Global ab initio ground-state potential energy surface of N₄

We present a global ground-state potential energy surface for N₄ suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation in N₂-N₂ collisions. To obtain the surface, complete active space second-order perturbation theory calculations were performed for the ground singlet state with an active space of 12 electrons in 12 orbitals and the maug-cc-pVTZ triple zeta basis set. About 17 000 ab initio data points have been calculated for the N₄ system, distributed along nine series of N₂ + N₂ geometries and three series of N₃ + N geometries. The six-dimensional ground-state potential energy surface is fitted using least-squares fits to the many-body component of the electronic energies based on permutationally invariant polynomials in bond order variables.