CCL: any soft to calc. subst. const. ? -ACD/Labs

Dear all,

I was trying to calculate the Hammett constant of some special groups (no experiment data). while the reviewer told me it could be done by software. what he/she said was 'approach described by the authors looks very similar to what is done by ACD software Sigma module'. I obtain the sigma through it's correlation with charges. Is anyone aware of how the soft does it ? which one is more rational ?
what are the soft commonly known to be calculating sigma? In case of being criticized by the reviewer next time I submit  it, I should be aware of the whole picture. Any hint ?


PS before I calculated the Hammett constants through charge, I have optimized the structure by DFT.