An ab initio semirigid bender calculation of the rotation and trans-tunneling spectra of (HF)₂ and (DF)₂

Using a purely ab initio minimum energy path for the trans-tunneling motion in the HF dimer, the energy levels for the A-type rotation and trans-tunneling motion for (HF)₂ and (DF)₂ are calculated with a one-dimensional semirigid bender Hamiltonian and no adjustable parameters. The transition moments for rotation-tunneling transitions are calculated, using our ab initio value for the dipole moment of an isolated HF molecule, and we also calculate B̄ values. The energy levels we obtain are in close agreement with experiment; for example, the K = 0 tunneling splitting in (HF)₂ is calculated as 0.65 cm^-1 compared to the experimental value of 0.658 69 cm^-1. As well as showing that our ab initio minimum energy path is very good, the calculation demonstrates that the semirigid bender formalism is able to account quantitatively for the unusual K dependence of the rotational energies resulting from the quasilinear behavior, and that the trans-tunneling motion is separable from the other degrees of freedom. We use the results to predict the locations, and transition moments, of the ΔK = 0 and ± 1 subbands in the tunneling spectra of (HF)₂ and (DF)₂, many of which have not yet been observed.