Simulating varicap diode in LTSpice
Maybe you can solve your problem by using variable capacitance models (intially found on the LTSpice Yahoo group I think ).

The equation governing the charge and voltage of a capacitor is
\begin{equation}
Q = CV
\end{equation}
In LTSpice, you cannot change the capacitance value. Rather you specify the charge of the capacitor. Hence, you specify your capacitor value as:
\begin{equation}
q = V{(\text{your capacitance node})} \cdot x
\end{equation}
where x is the voltage over the capacitor in question. This is covered in the LTSpice Help file under C. Capacitor