To obtain more accurate VEEs, dynamic correlation was included by using second order pertubation theory (PT2) with the multi state formalism (MS-CASPT2),48 with the SA(4)-CASSCF wave functions as a reference. We used the standard value of 0.25 for the ionisation potential-electron affinity (IPEA) shift as this has been shown to give more accurate dissociation and excitation energies.49 To minimise the effect of intruder states, an imaginary level shift of 0.2 a.u. was used.50