Chemistry Reference
In-Depth Information
n
-body decomposition (NBD) scheme,
57
and the multicentered integrated
method (MC QM:QM or MC ONIOM) for clusters.
58-60
]
Some readers may have noticed that methods based on intermolecular per-
turbation theory such as symmetry-adapted perturbation theory (SAPT),
61-64
have not been mentioned. These methods are not discussed in this chapter
because most versions of SAPT are actually 2-body methods. (See below for a
description of 2-body, 3-body, and many-body interactions in clusters.) SAPT is
not inherently limited to 2-body interactions; a 3-body implementation
exists.
65-67
However, the author is not aware of higher order SAPT programs,
let alone a general
n
-body program for clusters composed of
n
fragments. While
SAPT can certainly be used to study trimers, tetramers, and larger clusters, such
applications require a great deal of a priori knowledge about the nonadditivity
(or cooperativity) in the system and are certainly not for novices.
Molecular mechanics methods are also omitted from the present discus-
sion for similar reasons. Although very sophisticated force fields are available
for water (including polarizable models), most force fields for weakly bound
clusters are essentially 2-body (dimer) potentials that have been adjusted
empirically to reproduce bulk-phase properties.
68-71
This procedure leads to
very reliable descriptions of liquid water, but diminishes the quality of results
for small clusters. Although force fields that include 3-body interactions are
beginning to appear,
69,70
the effects of higher order interactions (4-body,
5-body, etc.) are still untested. Furthermore, the composition of a weakly
bound cluster, not just its size, is a major concern with molecular mechanics
force fields. The highly refined potentials that have been developed for
water
68,71
are not necessarily transferable to other weak noncovalent systems
(methanol, acetone, etc.).
WEAK NONCOVALENT INTERACTIONS
This section presents an overview of the nature of weak noncovalent
interactions between molecules (and atoms). Readers interested in more detail
are directed to classic references such as the 1954 text by Hirschfelder, Curtiss
and Bird,
72
the 1971 topic by Margenau and Kestener,
73
the 1996 monograph
by Stone,
4
as well as some more recent sources.
7,27,31,74
Historical Perspective
Theoretical treatments of attractive forces between molecules (and/or
atoms) in the gas phase can be traced as far back as 1873 to the efforts by
van der Waals to describe the deviation of real gases from ideal behavior at
relatively high densities.
75
By the early 1930s, theoretical explanations of
the origins of van der Waals' attractive forces began to emerge from the likes
of Keesom,
76,77
Debye,
78,79
Falckenhagen,
80
and London.
81,82
This body of
