MOLPRO

20 THE CI PROGRAM 150
and double excitations. If not given, the information defaults to that from the most recent SCF,
MCSCF or CI calculation. For calculations with closed-shell reference function (closed=occ),
see CISD, QCI, and CCSD.
20.2.4 Defining the orbitals
ORBIT,name.file,[specifications];
name.file specifies the record from which orbitals are read. Optionally, various specifications
can be given to select specific orbitals if name.file contains more than one orbital set. For details
see section 4.11. Note that the IGNORE ERROR option can be used to force MPn or triples
calculations with non-canonical orbitals.
The default is the set of orbitals from the last SCF, MCSCF or CI calculation.
20.2.5 Defining the state symmetry
The number of electrons and the total symmetry of the wavefunction are specified on the WF
card:
WF,elec,sym,spin
where
elec:
sym:
spin:
is the number of electrons
is the number of the irreducible representation
defines the spin symmetry, spin= 2S (singlet=0, doublet=1, triplet=2,
etc.)
The WF card must be placed after any cards defining the orbital spaces OCC, CORE, CLOSED.
The REF card can be used to define further reference symmetries used for generating the configuration
space, see REF.
20.2.6 Additional reference symmetries
REF,sym;
This card, which must come after the WF directive, defines an additional reference symmetry
used for generating the uncontracted internal and singly external configuration spaces. This is
sometimes useful in order to obtain the same configuration spaces when different point group
symmetries are used. For instance, if a calculation is done in C s symmetry, it may happen that
the two components of a Π state, one of which appears in A ′ and the other in A ′′ , come out not
exactly degenerate. This problem can be avoided as in the following example:
for a doublet A ′ state:
WF,15,1,1; !define wavefunction symmetry (1)
REF,2; !define additional reference symmetry (2)
and for the doublet A” state: