spectre captab: 1) negative capacitance 2) not reciprocal values

HI everyone,

I wanted to get some node-to-node capacitance values of a transistor with the help of spectre's captab option. Unfortunately I faced two strange issues:

1) In some cases I got negative capacitance values for the drain-source capacitance of a transistor.

2) C_AB differ from C_BA where B and A are two arbitrary nodes. In some cases the error is more than 10x.

The results are unsettling to me. Either I set up something wrong, or the transistor models behave very strange.The formula for calculating the capacitances is written in the Spectre's User Guide. What I do not know, how is it determined? Do Spectre run AC analysis on the circuit to determine the capacitances? How error-proof the process is? Is it easy to miss something?

Thanks in advance!
Zoltan