where V ( r ) is the external potential, the second term is the Hartree term describing the electron–electron Coulomb repulsion, and the last term V XC is the exchange–correlation potential. Here, V XC includes all the many-particle interactions. Since the Hartree term and V XC depend on n (r), which depends on the φ i, which in turn depend on V s, the problem of solving the Kohn–Sham equation has to be done in a self-consistent (i.e., iterative) way. Usually one starts with an initial guess for n (r), then calculates the corresponding V s and solves the Kohn–Sham equations for the φ i. From these one calculates a new density and starts again. This procedure is then repeated until convergence is reached. A non-iterative approximate formulation called Harris functional DFT is an alternative approach to this.