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I am calculating the magnetic susceptibility, essentially a second derivative evaluated at zero magnetic field. The problem is that sometimes Mathematica works perfectly, and sometimes it gives me a "1/0 complexinfinity" error message, with no obvious reason. The problem can arise simply by changing the value of a parameter, but once I return to the parameter values for which it worked fine, Mathematica no longer gives me back the results (it still gives me the same error message). I suspect that it might be an internal problem, i.e. perhaps I need to clear my (user-defined) functions some way.

Chi[Δ_, λ_, θ_, β_] = -D[F, {h, 2}] /. {h -> 0}; (* This works fine *)

Chi[0.4, 0.6, 0.2, β_] (* The problem is here; in particular, when I change the value of θ. Last time the problem occurred because I gave θ a value outside the function (so that I could use in other functions too);

Any help will be greatly appreciated!