You are not saying many about your problem, here general solution. I hate to use 'real' and 'ideal' because any model is always **ideal**, but i will use standard notation

C_{V, real} = C_{V, ideal} + A

A = I_{infinite}^{V} T (d^{2}_{T}p) dV

"I" is integral and (d^{2}_{T}) denotes second derivative of p respect to T.

Once known C_{V. real} can compute C_{P. real}. Do you need or either equation of state of your model (there are thousand of equations available in engineering literature, Geodome can explain this better than me) or an empirical fit with you system.