[CST or HFSS] Passive Inter Modulation PIM calculations?

Hello Members

I designed a GSM antenna in CST MWS. I would like to know about the Passive inter Modulation (PIM) of the antenna.
Does someone have any idea on how to calculate it using simulators?

http://electronicdesign.com/wireless...-you-need-know

Awaiting your reply,

Regards