how to apply transverse electric field in castep 4.4

dear all,

how can we apply transverse electric fielf  for determining electronic and optical properties of nanotubes?

electronic properties under transverse electric field  are determined by in the paper

Nanotechnology

15

Chun-Wei Chen

Ming-Hsien Lee

and S J Clark

Band gap modification of single-walled

carbon nanotube and boron nitride

nanotube under a transverse electric field

(2004) 1837–1843 PII: S0957-4484(04)84638-5