$CASCI group         (relevant to SCFTYP=RHF MPLEVL=2)
 
   This group carries out the Improved Virtual Orbital -
Complete Active Space CI method of Freed, Chaudhuri, and
co-workers.  IVO-CASCI starts with a RHF reference, and
then generates IVOs, which are used in a CI computation
within an active space chosen by the user.  The input
consists of this group, a $MCQDPT, and perhaps a $IVOORB
input, along with SCFTYP=RHF and MPLEVL=2.  MULT in $CONTRL
applies to the SCF reference, while MULT in $MCQDPT selects
the spin of the IVO-CASCI state(s).  Doublets are treated
by using a cation RHF reference.
 
IVOCAS = a flag to turn on IVO-CASCI computation.  This is
         usually the only input required (default=.FALSE.)
 
MOLIST = a flag to request complete control over the
         active space specification.  The default uses the
         parameters in $MCQDPT to select from the IVOs
         with the lowest energy.  (default=.FALSE.)
 
DEGENR = a flag to indicate the HOMO is degenerate.
         The program should set this for you.
 
PRINT  = a flag to print debugging info (default=.FALSE.)
 
The user should request IFORB=0 in $MCQDPT to suppress its
generation of canonical orbitals, so that the IVOs are
used.  A Huckel guess is usually fine.  The $MCQDPT should
define the active orbitals taken from the IVO set by giving
NMOFZC, NMODOC, and NMOACT, and the electronic state is
specified by that group's MULT, NSTATE, and NSTSYM.
 
References:
 
D.M.Potts, C.M.Taylor, R.K.Chaudhuri, K.F.Freed
  J.Chem.Phys.  114, 2592-2600(2001)
R.K.Chaudhuri, K.F.Freed, S.A.Abrash, D.M.Potts
  J.Mol.Spectrosc. 547, 83-96(2001)
R.K.Chaudhuri, K.F.Freed
  J.Chem.Phys.  126, 114103/1-6(2007)
 
A simple example follows,
 
 $contrl scftyp=rhf mplevl=2 runtyp=energy ispher=1 $end
 $casci  IVOCAS=.true. $end
 $mcqdpt mult=3 stsym=b1 nstate=1 iforb=0
         nel=8 nmofzc=1 nmodoc=2 nmoact=2 $end
 $basis  gbasis=ccd $end
 $guess  guess=huckel $end
 $data
Methylene...3-B-1 state...RHF/cc-pVDZ
Cnv  2
 
C   6.0   0.0  .0000000000  .0289123030
H   1.0   0.0  .9813851814  .4758735367
 $end
 
The result for the 1st order energy will be -38.9156231594,
which is a full CI within a two orbital space, generated by
the IVO process, rather than a more expensive MCSCF run.