Gradient-dependent approximations to the functional of the kinetic energy of non-interacting electrons (Ts[ρ]), which reflect various properties of the exact functional, are considered. For specially constructed pairs of electron densities, for which the analytic expression for the differences of Ts[ρ] is known, it is shown that the accuracy of the quantities derivable from a given approximation to Ts[ρ]: energy differences and their functional derivatives, does not reflect that of Ts[ρ] itself. The comparisons between the exact values of the kinetic energy in such cases are proposed as an independent condition/criterion for appraisal of approximations to Ts[ρ].

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Last update Tuesday March 26 2024