fixed ARAP

Author: miguelmaso

src/PhysicalModels/MechanicalModels.jl

@@ -940,7 +940,7 @@ struct ARAP2D_regularized <: IsoElastic
     J1 = 0.5 * (1.0 + sqrt(1.0 + δ^2))
     ∂J1 = 0.5 * (1.0 + 1.0 / sqrt(1.0^2 + δ^2))
     β = μ * (J1^(-1) - J1^(-2) * ∂J1)
-    Ψ(F) = μ * 0.5 * J(F)^(-1) * (tr((F)' * F)) - β * log(J_(F))
+    Ψ(F) = μ * 0.5 * J(F)^(-1) * (tr((F)' * F)) - β * log(J_(F)) -μ*J(I2)^-1
 
     ∂Ψ1_∂J(F) = -μ / 2 * (tr((F)' * F)) * J(F)^(-2)
     ∂Ψ2_∂J(F) = -β / J_(F)
@@ -972,7 +972,7 @@ struct ARAP2D <: IsoElastic
     μ = obj.μ
     J(F) = det(F)
     H(F) = det(F) * inv(F)'
-    Ψ(F) = μ * 0.5 * J(F)^(-1) * (tr((F)' * F))
+    Ψ(F) = μ * 0.5 * J(F)^(-1) * (tr((F)' * F)) -μ
     ∂Ψ_∂F(F) = μ * F * J(F)^(-1)
     ∂Ψ_∂J(F) = -μ / 2 * (tr((F)' * F)) * J(F)^(-2)