Sorry to clog your inbox. In my previous email I forgot to tell that I used interpolator_on_mesh_fem to get the gradient and hessian. Basically I would like the get the first derivative and second derivative of the displacement field at some points. I understand that the gradient is definitely discontinuous across elements so we have to use a discontinuous fem as the targeted fem if the compute_gradient() is used. But I am not sure about how the gradient and hessian is calculated when calling interpolator_on_mesh_fem.eval(...) and .eval_hess(...). As I said, if the linear element is used, are those results still correct? Thanks.
Regards,
Wen