$DFTB group                  (relevant for GBASIS=DFTB)
 
Density-functional tight-binding (DFTB) is turned on by
selecting GBASIS=DFTB in $BASIS.  $DFTB controls optional
parameters for a DFTB calculation.  DFTB is formulated in a
two-center approximation utilizing implicitly a minimal
pseudoatomic orbital basis set with corresponding,
pretabulated one- and two-center integrals.   Because of
this, many properties (for instances, multipoles higher
than dipoles) and many options are ignored or not available
in the current implementations of DFTB.  DFTB also uses an
independent SCF driver (SCF in DFTB is also called SCC, see
below), so most SCF options are not available for DFTB.
 
Only SCFTYP=RHF and UHF are implemented. SCFTYP=ROHF is
available, only when all SPNCST values are zero. DFTB does
not explicitly use symmetry (C1 throughout) since integrals
are never computed during the calculations.  Slater-Koster
tables are only defined for spherical functions (5d) so
DFTB sets ISPHER=1.  Most $GUESS options do not work for
DFTB (DFTB does not use initial orbitals in the usual
sense).  Other than the default (METHOD=HUCKEL, which is
ignored), only METHOD=MOREAD works (note that SCC-DFTB can
use initial charges on atoms, derived from the orbitals).
 
RUNTYP=OPTIMIZE, HESSIAN and RAMAN are available for full
(non-FMO) DFTB and FMO-DFTB. Excited state calculations for
full DFTB may be performed through the standard (linear-
response) time-dependent formalism (only closed shell). PCM
can be used for both ground and excited state calculations,
and energy and gradient can be evaluated.
 
In DFTB calculation, the atom type is determined by its
name, not its nuclear charge as elsewhere in GAMESS. The
nuclear charge (the second column in $DATA) is used only in
population analysis, but not in SCF.  DFTB uses a notion of
"species", which means an atomic type.  The species are
numbered according to the order in which atoms appear in
$DATA. For instances, in water there are two species, O and
H.  An atomic type of each species needs MAXANG, which for
most but not all atoms is set automatically.
 
 
NDFTB  order of the Taylor expansion of the total energy
       around a reference density in the DFTB model.
       = 1 NCC-DFTB, also called DFTB1.
           NCC stands for non-charge-consistent, i.e., no
           explicity charge-charge interaction term is
           included in the energy calculation.
       = 2 SCC-DFTB, also called DFTB2.
           SCC means a self-charge-consistent approach,
           and SCC implies that SCF iterations are carried
           out that converge monopolar charges towards
           self-consistency.
       = 3 DFTB3, including 3rd order correction using
           Hubbard derivatives (HUBDER).
           In order to reproduce the published DFTB3
           approach, it is necessary to also specify
           DAMPXH=.TRUE. to add other terms.
           Gaus, M. et al. J. Chem. Theory Comput. 2011,
           7, 931-948 is referred to as Gaus2011 below.
           Default: 2.
 
LCDFTB =  a flag to turn on long range corrected DFTB, as
          derived from LC-DFT. At present, only DFTB2 can be
          combined with this option. See EMU below.
          Note that LC-DFTB requires parameters optimized
          for it.
          Default: .FALSE.
 
EMU    =  Parameter for LC-DFTB. To use LC-DFTB properly,
          some non-zero value (for example, 0.3) should be
          assigned to EMU.
          Default: 0.
 
DAMPXH =  a flag to include the damping function for X-H
          atomic pair in DFTB3. See also DAMPEX, and eq 21
          in Gaus2011.
          The damping function is used when at least one
          atom in a pair is "H". "HYDROGEN" and any other
          name will turn off the damping.
          Default: .FALSE.
 
DAMPEX =  an exponent used in the damping function for X-H
          atomic pairs.  The default value is 4.0 (taken
          from the 3OB parameter set).
 
SRSCC  =  a flag to perform shell-resolved SCC calculation.
          If set to .FALSE., the code uses the Hubbard
          value for an s orbital for p and d orbitals,
          ignoring their Hubbard values defined in Slater-
          Koster tables.
          Using .TRUE. enables the use of proper Hubbard
          values for p and d orbitals, implemented only
          for DFTB1 and DFTB2.
          Default: .FALSE.
 
ITYPMX    Convergence method of SCC calculations.
       = -1 Use standard GAMESS convergence methods.
            SOSCF and DIIS are supported, but DEM is not.
       = 0  Broyden's method.
            Interpolation is applied for atomic
            (or shell-resolved when SRSCC=.TRUE.)
            charges, but not Hamiltonian matrix.
       = 1  (reserved)
       = 2  DIIS for charges.
            Default: 0.
 
ETEMP  = electronic temperature in Kelvin. Non-zero values
         of ETEMP help SCF convergence of nearly-degenerate
         systems by smearing occupation numbers around the
         Fermi-level. Only the Fermi-Dirac distribution
         function is available as a smearing function.  The
         default value is 0 Kelvin, meaning the smearing
         function is not used.
         ETEMP is implemented only for SCFTYP=RHF and when
         FMO is not used.
 
OCCTHR = threshold for identifying doubly occupied, partially
         occupied or unoccupied molecular orbitals with ETEMP.
         If an occupation number n is OCCTHR