Kilian (Van der Waals) Hyperelastic Model

I am interested in learning more and trying out the Kilian hyperelastic model that is available in ABAQUS. When reading through the ABAQUS theory manual, however, it appears that the strain energy density that is given does not have a minimum in the undeformed state. Does anyone know what strain energy density that the Kilian model is based on? Also, does anyone actually use the Kilian hyperelastic model?

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I try to plot the Kilian strain energy density VS 1st invariant. It shows that the energy reaches its minimum when the undeformed state (I1=3).

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