Symmetry-adapted perturbation theory (SAPT) calculations were performed to determine a two-dimensional potential for the interaction of the helium atom with the nitrous oxide molecule. For selected geometries, correlated supermolecular calculations were used to verify SAPT data. The ab initio interaction energies were fitted to an analytic function and rovibrational energy levels of He-N2O were computed on the resulting surface. Extensive comparisons were made with a literature ab initio He-CO2 potential and rovibrational states in order to rationalize the counterintuitive observations concerning spectra of N2O and CO2 in superfluid helium nanodroplets. We conjecture that the greater reduction of the N2O rotational constant than that of CO2 is related to the greater potential depth in the former case and the resulting greater probability of attaching helium atoms. An additional factor could be that the secondary minimum on the O side of N2O is 30% deeper than the linear minima in the case of CO2. As a by-product of this work, accurate multipole moments of N2O have been computed. The quadrupole, octupole, and hexadecapole moments are significantly different from experimental values and are probably more accurate than the latter. (C) 2003 American Institute of Physics.