Improved calculations of the lowest vibrational transitions in HeH+

More accurate variational calculations of the lowest three pure vibrational states (v=0,1,2) of the (HeH+)-He-4 molecular ion have been carried out without assuming the Born-Oppenheimer approximation. In the calculations we included the complete set of alpha(2) relativistic corrections, i.e., mass-velocity, Darwin, spin-spin, and orbit-orbit. This allowed us to improve the agreement between the theory and the experiment for the vibrational frequencies of the 1 -> 0 and 2 -> 1 transitions as compared to our previous calculations [Stanke , Phys. Rev. Lett. 96, 233002 (2006)].