How to implement AHO model with CASTEP?

Dear MS community,

My question is particularly addressed to CASTEP users: I'm trying to calculate

the second order susceptibilities to compare with experimental data according to

the anharmonic oscillator model (AHO), Journal of Molecular Structure: THEOCHEM,

Volume 954, Issues 1–3, 30 August 2010, Pages 22-35, and references therein.

How post-treatment of requested Optical Properties look like in the simplest case?

Who has similar experience, please, respond!

Kindly yours,

Artem