$CONICL group               (relevant if RUNTYP=CONICAL)
 
   This group governs a search for the lowest energy on the
3N-7 dimensional "seam" of intersection of two electronic
potential energy surfaces of the same spin and space
symmetry.  Such Conical Intersections (CI) are important in
photochemistry, where they serve as "funnels" for the
transfer from an excited state to a lower state.  See
RUNTYP=MEX and the $MEX input for the simpler case where
the two surfaces differ by either space or spin symmetry.
 
   Three search procedures are given, one of which requires
the non-adiabatic coupling matrix element (NACME), and two
others which do not require NACME information.  The conical
intersection search is available only for MCSCF (for which
NACME are available) or for TD-DFT potential surfaces
(where NACME are not available).  The TD-DFT must be used
in the Tamm/Dancoff approximation (see TAMMD in $TDDFT),
but can be either conventional or spin-flip.
 
   The search utilizes some of the options of $STATPT, but
note that the Schlegel stepper and HESS=CALC are not
permitted.  It may be reasonable to try the RFO stepper
sometimes.  The search can only be run in Cartesian
coordinates.  Restarts are possible only by updating the
coordinates in $DATA.
 
   At present, the only solvation model that is supported
is conventional TD-DFT with EFP1.
 
OPTTYP = search procedure choice, see references below!
       = GPWNAC  Gradient Projection with NACME, so this
                 is only available for MCSCF.
       = BPUPD   branching plane updating method (default)
       = PENALTY penalty-constrained optimization method
 
Note that for MCSCF surfaces, if state-averaging is used,
the program executes the code needed to produce NACME
vectors, to producing the state-averaged gradients.  There
is essentially no extra time required to produce also the
NACME, hence the GPWNAC stepper might as well be used.
 
IXROOT = array of two states whose CI point is sought.
         For example, this might be IXROOT(1)=2,3
         The roots are counted exactly the same as IROOT in
         the $DET or $TDDFT input groups.  For the latter
         case, set IXROOT to 0 if you want the ground state
         to be one of the two surfaces searched on.
         There is no default for IXROOT!
 
SYMOFF = flag to switch off point group symmetry,
         the default is .TRUE.
 
DEBUG  = flag to print debugging info, default is .FALSE.
 
The following are meaningful only for OPTTYP=PENALTY:
 
TOLSTP = energy difference tolerance
         default=1d-6 Hartree
 
TOLGRD = gradient convergence tolerance
         default=5d-3 Hartree/Bohr
 
ALPHA  = parameter ensuring a singularity free penalty,
         default=0.02 Hartree
 
SIGMA  = Lagrange multiplier for the penalty term.  In
         case the energy gap between the states is not
         acceptable at the CI point, increase the value.
         default = 3.5 (unitless)
 
An understanding of the search procedures can be gained by
reading the following papers:
 
  Gradient Projection with NACME:
    M.J.Bearpark, M.A.Robb, H.B.Schlegel
      Chem.Phys.Lett. 223, 269(1994)
  Branching Plane Updating method:
    S.Maeda, K.Ohno, K.Morokuma
      J.Chem.Theor Comput. 6, 1538(2010)
  Penalty constrained update method:
    B.G.Levine, C.Ko, J.Quenneville, T.J.Martinez
      Mol.Phys. 104, 1039(2006)
    B.G.Levine, J.D.Coe, T.J.Martinez
      J.Phys.Chem.B 112, 405(2008)
 
A comparative study of the first two procedures is
  T.W.Keal, A.Koslowski, W.Thiel
  Theoret.Chem.Acc. 118, 837(2007)