Resonance frequency of a piezoelectric cube

Hello,

I am trying to find the resonance frequency of a piezoelectric cube, using the Piezoelectric Devices physics of COMSOL 4.0.

The edge of the cube is 500e-9m long, and its material is Zync Oxide.

I mean to do so by two studies:

1- Frequency Domain study.

I put a voltage of 40mV on a face of the cube, the opposed face being the ground. This potential induces a deformation in the cube.
I do the study from 1e9 to 10e9 Hz with a 10e6Hz step.

I don't really know how to see the resonance... So I guess the best solution must be to plot the displacement of a corner depending on the frequency ( disp=f(freq) )?
I see something looking like a resonance frequency around 3.4GHz, but I see several other maximums at higher frequencies (8.1GHz, 8.3GHz....) , that I can't explain.

Could you tell me if it can be because of some wrong or missing parameters? or else they would be normal..

2- Eigenfrequency

I try to find eigenfrequencies around the 3.4GHz seen before (6 values by def.) , but the software seems to give me random values around this 3.4GHz frequency.
Even stranger, I will get eigenfrequencies for any eigen value I put (like 5 or 6 GHz).

Finally, sometimes I get unexplainable Complex eigenfrequencies... Do you know why?

Thanks for your time,
Regards

Michel