  A very accurate, (HF)$_2$ potential energy surface (PES) due to Huang et al.
(J. Chem. Phys., 150, 154302 (2019)) is used to calculate the energy levels of
the HF dimer by solving the nuclear-motion Schr\"{o}dinger equation using
variational program WAVR4. Calculations on an extended range of rotational
states show very good agreement with experimental data. In particular the known
empirical rotational constants for the ground and some observed excited
vibrational states are reproduced with an accuracy of about 50 MHz. This level
of accuracy is shown to extend to higher excited inter-molecular vibrational
states $v$ and higher excited rotational quantum numbers $(J,K_a)$. These
calculations allow the assignment of 3 new $J$-branches in an HF dimer
tunneling-rotation spectra recorded 30 years ago. These branches belong to
excited $K_a$ = 4 state of the ground vibrational state, and $K_a$ = 0 states
of excited inter-molecular vibrational states.
