Chemistry Reference
In-Depth Information
At one extreme (bottom of the figure), one finds complexes held together pri-
marily by dispersion forces (rare gas dimers, He nanodroplets, etc.). On the
opposite extreme (top of the figure) are clusters dominated by electrostatic
interactions such as hydrogen bonding (formic acid dimer, water clusters,
etc.). Of course, most interfragment interactions fall somewhere between these
two extremes. [In this work, the term
interfragment
(or
intermonomer
) inter-
action is used because it is more general than and implicitly includes both
interatomic
and
intermolecular
interactions. Note that some researchers object
to the latter adjective when describing weakly bound clusters because it is
technically incorrect. For example, if (HF)
3
is considered an independent
molecular species then, by definition, there can be only intramolecular interac-
tions.] A more detailed analysis of this continuum of weak noncovalent inter-
actions is presented below.
Given the current flurry of activity in the area of
-type interactions and
halogen bonding (a specific case of sigma-hole interactions), special attention
will be paid to these two types of weak interactions. In fact, an entire chapter in
this volume of
Reviews in Computational Chemistry
is dedicated to noncovalent
p
p
interactions.
49
It should be noted that, although most examples are for relatively
small (dimers, trimers, tetramers, and pentamers) homogeneous clusters, the
principles discussed here can readily be extended to larger, heterogeneous systems.
Computational Methods
Although a wide variety of theoretical methods is available to study
weak noncovalent interactions such as hydrogen bonding or dispersion forces
between molecules (and/or atoms), this chapter focuses on size consistent elec-
tronic structure techniques likely to be employed by researchers new to the
field of computational chemistry. Not surprisingly, the list of popular electro-
nic structure techniques includes the self-consistent field (SCF) Hartree-Fock
method as well as popular implementations of density functional theory
(DFT). However, correlated wave function theory (WFT) methods are often
required to obtain accurate structures and energetics for weakly bound clus-
ters, and the most useful of these WFT techniques tend to be based on
many-body perturbation theory (MBPT) (specifically, Møller-Plesset pertur-
bation theory), quadratic configuration interaction (QCI)
theory, and
coupled-cluster (CC) theory.
This review concentrates on the fundamentals of supermolecule model
chemistries for clusters of atoms/molecules held together by weak chemical
forces. The principles behind the appropriate selection of theoretical method
and basis set for a particular class of weak noncovalent interactions provide
the foundation for understanding more complex computational schemes that
might require the user to specify more than just a method and/or basis set, such
as highly efficient fragmentation schemes [e.g., the effective fragment potential
(EFP) method,
53,54
the fragment molecular orbital (FMO) method,
55,56
the
