Computational Chemistry

outside the active space, are not subjected to the CI calculation, but instead are
treated at the Hartree–Fock level. The CASSCF calculation is said to treat properly
static correlation, but notdynamic correlation(Section 5.4.1). To account more
completely for dynamic correlation, a single-point perturbational calculation based
on the CASSCF wavefunction is frequently done. This is a CAS perturbational
theory, second order, or CASPT2, calculation. The most popular implementation of
this is CASPT2N (N¼nondiagonal one-particle operator) [ 66 ]. For programs that
will do CASPT2-type calculations, seeSection 9.3. Improving a CASSCF energy
with a CASPT2 calculation is analogous to improving a Hartree–Fock- (i.e. SCF-)
level calculation with a single-point MP2 calculation (Section 5.4.2).

CASSCF Calculations on 1,5-Pentanediyl and Cyclopentane

I outline another example of CASSCF calculations: a comparison of the energy of
1,5-pentanediyl and cyclopentane:

..

E(C – C)

(1.483)

(1.619)

1.492

.

1.568

1

2

2

3

4

1

7

C 1 C 2 C 3 C 4 = 180 (180) 8

H 1 C 1 C 2 C 3 = 75.7 (76.0)

H 2 C 1 C 2 H 1 = 151.4 (208.0)

H 7 C 4 C 3 C 2 = 75.6 (76.0)

H 8 C 4 C 3 H 7 = 151.3 (208.0)

This work

Doubleday

Fig. 8.11 The C2h1,4-butanediyl diradical relative minimum (no imaginary frequencies), as
calculated by CASSCF(2,2)/6-31G (this work) and CASSCF(4,4)/6-31G (Doubleday [ 61 ],
Fig. 1 and Table III)

8.2 Singlet Diradicals 543