$FORCE

$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 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.

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. 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 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.


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