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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.

Examples

analytic calculation of harmonic vibrational frequencies
numerical calculation of harmonic vibrational frequencies

Harmonic vibrational frequencies for non-standard isotopomers

Recommendations for the calculation of harmonic frequencies