Calculation Of Harmonic Frequencies And IR Intensities
The calculation of harmonic vibrational frequencies and infrared intensities are requested via the keyword VIB.
For quantum chemical methods, for which analytic second derivatives are available,
VIB=ANALYTIC (in the current release not available, use VIB=EXACT)
requests a harmonic force-field based on analytic second derivatives, while for methods for which analytic gradients are available
VIB=FINDIF (Please do not use VIB=2!)
requests a force-field calculation based on numerical differentiation of analytically evaluated forces.
Finte-difference calculations are controlled by the keywords FD_CALCTYPE, FD_STEPSIZE, FD_PROJECT, and FD_IRREPS.