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We discuss the relation of the specific heat, the energy density and thethermodynamic Casimir effect in the case of thin films in the three dimensionalXY universality class. The finite size scaling function $\theta(x)$ of thethermodynamic Casimir force can be expressed in terms of the scaling functionsh'(x) and h(x) of the excess energy density and the excess free energy density.A priori these quantities depend on the reduced temperature t and the thicknessL_0 of the film. However finite size scaling theory predicts that the scalingfunctions depend only on the combination x=t [L_0/\xi_0]^{1/\nu}, where \nu isthe critical exponent and $\xi_0$ the amplitude of the correlation length. Weexploit this fact to compute \theta from Monte Carlo data for the excess energydensity of the improved two-component \phi^4 model on the simple cubic latticewith free boundary conditions in the short direction. We repeat this exerciseusing experimental data for the excess specific heat of 4He films. The finitesize scaling behaviour of the excess specific heat is governed by h''(x), whichis proportional to the scaling function $f_2$ discussed in the literature. Wecompare our results with previous work, where the Casimir force has beencomputed by taking the derivative of the excess free energy with respect to thethickness of the film. As a preparative study we have also computed the scalingfunctions h'(x) and h(x) for finite L^3 systems with periodic boundaryconditions in all directions, where L is the linear extension of the system.

The specific heat, the energy density and the thermodynamic Casimir force in the neighbourhood of the lambda-transition