  We present four-dimensional ab initio potential energy surfaces for the three
spin states of the NH-NH complex. The potentials are partially based on the
work of Dhont et al. [J. Chem. Phys. 123, 184302 (2005)]. The surface for the
quintet state is obtained at the RCCSD(T)/aug-cc-pVTZ level of theory and the
energy diferences with the singlet and triplet states are calculated at the
CASPTn/aug-cc-pVTZ (n = 2; 3) level of theory. The ab initio potentials are
fitted to coupled spherical harmonics in the angular coordinates, and the long
range is further expanded as a power series in 1/R. The RCCSD(T) potential is
corrected for a size-consistency error prior to fitting. The long-range
coeficients obtained from the fit are found to be in good agreement with
perturbation theory calculations.
