Fix bad bug in potential_energy_fn

Author: mstoelzle

pyproject.toml

@@ -17,7 +17,7 @@ name = "jsrm"  # Required
 #
 # For a discussion on single-sourcing the version, see
 # https://packaging.python.org/guides/single-sourcing-package-version/
-version = "0.0.13"  # Required
+version = "0.0.14"  # Required
 
 # This is a one-line description or tagline of what your project does. This
 # corresponds to the "Summary" metadata field:

src/jsrm/systems/planar_pcs.py

@@ -417,7 +417,7 @@ def potential_energy_fn(
         # compute the stiffness matrix
         K = stiffness_fn(params, B_xi, formulate_in_strain_space=True)
         # elastic energy
-        U_K = (xi - xi_eq).T @ K @ (xi - xi_eq)  # evaluate K(xi) = K @ xi
+        U_K = 0.5 * (xi - xi_eq).T @ K @ (xi - xi_eq)  # evaluate K(xi) = K @ xi
 
         # gravitational potential energy
         params_for_lambdify = select_params_for_lambdify_fn(params)