All-electron CCSD(T)/cc-pCVTZ theory was used to calculate the equilibrium geometry and a 68 point discrete potential energy hypersurface for the dihelium oxene dication He 2 O 2 + . The CCSD(T) optimised geometry was of C 2 v symmetry with anR O - H e bond length of 1.169 9 and an included bond angle of 92.6°. An analytical potential function was obtained from this surface using an Ogilvie Pade (4,5) power series expansion, which yielded a (x 2 ) 1 2 value of 2.810 10 - 5 a.u. The analytical function was then embedded in the Eckart-Watson Hamiltonian, which was solved variationally. Within the anharmonic approximation, the fundamental frequencies for the breathe, bend and asymmetric stretch vibrations were calculated to be 1247.3 cm - 1 , 816.8 - 1 and 1275.6 cm - 1 , respectively. Using a 560 configuration basis involving products of vibrational eigenfunctions and plus/minus combinations of regular symmetric-top rotor functions, the low-lying rovibrational states of the 1 A 1 electronic state of He 2 O 2 + were determined.