Chemistry Reference
In-Depth Information
Table 6
SCF, MP2, and CCSD(T) Interaction Energies (
E
int
)of
(HF)
3
Obtained with a Series of Correlation-Consistent Basis Sets
a
Basis set
b
CCSD
ð
T
Þ
SCF
MP2
d
CCSD(T)
MP2
haDZ
50
:
60
60
:
89
0
:
82
61
:
71
haTZ
47
:
30
60
:
50
1
:
37
61
:
87
haQZ
46
:
61
59
:
94
1
:
49
61
:
44
ha5Z
46
:
34
59
:
75
—
—
ha6Z
46
:
30
59
:
71
—
—
31
c
70
c
CBS
a
CCSD(T) corrections for higher order correlations effects are also reported
relative to the MP2 values (
46
:
59
:
½
1
:
49
½
61
:
19
Þ
MP2
). All values are in kj mol
1
. Square
brackets denote values obtained with the additive approximation described
in the text.
b
ha
X
Z denotes cc-pV
X
Z for H and aug-cc-pV
X
Z for F.
c
Obtained from data in Table 5.
CCSD
ð
T
d
CCSDðTÞ
MP2
(T) can be reasonably estimated by adding the
d
correction obtained
from smaller basis sets to the MP2 CBS limit of
E
int
:
E
CCSDðTÞ=CBS
int
E
MP2=CBS
int
CCSD
ð
T
Þ
þ
d
½
47
MP2
49 kJ mol
1
obtained with
the haQZ basis set is combined with the MP2 CBS limit to produce an estimate
of
For this example, the CCSD(T) correction of
1
:
19 kJ mol
1
for the CCSD(T) CBS interaction energy of (HF)
3
. Square
brackets have been placed around these numbers in Table 6 to denote that they
are based upon the additive approximation in Eq. [47] rather than an extra-
polation of the correlation energy.
Step 3 in the tutorial associated with the section is to calculate the SCF
and MP2 interaction energies (including the CBS values) in Table 6 from the
energies given in Table 5.
Step 4 in the tutorial is to calculate the CCSD(T)
E
int
and
61
:
CCSD
ð
T
Þ
d
values
MP2
for the haDZ, haTZ, and haQZ basis sets.
Step 5 in the tutorial is to use Eq. [47] to estimate the CCSD(T) CBS limit
of
E
int
for (HF)
3
.
Accurate Potential Energy Surfaces
The systematic computational strategy outlined in this section of the
review is necessary albeit demanding. The approach provides an accurate
description of the entire spectrum of noncovalent interactions between frag-
ments in a cluster. One can be confident in the calculated results regardless
of cluster composition [i.e., whether examing the (H
2
O)
6
,
C
6
H
6
Þ
2
, or a mix-
ture of the two]. Less obviously but more importantly, one can also be confi-
dent in the calculated results across the entire (intermolecular) potential energy
ð
