(15) where we use a four point finite difference stencil of ξ to reduce the requirements for accurate forces from the SCF cycle. Two point-stencils may be used, while paying special attention to the convergence of the atom centered forces (see SI). Superscripts of F denote the order of the applied field, such that F j+2 are the forces at ξ + 2∆ξ, F j+1 are the forces at ξ + ∆ξ and so on.