How to solve the Schrödinger equation of 1D hindered rotor?

I have the following equation:


$$-\frac{{{\hbar }^{2}}}{2I{}\_{r}}\frac{{{d}^{2}}\psi }{d{{\theta }^{2}}}+V\left( \theta \right)\psi =E\psi, $$


with 


$$V\left( \theta \right)=a+\sum\limits\_{n=1}^{N}{{{b}\_{n}}\cos \left( n\theta \right)}+\sum\limits\_{m=1}^{M}{{{c}\_{m}}\sin \left( m\theta \right)}$$


where the notations are straightforward ($\theta \in \left[ 0,2\pi \right)$). I would like to get $K$ eigenvalues of the spectrum. How to integrate it (with some derivations)? Is there any non-commercial program which is able to provide them? I used MSTOR by Truhlar's group, but the potential does not contain the sine-terms.


