The infrared spectrum of H +3 and its isotopomers. A challenge to theory and experiment
H+3 and its isotopomers are seen as a benchmark for the comparison of theory and experiment. The stages of an ab initio calculation (electronic structure, potential fitting and nuclear motion) are discussed and the sources of error highlighted by consideration of recent results. Fitting of the electronic structure data is seen to be an area needing more development. New results are presented for the low-lying rovibrational states of D+3. While rovibrational properties can be calculated in the low-energy region to within ca. 0.1 % accuracy, the high-energy region is still largely unexplored. New developments in the calculation of excited rotational states (J⩽ 20) are outlined and the possibility of quantum-mechanical calculations of vibrational levels in the near-dissociation region is discussed in the light of illustrative calculations.