiscale Molecular Methods in Applied Chemistry - page 225

RESP method. Bond lengths and bond angles were constrained to reproduce the
optimum geometry from QM calculations. The LJ parameters were optimized to
experimental values of shear viscosity, self-diffusion coefficient, and liquid density.
A review on force fields for the simulation of imidazolium-based ionic liquids can
be found in [ 175 ].
Liu et al. [ 176 ] developed force fields for guanidinium-based ionic liquids
following the AMBER force field approach. The intramolecular interactions of
their force fields include harmonic bond stretching and angle bending, together with
torsional motions. The equilibrium bond lengths and bond angles were taken from
QM calculations at the HF/6-31+G(d) level. The force constants were adjusted to
vibrational frequencies obtained by ab initio calculations or from experiment.
Single point MP2/6-31+G(d) calculations were taken to parameterize the torsional
potential and QM calculations at the B3LYP/6-31+G(d) level to obtain RESP
charges. The LJ parameters were transferred from the AMBER99 force field.
4.2.3 Ab Initio Force Fields
Hellmann et al. [ 23 , 177 , 178 ] have proposed ab initio force fields for several small
molecules, such as helium, neon, or methane, based on the Tang and Toennies
potential (9) and coulombic terms (14). With these force fields, gas phase properties
like second virial coefficient, shear viscosity, thermal conductivity, or self-diffusion
coefficient can be predicted extremely accurately. Typically, the generated data are
within the experimental uncertainty.
Doma ´ ski et al. [ 179 ] developed an ab initio force field for liquid carbon dioxide
by fitting the LJ parameters and the coulombic terms to the potential energy surface
calculated with QM at the MP2 level of theory and the 6-31G * basis set. Unfortu-
nately, their model does not reproduce the thermodynamic behavior of the liquid
state so that an empirical scaling factor had to be adjusted to experimental data.
Hloucha et al. [ 24 ] developed force fields for methanol and acetonitrile from ab
initio calculations for the prediction of macroscopic properties. These all-atom
force fields include LJ 12-6 or modified Buckingham exponential 6 sites plus
partial charges. Interaction energies for many hundreds of configurations calculated
via symmetry adapted perturbation theory (SAPT) were employed for the parame-
terization of the LJ and Buckingham terms. To cover electrostatics, the charges
were fitted to the ESP from quadratic configuration interaction with single and
double substitution (QCISD) calculations and the augmented correlation-consistent
polarized valence double-zeta basis set (aug-cc-pVD). Despite the fact that the
force field for acetonitrile yields a reasonable agreement with the experiment for
vapor-liquid equilibrium properties, for methanol the saturated liquid density was
strongly underpredicted and the vapor pressure was overpredicted by one order of
magnitude. Cabaleiro-Lago and R´os [ 20 ] proposed a similar ab initio force field
for acetonitrile optimized at the MP2/6-311+G * level of theory. However, their
force field gives a poor prediction of the phase behavior [ 24 ]. Further examples of
ab initio force fields can be found, e.g., in [ 69 ].