Biomedical Engineering Reference
In-Depth Information
4. The first higher level correction is a complete MP4/6-31G(d) single point, which is then
modified to allow for diffuse functions, higher polarization functions, higher correlation
terms using QCI, larger basis sets and the effect of nonadditivity.
5. TheMP4/6-31G(d) energy and the four corrections are combined along with a spin-orbit
correction.
6. The usual higher level correction is added to take account of remaining deficiencies:
B
n
α
−
n
β
Δε (HLC)
=−
An
β
−
for molecules
(21.2)
D
n
α
−
n
β
Δε (HLC)
=−
Cn
β
−
for atoms
A
through
D
are constants chosen to give the smallest average deviation from experiment
for the G2 test set.
7. The total energy at0Kisobtained by adding the zero-point energy from step 2.
The overall agreement with experiment for 299 test energies is 1.02 kcal mol
−
1
compared
to 1.48 kcal mol
−
1
for G2 theory. Use of MP3 rather than MP4 gives a saving in computer
resource but the agreement with experiment becomes 1.22 kcal mol
−
1
.
21.3.1 G3 Variations
In view of the discussion for G2, you will not be surprised to learn that there is a G3(MP2)
version which aims to give a saving in computer resource by reducing the MP4 basis set
extension calculations to MP2.
Given the steady rise in density functional theory, you might like to read the work of
Baboul
et al.
(1999), who use B3LYP geometries and frequencies. The authors report that
the use of density functional geometries and zero-point energies in G3 and G3(MP2) is a
useful alternative to MP2 geometries and HF zero-point energies.
There have been a number of variations on similar themes over the years, and these are
nicely summarized in Baboul
et al.
(1999).
References
Baboul, A.G., Curtiss, L.A. and Redfern, P.C. (1999)
J. Chem. Phys.
,
110
, 7650.
Curtiss, L.A., Raghavachari, K., Trucks, G.W. and Pople, J.A. (1991)
J. Chem. Phys.
,
94
, 7221.
Curtiss, L.A., Raghavachari, K., Redfern, P.C.
et al.
(1998)
J. Chem. Phys.
,
109
, 7764.
Curtiss, L.A., Redfern, P.C., Raghavachari, K.
et al.
(1999)
J. Chem. Phys.
,
110
, 4703.
Pople, J.A., Head-Gordon, M., Fox, D.J.
et al.
(1989)
J. Chem. Phys.
,
90
, 5622.
