Essentials of Computational Chemistry

11
Implicit Models for Condensed
Phases
11.1 Condensed-phase Effects on Structure and Reactivity
The gas phase is delightful in its simplicity. At low to moderate pressures, molecules may
be treated as isolated, non-interacting species, and this facilitates theoretical modeling enormously,
insofar as the system of interest is entirely defined by the molecule itself. Were
theory to restrict itself to the gas phase, however, it would be inapplicable to vast tracts of
chemistry, to include essentially all of biochemistry.
Of course, one can carry out accurate gas-phase calculations and then make broad generalizations
about how one might expect a surrounding condensed phase to affect the results.
Indeed, this modus operandi was much employed well into the 1980s and still sees modest
use today. Provided one can be reasonably confident that condensed-phase effects are small
for the particular properties being studied (either in an absolute sense or through cancellation
by judicious comparisons), such an approach can still be useful, particularly in a qualitative
sense. However, significant developmental efforts over the last two decades combined
with growth in the computational power required to implement them have resulted in the
widespread availability of condensed-phase models designed to more accurately describe the
physical nature of condensed-phase systems. This chapter considers one such class of these
models, namely, implicit solvation models, which are also often called continuum solvation
models.
At first thought, of course, it might seem that the modeling of a condensed-phase system
should be trivial. Take for example a liquid solution (which we will take as our ‘default’
condensed phase in this and the next chapter, although others will be discussed). If our
solution is dilute, then the ‘obvious’ way to construct a model is to surround our solute
with a number of solvent molecules. But a critical question is, how many? If we want to
consider glucose in water, for instance, it seems clear that we would want at least the entire
surface of the glucose molecule to be covered. This might take, say, 14 water molecules,
which we could place approximately at the corners and faces of an imaginary cube about
our solute. However, it would be something of a stretch of faith to imagine this as true
aqueous solvation – none of the water molecules is interacting with a second solvation
Essentials of Computational Chemistry, 2nd Edition Christopher J. Cramer
© 2004 John Wiley & Sons, Ltd ISBNs: 0-470-09181-9 (cased); 0-470-09182-7 (pbk)