The Raman tensor calculated by CASTEP

Hi,

I have performed the Raman spectra calculation by using the module CASTEP on some small molecules which have been put into a 15A box. I found that the raman activity given in the result file (.castep) can not be calculated directly from the corresponding Raman tensor, but must be multiplied by a factor of 3.57. I want to know where does this factor comes from? Is it a bug?

Thank you.