MOLPRO

39 THE VB PROGRAM CASVB 283
civb: record name for valence bond full CI vector defined in terms of the CASSCF MOs (default
is 3300.2). Saving this vector is necessary for the calculation of further properties, geometry
optimization, etc.
vbao: record name for valence bond wavefunction in the AO basis. Note that specifying orb in
the START directive is a precondition for this keyword. It may be useful for plotting of orbitals,
or for providing a guess to be used in the interpretation of a CASSCF solution employing a
different active space.
It is normally advisable to use records on file 2 for vb, civb, and vbao.
39.7 Specifying a guess
GUESS;key-1,. . . ;key-2,. . . ;. . .
The GUESS keyword initiates the input of a guess for the valence bond orbitals and structure coefficients.
key-i can be either ORB, STRUC or READ. These keywords modify the guess provided
by the program, or specified by the START directive. It is thus possible to modify individual
orbitals in a previous solution to construct the starting guess.
39.7.1 Orbital guess
ORB,i, c 1 , c 2 ,. . . c mact ;
Specifies a starting guess for valence bond orbital number i. The guess is specified in terms
of the mact active MOs defining the CASSCF wavefunction. (Note that the definition of these
MOs will depend on how the CI vector was dumped – i.e. which of the SAVE, NATORB,
CANONICAL, or LOCALI directives was used (see section 19.5.4). Use of one of the three
latter keywords is recommended.)
39.7.2 Guess for structure coefficients
STRUC,c 1 , c 2 ,. . . c NV B ;
Specifies a starting guess for the NV B structure coefficients. If this card is not provided, and no
guess specified by START, the perfect-pairing mode of spin coupling is assumed for the spatial
configuration having the least number of doubly occupied orbitals. Note that the definition of
structures depends on the value of SPINBASIS. Doubly occupied orbitals occur first in all
configurations, and the spin eigenfunctions are based on the singly occupied orbitals being in
ascending order.
39.7.3 Read orbitals or structure coefficients
The READ keyword can take one of the following forms:
READ,ORB,iorb1[,TO,iorb2] [,AS,jorb1[,TO,jorb2]] [,FROM,record];
READ,STRUC,istruc1[,TO,istruc2] [,AS,jstruc1[,TO,jstruc2]] [,FROM,record];
READ,ALL [,FROM,record];
In this way a subset of orbitals and/or structure coefficients may be picked out from a previous
calculation. Renumbering of orbitals or structures can be done using the “AS” construct as
outlined above. If the VB wavefunction was previously saved in the AO basis, the orbitals will