$DETPT group      (relevant if SCFTYP=MCSCF and MPLEVL=2)
 
    This input group applies to the determinant-based
multi-reference perturbation theory program, if chosen by
MRPT=DETMRPT in $MRMP.
 
    When applied to only one state, the theory is known as
multi-reference Moller-Plesset (MRMP), but the term MCQDPT
is used when this theory is used in its multi-state form.
Please note that this perturbation theory is not the same
thing as the CASPT2 theory, and should -NEVER- be called
that.  A more complete discussion may be found in the
'Further Information' chapter.
 
NVAL   = number of filled valence orbitals in the MCSCF to
         be included in the dynamic correlation treatment.
         This is analogous to NMODOC in the $MCQDPT input.
         The number of frozen cores orbitals is found by
         subtracting NVAL from NCORE in $DET, so that you
         need not specify the chemical core's size.  Also,
         there is no input for specifying the active space,
         which is inherited from $DET.  The default for
         NVAL correlates valence orbitals, but freezes any
         chemical cores.
 
NEXT   = number of external orbitals to use.  The default
         means to use all of them (default=-1).
 
NOS    = a flag to use MCSCF natural orbitals rather than
         canonicalized orbitals as the basis of the PT.
         This changes the numerical results!!!
 
Omitting NPTST, IPTST, and WPTST is the simplest option,
meaning that any state with a non-zero WSTATE in $DET is
included in the pertubation.  Canonicalization of the
orbitals is normally done by the MCSCF program, see CANONC
in $MCSCF.  However, if not, or if the state weights are
changed, the canonicalization is done in the perturbation
code, according to CANON in this group.  The default is the
most computationally efficient.
 
CANON  = flag to request canonicalization.  Default=.TRUE.
         Turning off canonicalization is for experimental
         purposes, so most runs should not avoid it.  The
         canonicalization will be done in the perturbation
         code under three circumstances,
             RDVECS=.TRUE. was used, at the first geometry,
             the MCSCF step skipped canonicalization, or
             you enter NPTST/IPTST/SPTST information.
         Canonicalization uses the state averaged density
         matrix to build the "standard Fock operator", and
         involves diagonalizing its diagonal sub-blocks.
 
NPTST  = the number of states to include in generation of
         the unperturbed CAS states.  If NPTST is chosen,
         spins of the states will be ignored, like using
         PURES=.F. in $DET, so you must be careful in your
         matching IPTST input.
 
IPTST  = an array of CAS-CI states to be included in the
         perturbation theory, give NPTST values.
 
WPTST  = an array of state weights.  Like NPTST/IPTST, the
         default for WPTST is derived from WSTATE in $DET.
 
example: NPTST=3 IPTST(1)=1,3,5 might be used to include
three singlets, S0,S1,S2 in a MCQDPT-type treatment, but
skip over T1 and T2.  You will have done an earlier CI or
MCSCF run, in order to know that you need NPTST five or
higher to capture the lowest three singlets, and that these
singlets appear where they do.  NSTATE in $DET must be at
least 5 in this example, to find enough roots.
 
EDSHFT  is the same as the same keyword in $MCQDPT.  The
        denominators D are changed to D + EDSHFT/D.
        Reasonable values are 0.02 to 1D-4, if you need
        any shift at all.  The default is 0.0.
 
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