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12 Frequency calculations

Harmonic vibrational frequencies can be computed automatically using the frequency directive. Normally, this should be preceded by a geometry optimization. A typical input reads as follows.

hf                     !Perform HF calculation
mp2                    !Perform MP2 calculation
optg                   !Optimize geometry for MP2
frequencies,sym=auto   !Perform frequency calculation for MP2; use symmetry in hessian calculation

The second energy derivatives are computed by finite differences using analytical energy gradients when available (see section 11), otherwise from energies (which may take long time and is less accurate!). Note that during frequency calculations the symmetry of the molecule may be lowered, and then the calculation may fail. It is therefore advisable to use the nosym directive as first line in the geometry input.

In the following cases the second derivatives can be computed analytically:

Closed shell Hartree-Fock (RHF)
Single state MCSCF and CASSCF without symmetry (i.e. using the nosym directive in the geometry input).

In these cases the input is

frequencies,analytical      !Perform frequency calculation 2017-12-11