Chemistry Reference
In-Depth Information
benchmark values.
143
Even for high-energy saddle points and structures with
bifurcated hydrogen bonds, the deviations still tend to be less than a few tenths
of a kilocalorie per mole per hydrogen bond.
118,119
Only for cyclic hydrogen-
bonding motifs (like that found in the formic acid dimer), do MP2 interaction
energies deviate substantially from CCSD(T) values.
143
At the other end of the spectrum of weak interactions (Figure 1), the
MP2 method can still provide a reasonable description of dispersion-bound
clusters despite having a tendency to slightly overestimate the interaction ener-
gies between molecules relative to CCSD(T) results. For example, MP2 calcu-
lations yield interaction energies that are just a few tenths of a kilocalorie per
mole larger than the CCSD(T) values for
n
-alkane dimers
142
and even some
simple
Þ
2
.
120
systems are
delocalized, however, the MP2 errors can become massive (
vida infra
). Even
the interactions between rare gas atoms are described reasonably well by the
MP2 method.
144
However, MP2 tends to underbind the Ne
2
and He
2
by an
amount that is small in an absolute sense but large in a relative sense, particu-
larly in the case of He
2
.
The MP2 method is not perfect, however, and the most notable (and fairly
dramatic) failure of the MP2 method in the field of weak noncovalent interac-
tions occurs for delocalized
p
-stacked dimers such as
ð
N
2
Þ
2
and
ð
C
2
H
2
If the
p
stacking.
137-139
In fact, a separate chapter of this
p
-type interactions.
49
The MP2 method overesti-
mates dramatically the stability of ''face-to-face'' or ''stacked''-type configura-
tions relative to ''edge-to-face'' or ''T-shaped'' orientations that leads to a
qualitatively incorrect description of delocalized
volume is dedicated to these
p
stacking as illustrated with
the benzene dimer in described above. Consider the parallel-displaced stacked
and T-shaped configurations of the diacetylene dimer (H
p
C
C
H)
2
(shown in Figure 7) along with the analogous configurations of the acetylene
C
C
Figure 7
Parallel displaced and T-shaped configurations of the diacetylene dimer.
