· It is
important to remember also that calculation of the vibrational frequencies can
only be done meaningfully at a stationary point - i.e. the geometry must be
optimized first.
· A frequency calculation
on a structure produced by a normal mode analysis can tell us if the structure
really is a minimum.
· At a minimum, all
the 3N-6 eigenvalues are positive, so
all the calculated frequencies will be positive. For the calculation on water, all the frequencies are positive,
so it really is a minimum energy geometry.