def get_func_deriv(k):
"""
Get the energy value and its derivative for given k value
"""
f = np.pi
spr = Spring(k)
u = spr.solve(0, f)
E = spr.F(u)
dRdk = spr.dRdk(u)
dFdk = spr.dFdk(u)
dRdq = spr.dRdq(u)
dFdq = spr.dFdq(u)
lam = spr.adjoint_solve(u)
DFDx = dFdk + lam * dRdk
return E, DFDx
