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To obtain harmonic vibrational frequencies and thermodynamic properties,
make sure you have the correct equilibrium geometry for the given level of
theory and that the level of theory has been specified. Then simply
select QChem Vibrations under the Compute menu.
Q-Chem will run in a separate
window and will perform a second derivative calculation (either an analytic
second derivative for Hartree-Fock, or a numerical second derivative by
differentiating first derivatives for DFT and MP2). This will provide
all the information needed for harmonic vibrational frequencies and
thermodynamic properties, which will be printed out towards the end of the
output. As a check, for an analytic second derivative computation,
your total energy should be exactly the same as at the end of the optimization.
For a numerical second derivative calculation, a series of gradients will
be evaluated, and the energy at each point should be within 0.001 hartree
of the equilibrium total energy found at the end of optimization. If not,
you have the wrong geometry or the program is finding the wrong solution.
C. David Sherrill