Low-temperature collisions of NH(X3 Sigma(-)) molecules with He atoms in a magnetic field: An ab initio study

Ab initio calculations of cross sections and rate constants for elastic scattering and Zeeman relaxation in collisions of NH(X (3)Sigma(-)) molecules with He-3 atoms in a magnetic field are presented and the mechanism for Zeeman transitions in ground-state (3)Sigma molecules is established. The potential surface for He-NH interaction is obtained using all-electron coupled-cluster calculations with single, double, and noniterative triple excitations in a basis set augmented by bond functions. The ratio of the rates for elastic energy transfer and Zeeman relaxation in NH-He-3 collisions exceeds 10(5) at temperatures between 0.5 and 1 K.