Chemistry Reference
In-Depth Information
more fully than is done with atomistic classical simulations. Whereas quantum
calculations are implemented regularly to develop the classical force fields used
in condensed-phase simulations, one can also conduct simulations in which a
quantum calculation is used to compute the energy and forces on each atom
present in the system. Such methods go by various names including ab initio
molecular dynamics or first-principles molecular dynamics and have become
popular since the release of easy-to-use software. Two of the more commonly
used programs are CPMD
133
and SIESTA,
134
both of which use density func-
tional theory with pseudopotentials. Because ab inito MD is significantly more
demanding of computational resources than is classical MD, only very small
system sizes can be examined and only for very short periods of time. This
limits its usefulness significantly when it comes to computing properties of
ionic liquids. Moreover, although ab initio methods are in principle ''exact,''
in practice various approximations go into the methods used in ab initio MD,
and the treatment of attractive dispersion interactions in density functional
theory is questionable. Nevertheless, the calculations can provide some insight
about local interactions present in these systems that could be useful in under-
standing solvation and reaction.
Del Popolo, Lynden-Bell and Kohanoff
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conducted the first ab initio
MD calculation of a condensed-phase ionic liquid in 2005. They modeled
[C
1
mim][Cl] using the SIESTA code. Although they state that this is a room
temperature ionic liquid, it is actually a solid at room temperature, but it
does melt below 100
C and so fits our definition of an ionic liquid. The
bulk of their calculations were on eight ion pairs, with six trajectories, each
run for up to 7 ps. It is impossible to equilibrate a system in only 7 ps due
to the slow dynamics of ionic liquids. Therefore, classical simulations were
first run to generate equilibrated systems, and then these configurations
were used as initial conformations for the SIESTA runs. The authors simulated
the crystalline phase and liquid phase and examined overall structure and
orientation of the phases. Later, Bhargava and Balasubramanian
136
carried
out CPMD simulations on 32 ion pairs of [C
1
mim][Cl]. They computed the
liquid structure and found good agreement between quantum-derived radial
distribution functions and those obtained from classical MD. They also com-
puted the vibrational density of states from the Fourier transform of the velo-
city autocorrelation function and made frequency assignments for each of the
modes. As was the case with the work by Del Popolo, Lynden-Bell and Kohan-
off,
135
however, deuterium was substituted for hydrogen to enable larger time
steps to be taken, and so the calculated frequencies are shifted.
Ab initio MD methods are certain to gain popularity as computational
power grows, but they are presently too expensive to use to obtain quantita-
tive estimates of properties. Quantum MD is most useful for computing
spectra, for helping validate and improve classical force fields, and for
studying reactivity in ionic liquids, something classical simulations cannot do.
