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Near dissociation states for H$_2^+$-He on MRCI and FCI potential energy surfaces

A new analytical potential energy surface (PES) has been constructed for H$_2^+$-He using a reproducing kernel Hilbert space (RKHS) representation from an extensive number of $ab initio$ energies computed at the multi-reference and full configuration interaction level of theory. For the MRCI PES the long-range interaction region of the PES is described by analytical functions and is connected smoothly to the short-range interaction region, represented as a RKHS. All ro-vibrational states for the ground electronic state of H$_2^+$-He are calculated using two different methods to determine quantum bound states. Comparing transition frequencies for the near-dissociation states for $ortho$- and $para$-H$_2^+$-He allows assignment of the 15.2 GHz line to a $J=2$ $e/f$ parity doublet of $ortho$-H$_2^+$-He whereas the experimentally determined 21.8 GHz line is only consistent with a $(J=0)$ $\rightarrow$ $(J=1)$ $e/e$ transition in $para$-H$_2^+$-He.