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:
In these cases the input is
frequencies,analytical !Perform frequency calculation