This keyword provides an estimate of the average phonon frequency at the gamma point. It is used to initialize the Hessian for BFGS geometry optimization with ionic relaxation. One expects to achieve faster convergence of the geometry optimization if the value is realistic. The idea is that this single number represents the whole of the Hessian in a compact form.
The value of the estimated frequency is provided in the CASTEP output after the geometry optimization run, and this value can be used as an input parameter for a new calculation on a similar system. An example would be a study of different configurations of a self-interstitial defect in silicon.
The value of the estimated frequency as returned by CASTEP should not be interpreted as an actual frequency of any real vibrational mode.
GEOM_FREQUENCY_EST : 17.54 THz
CASTEP keyword glossary
CASTEP parameters keywords