$FORCE group
 
(optional, relevant for RUNTYP=HESSIAN,OPTIMIZE,SADPOINT)
 
    This group controls the computation of the hessian
matrix (the energy second derivative tensor, also known
as the force constant matrix), and an optional harmonic
vibrational analysis.  This can be a very time consuming
calculation.  However, given the force constant matrix,
the vibrational analysis for an isotopically substituted
molecule is very cheap.  Related input is HESS= in
$STATPT, and the $MASS, $HESS, $GRAD, $DIPDR, $VIB inputs.
Calculation of the hessian automatically yields the dipole
derivative tensor, giving IR frequencies.  Raman
intensities are obtained by following with RUNTYP=RAMAN.
 
METHOD = chooses the computational method:
       = ANALYTIC is a fully analytic calculation.  This
                  is implemented for SCFTYP=RHF, UHF, ROHF,
                  GVB (for NPAIR=0 or 1, only), and
                  MCSCF (for CISTEP=ALDET or ORMAS, only).
                  R-DFT and U-DFT are also analytic.
                  This is the default for these cases.
       = SEMINUM  does numerical differentiation of
                  analytically computed first derivatives.
                  This is the default for UHF, MCSCF using
                  other CISTEPs, all solvent models,
                  relativistic corrections, and most MP2 or
                  CI runs.
       = FULLNUM  numerically differentiates the energy
                  twice, which can be used by all other
                  cases.  It requires many energies (a
                  check run will tell how many) and so
                  it is mainly useful for systems with
                  only very few symmetry unique atoms.
 
The default for METHOD is to pick ANALYTIC over SEMINUM if
that is programmed, and SEMINUM otherwise.  FULLNUM will
never be chosen unless you specifically request it.
 
RDHESS = a flag to read the hessian from a $HESS input,
         rather than computing it.  This variable pertains
         only to RUNTYP=HESSIAN.  See also HESS= in the
         $STATPT input group.  (default is .FALSE.)
 
PURIFY = controls cleanup
         Given a $ZMAT, the hessian and dipole derivative
         tensor can be "purified" by transforming from
         Cartesians to internals and back to Cartesians.
         This effectively zeros the frequencies of the
         translation and rotation "modes", along with
         their IR intensities.  The purified quantities
         are punched out.  Purification does change the
         Hessian slightly, frequencies at a stationary
         point can change by a wave number or so.  The
         change is bigger at non-stationary points.
         (default=.FALSE. if $ZMAT is given)
 
PRTIFC = prints the internal coordinate force constants.
         You MUST have provided $ZMAT input to use this.
         (Default=.FALSE.)
 
HSSTYP selects the integral package used for integral
       derivatives in analytic hessians.  This is
       used only for debugging purposes.
       = BEST  use experimental routines, not fully
               tested.
       = GENERAL use slower safe code (default).
 
  --- the next four apply to numeric differentiation ----
 
NVIB   =    The number of displacements in each Cartesian
            direction for force field computation.  This
            pertains only to METHOD=SEMINUM, as FULLNUM
            always uses double difference formulae.
       = 1  Move one VIBSIZ unit in each positive
            Cartesian direction.  This requires 3N+1
            evaluations of the wavefunction, energy, and
            gradient, where N is the number of SYMMETRY
            UNIQUE atoms given in $DATA.
       = 2  Move one VIBSIZ unit in the positive direction
            and one VIBSIZ unit in the negative direction.
            This requires 6N+1 evaluations of the
            wavefunction and gradient, and gives a small
            improvement in accuracy.  In particular, the
            frequencies will change from NVIB=1 results by
            no more than 10-100 wavenumbers, and usually
            much less.  However, the normal modes will be
            more nearly symmetry adapted, and the residual
            rotational and translational "frequencies"
            will be much closer to zero. (default)
 
VIBSIZ =    Displacement size (in Bohrs). This pertains to
            Both SEMINUM and FULLNUM.  Default=0.01
 
       Let 0 mean the Vib0 geometry, and
       D mean all the displaced geometries
 
NPRT   = 1  Print orbitals at 0 and D
       = 0  Print orbitals at 0 only (default)
 
NPUN   = 2  Punch all orbitals at 0 and D
       = 1  Punch all orbitals at 0 and occupied orbs at D
       = 0  Punch all orbitals at 0 only (default)
 
 
  ----- the rest control normal coordinate analysis ----
 
VIBANL = flag to activate vibrational analysis.
         (the default is .TRUE. for RUNTYP=HESSIAN, and
         otherwise is .FALSE.)
 
SCLFAC = scale factor for vibrational frequencies, used
         in calculating the zero point vibrational energy.
         Some workers correct for the usual overestimate
         in SCF frequencies by a factor 0.89.  ZPE or other
         methods might employ other factors, see
            J.P.Merrick, D.Moran, L.Radom
            J.Phys.Chem.A  111, 11683-11700 (2007).
         The output always prints unscaled frequencies, so
         this value is used only during the thermochemical
         analysis.  (Default is 1.0)
 
TEMP   = an array of up to ten temperatures at which the
         thermochemistry should be printed out.  The
         default is a single temperature, 298.15 K.  To
         use absolute zero, input 0.001 degrees.
 
FREQ   = an array of vibrational frequencies.  If the
         frequencies are given here, the hessian matrix
         is not computed or read.  You enter any imaginary
         frequencies as negative numbers, omit the
         zero frequencies corresponding to translation
         and rotation, and enter all true vibrational
         frequencies.  Thermodynamic properties will be
         printed, nothing else is done by the run.
 
PRTSCN = flag to print contribution of each vibrational
         mode to the entropy.  (Default is .FALSE.)
 
DECOMP = activates internal coordinate analysis.
         Vibrational frequencies will be decomposed into
         "intrinsic frequencies", by the method of
         J.A.Boatz and M.S.Gordon, J.Phys.Chem., 93,
         1819-1826(1989).  If set .TRUE., the $ZMAT input
         may define more than 3N-6 (3N-5) coordinates.
         (default=.FALSE.)
 
PROJCT = controls the projection of the hessian matrix.
         The projection technique is described by
         W.H.Miller, N.C.Handy, J.E.Adams in J. Chem.
         Phys. 1980, 72, 99-112.  At stationary points,
         the projection simply eliminates rotational and
         translational contaminants.  At points with
         non-zero gradients, the projection also ensures
         that one of the vibrational modes will point
         along the gradient, so that there are a total of
         7 zero frequencies.  The other 3N-7 modes are
         constrained to be orthogonal to the gradient.
         Because the projection has such a large effect on
         the hessian, the hessian is punched both before
         and after projection.  For the same reason, the
         default is .FALSE. to skip the projection, which
         is mainly of interest in dynamical calculations.
 
==========================================================
 
 
There is a program ISOEFF for the calculation of kinetic
and equilibrium isotope effects from the group of Piotr
Paneth at the Technical University of Lodz.  This program
will accepts data computed by GAMESS (and other programs),
and can be requested from paneth@p.lodz.pl
 
 
 
 
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