Spline model for strain energy

variational formulation
15

In the context of the post Incompressible Humphrey hyperelastic material in Fencis the author defines strain energy density as -

psi = inner(dot(dgF,pkstrs), grad(_u))

In Hyperelasticity — DOLFIN documentation the author defines the strain energy density as -

# Stored strain energy density (compressible neo-Hookean model)
psi = (mu/2)*(Ic - 3) - mu*ln(J) + (lmbda/2)*(ln(J))**2

So, I think it depends on the user which model they want to choose.