The ground-state energy and external potential as functionals of the electron density and their derivatives

It is shown that the ground-state energy as a functional solely of the electron density is determined by the asymptotic value of the derivative of the degree-one homogeneous extension of the universal density functional F[n] at the given electron number. This has the consequence that its derivative cannot be properly determined. Carrying out the derivative of E[n[N,v]] with respect to v(r) leads to a paradox, which is resolved by the non-differentiability of E[n] if one follows traditional wisdom regarding the non-invertibility of the linear response function. However, considering the derivative of v[n[v]] through the one-electron case shows that this paradox has a more elementary origin, namely, an unaccounted restriction of the v(r) domain.

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