For probability distribution i am getting this error.

I am diagonalizing Hamiltonian which is square matrix and i am getting eigen values like Ek =diag(D);

After then i have defined S = Ek+1- Ek and accordingly i have calculated X =S/mean(S), so now i am trying to calculate P(x) =(pi*x/2)*exp(-pi*x.^2/4), but in return i am getting error that attempted to access (0); index must be a positive integer.

Please let me know where i am doing mistake.