MOLPRO

39 THE VB PROGRAM CASVB 282
configurations are defined in terms of the active orbitals only, and may be specified using one or
more CON cards (note that the RESTRICT and SELECT keywords are not used in CASVB):
CON,n 1 ,n 2 ,n 3 ,n 4 ,...;
The configurations can be specified by occupation numbers, exactly as in MULTI (see section
19.4.3), so that n i is the occupation of the ith valence bond orbital. Alternatively a list of
Nact orbital numbers (in any order) may be provided – the program determines which definition
applies. The two cards CON,1,0,1,2; and CON,1,3,4,4; are thus equivalent.
If no configurations are specified the single covalent configuration φ 1 φ 2 ···φ Nact is assumed.
39.4.2 Selecting the spin basis
SPINBASIS,key;
key may be chosen from KOTANI (default), RUMER, PROJECT or LTRUMER, specifying the
basis of spin eigenfunctions used in the definition of valence bond structures. PROJECT refers
to spin functions generated using a spin projection operator, LTRUMER to Rumer functions with
the so-called “leading term” phase convention.
39.5 Recovering CASSCF CI vector and VB wavefunction
The appropriate MOLPRO records may be specified explicitly using the START directive (an
alternative is the vbdump mechanism described in section 39.2.1):
START,ci,vb,orb,trnint;
ci: record name for the CASSCF CI vector. The CI vector must have been dumped previously
using either of the SAVE, NATORB, CANONICAL, or LOCALI directives (see section
19.5.4). A default value for ci is determined from the most recent vbdump record(s).
Note that if the ci record is not found, only an energy-based optimization of the VB wavefunction
can be carried out.
vb: record name for the valence bond orbitals and structure coefficients, as saved by a previous
CASVB calculation. If the VB wavefunction was previously saved in the AO basis the orbitals
will be projected onto the present active space (note that it is necessary to specify a record name
for the molecular orbitals (orb below) for this to be possible).
orb: record name for the molecular orbitals defining the CASSCF wavefunction. This information
is necessary if one wants to output the valence bond orbitals in the atomic orbital basis.
trnint: record name for the transformed CASSCF integrals. These are required for the energybased
criteria (i.e., if CRIT,ENERGY is specified), and can be saved inside MULTI by the
TRNINT sub-command (see 19.8.7). The default record name, both here and in MULTI, is
1900.1.
39.6 Saving the VB wavefunction
SAVE,vb,civb,vbao;
vb: record name for VB wavefunction (default is first available record after 3200.2), i.e., orbitals
and structure coefficients.