Note that least-squares solutions to equation 13 will generally have non-zero residual at all points except the final converged geometry. The Hessian-matrix products from equation 12 are stored in the matrix Yi = ȞL Ši . Additionally, to update the approximate potential energy Hessian matrix B after the diagonalization algorithm has converged, the potential energy Hessian-vector products ȞŠ i are stored separately in the matrix YE . The iterative diagonalization procedure is considered converged once the space spanned by the columns of Ši is found to contain the leftmost eigenvector of ȞL .