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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 groups.
METHOD = chooses the computational method.
= ANALYTIC is implemented only for SCFTYPs RHF,
ROHF, and GVB (when NPAIR is 0 or 1),
if the calculation does not use ECPs.
This is the default for these cases.
= NUMERIC is the default for all other cases.
IR intensities are available only for NUMERIC runs at present.
RDHESS = a flag to read the hessian from a $HESS group,
rather than computing it. This variable pertains
only to RUNTYP=HESSIAN. See also HESS= in the
$STATPT 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 defined a $ZMAT group to use this.
(Default=.FALSE.)
--- the next four apply only to METHOD=NUMERIC ----
NVIB = Number of displacements in each Cartesian
direction for force field computation.
= 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. (default)
= 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.
VIBSIZ = Displacement size (in Bohrs). 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. The output
always prints unscaled frequencies, this value
is used only in 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.
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 group
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 punched is the one
BEFORE projection. For the same reason, the
default is .FALSE. to skip the projection, which
is mainly of interest in dynamical calculations.