645916 rad, 2.467341 bohr, 0. bohr] and expressed as a sum of four n-mode contributions, V 0 (Q), ∆V 1 (Q), ∆V 2 (Q) and ∆V 3 (Q), which depend on the coupling between modes. The first contribution, V 0 (Q), is just the reference energy, i.e. the energy of the minimum. The other contributions are fitted from the energies generated similarly to the n-mode expansion: