They realized that the quantitatively most important term in the energy functional (eq. 2. energy 𝑇 KS [𝜌] of a model system which contains independent electrons (i.e., the electron-electron interaction 𝑉 ee in this system is zero) butby definitiondelivers the same density as the real system to be solved. The clue is that the kinetic energy for such a KS model system can be computed exactly by using orbitals 𝜑 (= one-electron wavefunctions) as shown in eq. 2.3. 𝑇 KS is a pretty good approximation to the actual kinetic energy T.