This data block contains electric field vector in Cartesian coordinates.
The data block has the following format:
%BLOCK EXTERNAL_EFIELD [units] Ex Ey Ez %ENDBLOCK EXTERNAL_EFIELD
The first line is optional and contains electric field units. The default unit is eV/Å/electron (eV/Ang/e in CASTEP notation). The next line contains three real numbers representing electric field vector components in Cartesian coordinates.
The external electric field is applied in a sawtooth formalism. Electric field discontinuity is on the cell boundary, so the only sensible systems to study using this approach are molecules or slabs (the field must be along the surface normal in the latter case). In both cases atoms should be positioned in the middle of the cell.
%BLOCK EXTERNAL_EFIELD HARTREE/BOHR/E 0.0 0.0 0.1 %ENDBLOCK EXTERNAL_EFIELD
CASTEP keyword glossary
CASTEP cell keywords and data blocks