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:

`hf`- Closed shell Hartree-Fock (RHF)
`mcscf`- 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

molpro@molpro.net 2018-09-21