How to represent a losses in lumped capacitor in HFSS

I am trying to simulate capacitors with certain Q values as lumped elements!
i am trying to give equivalent capacitor values as C_eq = C *(1-j/Q) but i am having difficulty in defining j as local variable. can some one help me in resolving it.
Thanks in advance

add a resistor in paralle to represent the losses.

I did that..but my losses are freq dependent...