Discrete hellmann-feynman theorem ?? The Helmann-Feynman theorem states that : derivative of eigenvalue with respect to a parameter = eigenfunction dagger * derivative of operator with respect to parameter * eigenfunction (assuming that the eigenfuncitons are normalized, otherwise the theorem would also include a normalization integration). Does anyone know if this works for the DISCRETE CASE ?? ie: derivative of eigenvalue with respect to parameter = eigenVECTOR dagger * derivative of MATRIX with respect to parameter * eigenVECTOR ?? I can't find a proof of it anywhere!