Dissociation limit in Kohn-Sham density functional theory

Abstract: We consider the dissociation limit for molecules of the type $X_2$ in the Kohn-Sham density functional theory setting, where $X$ can be any element with $N$ electrons. We prove that when the two atoms in the system are torn infinitely far apart, the energy of the system convergences to $\min \limits_{\alpha \in [0,N]} \big( I^{X}_{\alpha} + I^{X}_{2N-\alpha} \big)$, where $I^{X}_{\alpha}$ denotes the energy of the atom with $\alpha$ electrons surrounding it. Depending on the "strength" of the exchange this minimum might not be equal to the symmetric splitting $2I^{X}_{N}$. We show numerically that for the $H_2$-molecule with Dirac exchange this gives the expected result of twice the energy of a H-atom $2 I^{H}_1$.