I've simulated a simple rectangular waveguide (TE10 mode) in HFSS with a frequency sweep. I want to know the exact cutoff frequency of the waveguide (I know that it is analytically c/2a). Where can I find that value exactly calculated by HFSS? You can approximate the value by watchin the gamma constant in frequency domain, but I need the exact value.
The amount of attenuation per length that a cuttoff waveguide has is given by an equation for the propagation constant's real part. You have to curve fit the data to that equation. Since attenuation near cuttoff is gradually increasing with decreasing frequency, you will not have an all or nothing type of situation for attenuation vs frequency.
HFSS gives prop constant. From imaginary part of prop constant and the simulated frequency, the exact cutoff can be calculated. The formula to do so is given in many em books like Cheng's Field and wave electromagnetics