21.3.6 Specifying correlation of orbital pairs

PAIR,iorb1.isy1,iorb2.isy2,np;

is a request to correlate a given orbital pair.

np=1:

singlet pair

np=-1:

triplet pair

np=0:

singlet and triplet pair (if possible)

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.