Chemistry Reference
In-Depth Information
transition metals, and unimolecular bond-breaking reactions can cause diffi-
culties for CCSD(T) due to the presence of electronic near degeneracies,
46
such issues are not a concern for typical noncovalent interactions. Second, a
study by Hopkins and Tschumper
47
indicates that quadruple substitutions
make only small contributions to the interaction energies of small van der
Waals complexes (perhaps about 5% or less).
Basis Set Problem
Compounding the difficulty of accounting for electron correlation effects
properly, accurate computations of noncovalent interactions also require very
large basis sets. This is not surprising because London dispersion interactions
can be expressed in terms of the polarizabilities of the weakly interacting
molecules, and polarizability computations are known to have large basis
set requirements. In many weakly bound complexes, the dispersion terms
can be the dominant ones.
Figure 3 again considers the potential energy curve for the sandwich ben-
zene dimer, this time fixing the theoretical method to MP2 and varying the
basis set. We consider Dunning's augmented, correlation-consistent basis
sets
44
(i.e., aug-cc-pVDZ, aug-cc-pVTZ,
and aug-cc-pVQZ). These basis sets are the most commonly used for accurate
computations of noncovalent interactions because they have been designed to
converge smoothly to the complete (infinite) basis set limit. Note that these are
from double-
through quadruple-
2.0
1.0
0.0
-1.0
-2.0
-3.0
aug-cc-pVDZ
aug-cc-pVTZ
aug-cc-pVQZ
aug-cc-pVDZ (CP-corr)
aug-cc-pVTZ (CP-corr)
aug-cc-pVQZ (CP-corr)
-4.0
-5.0
-6.0
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
R (Å)
Figure 3
Interaction energy for the sandwich benzene dimer as a function of the
intermolecular distance using the MP2 method and various basis sets, with and without
counterpoise (CP) correction, using the rigid monomer geometries of Ref. 39.
