When numerical CSPs are used to solve systems of n equations with n variables, the interval Newton operator plays a key role: It acts like a global constraint, hence achieving a powerful contraction, and proves rigorously the existence of solutions. However, both advantages cannot be used for under-constrained systems of equations, which have manifolds of solutions. A new framework is proposed in this paper to extend the advantages of the interval Newton to under-constrained systems of equations. This is done simply by permitting domains of CSPs to be parallelepipeds instead of the usual boxes.