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Sliced contour plots of idealized electron densities of (a) sphere-like, (b) cylinder-like and (c) minimal surface like structures. Only in (a) is there a strictly convex upward density region where all eigenvalues of the Hessian matrix are negative. The strictly convex upward density region forms a closed isoelectron density surface. On the other hand, there is no closed isoelectron density surface in (b) and (c).
