Please find the attached formula and some reference papers to calculate Channel capacity loss (using S-parameter) for MIMO antenna.
This parameter cannot be directly calculated using HFSS/CST. You need to get the values of S-parameter from the software and plot the curve using MATLAB.
It's better to import the S-parameters from HFSS to MATLAB in .mat file and implement the channel, capacity, and other MIMO parameters equations in MATLAB. Put the value from .mat file in MATLAB equation to find desired parameter value w.r.t. frequency.