Gibbs ensemble simulations using ab initio intermolecular potentials are reported for the vapour–liquid phase coexistence of neon and argon. For neon two different quantum chemical ab initio potentials of well-known quality are used to investigate the effect of the quality of pair interactions. In addition calculations are also reported for neon using a potential that includes three-body interactions. For argon, simulations are compared with results obtained from NPH-ensemble molecular dynamics simulations. It is found that the results of a perfect pair potential must occur outside the experimental temperature–density phase envelope. Therefore, if a perfect pair potential is used, many-body interactions and quantum effects must be considered to obtain good agreement with experiment.