Derivatives of the eigenvalues of the hamiltonian to fifth order

Results of CPMET calculations on the He-He potential are presented. The effect of inter-intra coupling is shown to be far from negligible (about 30 per cent of the binding energy). Since the coupling must vanish with increasing R the previous striking agreement between experiment and ab initio calculations must be regarded as somewhat fortuitous. This is likely to be a principal reason for the present disagreement between ab initio and perturbation theoretical values for the long-range interatomic energy constants.