Full-configuration-interaction calculation of three-body nonadditive contribution to helium interaction potential.

The three-body nonadditive interaction energy between helium atoms was calculated at 253 trimer configurations using the full-configuration-interaction (FCI) method. The analytic potential fitted to these energies is the best current representation of the three-body nonadditive interactions between helium atoms. At the equilateral triangle configuration with R=5.6 bohr, near the minimum of the total potential, the nonadditive three-body energy calculated at the FCI level amounts to -88.5 mK, compared to -98.5 mK at the coupled cluster with single, double, and noniterative triple excitations [CCSD(T)] level. The uncertainty of the former result resulting from basis set incompleteness is estimated to be 1.5 mK. The relative uncertainty of our present complete three-body fit, including the uncertainties resulting from the fitting procedure, is estimated at 2%, a fivefold improvement over the previous best potential. Overall, the FCI contribution beyond CCSD(T) is rather important, being of the same order of magnitude as the uncertainty of the sum of two-body interactions. The inclusion of this contribution makes uncertainties of the total trimer interaction energies dominated by the uncertainties of the two-body component.