Editing Functional derivative (section)

====Thomas–Fermi kinetic energy functional====
The [[Thomas–Fermi model]] of 1927 used a kinetic energy functional for a noninteracting uniform [[free electron model|electron gas]] in a first attempt of [[density-functional theory]] of electronic structure:
<math display="block">T_\mathrm{TF}[\rho] = C_\mathrm{F} \int \rho^{5/3}(\mathbf{r}) \, d\mathbf{r} \, .</math>
Since the integrand of {{math|''T''<sub>TF</sub>[''ρ'']}} does not involve derivatives of {{math|''ρ''('''''r''''')}}, the functional derivative of {{math|''T''<sub>TF</sub>[''ρ'']}} is,<ref name=ParrYangP247A.6>{{harvp|Parr|Yang|1989|loc=p. 247, Eq. A.6}}.</ref>
<math display="block">\frac{\delta T_{\mathrm{TF}}}{\delta \rho (\boldsymbol{r}) }
= C_\mathrm{F} \frac{\partial \rho^{5/3}(\mathbf{r})}{\partial \rho(\mathbf{r})}
= \frac{5}{3} C_\mathrm{F} \rho^{2/3}(\mathbf{r}) \, .</math>