This keyword specifies whether to calculate the non-analytical contribution to the dynamical matrix from long-range electric field effects, which are responsible for the LO/TO splitting of phonon frequencies at the gamma point.
When this keyword is TRUE, the calculation of the dielectric permittivity is carried out via a linear response scheme for an applied electric field. The Born effective charges are also evaluated.
You should set this keyword to FALSE for molecule in a box systems, where there is no physical LO-TO splitting of phonon frequencies.
The value of the LO-TO splitting depends on the approach direction to gamma. It is possible to specify multiple directions using the PHONON_GAMMA_DIRECTIONS block.
PHONON_CALC_LO_TO_SPLITTING : FALSE
CASTEP keyword glossary
CASTEP parameters keywords