Chemistry Reference
In-Depth Information
dispersion term (DFT-D) appears to be promising.
187-189
The DFT-D method,
however, is still not nearly as reliable as high-accuracy WFT approaches such as
those outlined above. While the nonempirical approaches (such as those
of Becke
190-192
) are certainly appealing, they currently do not outperform
DFT-D. The last approach, which attempts to include dispersion in DFT via
modification of existing functionals, is by far the most popular of the
three.
174,177,193-195
Unfortunately, it seems that modifications to the exchange
functional leading to a successful description of dispersion interactions also
happen to destroy any ability to reliably describe hydrogen bonding. When
examining the performance of newly developed functionals, it is important to
note that rare gas dimers are a poor model for dispersion-bound molecular
clusters. Just because a functional gives reasonable results for rare gas
dimers does not mean that similar performance can be expected for molecular
dimers.
Guidelines
Although being an oversimplification, noncovalent interactions can be
divided into three categories to help select less demanding computational pro-
cedures when studying a particular weakly bound cluster.
Category 1
(
Easy
) Strong noncovalent interactions that are dominated by the
electrostatic component of the interaction energy tend to be fairly easy to
compute.
MP2 will provide excellent results while most DFT methods will
generally provide reliable results near minima on the PES
.
Category 2
(
Hard
) Interactions in which dispersion plays a non-negligible
role tend to be more difficult to compute.
MP2 will provide reasonable
results while conventional DFT methods will not even provide a
qualitatively correct description of these interactions across the PES.
Category 3 (Problematic
) Dispersion interactions involving one or more
delocalized
electron systems are exceptionally difficult to describe.
MP2
will overbind in a manner that is inconsistent across the PES, and
conventional DFT methods still provide an unphysical description of the
interactions.
p
OTHER COMPUTATIONAL ISSUES
Basis Set Superposition Error and Counterpoise
Corrections
As noted when introducing the Boys-Bernardi CP correction above,
BSSE is a concern whenever the supermolecule method is used to compare
the energies of fragments to the energy of the entire cluster [i.e., when comput-
ing the dissociation
ð
D
e
Þ
or interaction
ð
E
int
Þ
energy].
