Chemistry Reference
In-Depth Information
with other force fields, partial charges were fit to the electrostatic potential
obtained from the quantum calculations. The level of quantum theory used
by these authors was superior to that used in previous calculations. The
authors were careful to summarize previously developed force fields and
highlight the differences between their new force field and the others. To
validate their force field, the authors ran 200-ps flexible isothermal-isobaric
MD simulations using the DL_POLY package on five crystal structures for
which lattice constants were known. They then compared their computed
lattice constants with those obtained experimentally. They also conducted
liquid-phase simulations (300 ps under isothermal-isobaric conditions) on
nine different compounds, combining the [C
2
mim], [C
4
mim], and [C
6
mim]
cations with the PF
6
,NO
3
,andCl
anions. Of the nine liquids and five
solids, they were able to compare their results to six liquid densities and
five crystal densities. Densities deviated from experiment within the range
of 1-5% for all substances, which is about the level of accuracy obtained
in previous works. This was the first time, however, that comparison was
made to crystalline densities. In doing this, the ionic liquid modeling effort
came full circle, using the same criterion Tosi and Fumi
18
used to develop
alkali halide potentials. The force field developed by Canongia Lopes and
co-workers
57
is important because it has become one of the most widely
used force fields for this class of ionic liquids. This is mainly due to the rigor
with which it was developed, its wide applicability, and the generality and
popularity of the OPLS-type force field. The list of force field parameters
in the original study contained some typographical errors, some of which
were corrected in an corrigendum.
58
The updated and correct set of para-
meters are now located on the authors' website,
59
and it is recommended
that parameters be obtained from that source. This group has extended its
force field to include many more cations and anions, and those parameters
can also be found on the website.
59
Liu, Huang and Wang
60
proposed a ''refined'' force field for imidazo-
lium-based ionic liquids, shortly after the work of Canongia Lopes and co-
workers
57
appeared. Their force field consists of explicit atoms and is based
on the AMBER functional form. Unlike previous studies in which some
bond and angle force constants came from literature sources, these authors
determined all of these intramolecular terms from a quantum mechanical
frequency analysis of an energy-minimized structure. Vibrational wavenum-
bers for a large number of modes in the cation were computed and compared
to values given by their force field as well as from the standard AMBER force
field and from infrared (IR) data. The same protocol was used for PF
6
and
BF
4
anions. As was done by Canongia Lopes et al.,
57
some of the dihedral
angle parameters were fit to the energy profiles obtained from quantum calcu-
lations. Finally, the Lennard-Jones parameter for the hydrogen bonded to the
C2 carbon in the imidazolium ring was adjusted to enable the force field to
match ab initio energies calculated for several conformations.
