1-matrix functional for long-range interaction energy of two hydrogen atoms

The leading terms in the large-$R$ asymptotics of the functional of the one-electron reduced density matrix for the ground-state energy of the H${}_2$ molecule with the internuclear separation $R$ is derived thanks to the solution of the phase dilemma at the $R\to \mathrm{\infty }$ limit. At this limit, the respective natural orbitals (NOs) are given by symmetric and antisymmetric combinations of “half-space” orbitals with the corresponding natural amplitudes of the same amplitudes but opposite signs. Minimization of the resulting explicit functional yields the large-$R$ asymptotics for the occupation numbers of the weakly occupied NOs and the $C_6$ dispersion coefficient. The highly accurate approximates for the radial components of the $p$-type “half-space” orbitals and the corresponding occupation numbers (that decay like $R^{-6}$), which are available for the first time thanks to the development of the present formalism, have some unexpected properties.