MOLPRO

20 THE CI PROGRAM 154
Default is to correlate all electron pairs in active and closed orbitals. See also PAIRS card.
PAIRS,iorb1.isy,iorb2.isy,np;
Correlate all pairs which can be formed from orbitals iorb1.isy1 through iorb2.isy2. Core
orbitals are excluded. Either iorb2 must be larger than iorb1 or isy2 larger than isy1. If
iorb1.isy1=iorb2.isy2 the PAIRS card has the same effect as a PAIR card. PAIR and PAIRS
cards may be combined.
If no PAIR and no PAIRS card is specified, all valence orbitals are correlated. The created
pair list restricts not only the doubly external configurations, but also the all internal and semi
internals.
20.2.13 Restriction of classes of excitations
NOPAIR;
No doubly external configurations are included.
NOSINGLE;
No singly external configurations are included.
NOEXC;
Perform CI with the reference configurations only.
20.3 Options
20.3.1 Coupled Electron Pair Approximation
CEPA(ncepa);
(0 ≤ ncepa ≤ 3). Instead of diagonalizing the hamiltonian, perform CEPA calculation, CEPA
type ncepa. This is currently available only for single configuration reference functions.
20.3.2 Coupled Pair Functional (ACPF, AQCC)
ACPF,options
AQCC,options
where options can be
GACPFI=gacpfi
GACPFE=gacpfe
Instead of diagonalizing the hamiltonian, perform ACPF calculation or AQCC calculation. Using
the options GACPFI and GAPCPE The internal and external normalization factors gacpfi,
gacpfe may be reset from their default values of 1, 2/nelec and 1, 1-(nelec-2)(nelec-3)/nelec(nelec-
1), respectively.
The ACPF and related methods are currently not robustly working for excited states. Even
though it sometimes works, we do not currently recommend and support these methods for
excited state calculations.