Thermodynamic Properties for Applications in Chemical Industry via Classical Force Fields - Multiscale Molecular Methods in Applied Chemistry - page 217

for equilibrium geometries [ 10 , 93 ]. An important example is the Becke's three-
parameter density functional hybrid method combined with the Lee, Yang, and Parr
gradient-corrected correlation functional B3LYP [ 94 ].
QM is widely used to calculate relative energies of conformation sets and energy
barriers between them. Hence, bond length, bond angle, and torsional potential
terms can be fitted to reproduce intramolecular energy surfaces, the relative energy
of conformational pairs, or rotational energy profiles. The variation of energy for
different configurations can be calculated quite accurately with relatively small
basis sets. The rotational energy profiles are often taken as a basis to determine the
torsional interactions. For this purpose, the energy of a series of molecular struc-
tures generated by bond rotation is obtained from ab initio calculations. The
torsional potential is fitted to the resulting energy profile together with the Van
der Waals and electrostatic potentials [ 19 ]. Both HF and MP2, together with the
6-31G basis set, are often employed for such calculations [ 95 ]. It should be noted
that DFT with the B3LYP functional performs rather poorly for intermolecular
interactions and conformational energies [ 10 ].

3.2 Empirical Parameterization

Due to the difficulties of QM methods to describe correctly condensed phase
behavior, Van der Waals parameters and atomic point charges of molecular models
are often adjusted to reproduce experimental data on macroscopic properties of the
liquid state. Usually, they are fitted to thermodynamic properties determined by
means of molecular dynamics (MD) or Monte Carlo (MC) simulations.

3.2.1 Intermolecular Interactions

Intermolecular potential parameters can be optimized to different types of experi-
mental data. For engineering applications, liquid density and liquid enthalpy are
very often used. E.g., the liquid density strongly depends on the LJ size parameter
s , whereas the enthalpy of vaporization strongly depends on the LJ energy well

depth e [ 60 ]. Therefore, intermolecular parameters are frequently adjusted to
experimental data on both of these quantities, as in the OPLS force field [ 96 ].
The vapor pressure is even more sensitive to the intermolecular potential para-
meters so that, particularly in recent years, it was chosen together with the saturated
liquid density and the heat of vaporization to devise numerous generic force fields
of interest to chemical engineers like TraPPE, AUA, and NERD. The latter strategy
was also used for the development of a wide variety of specific molecular models
for engineering applications [ 97 - 99 ].
Many other physical properties may also be taken as targets for parameter
optimization of Van der Waals and electrostatic potentials: second virial coefficient