Calculated rotation-bending energy levels of CH5 (+) and a comparison with experiment.

We report J > 0 CH5 (+) levels computed by fixing stretch coordinates. They are computed by using a simple product basis, exploiting symmetry, and carefully parallelizing the calculation. The J > 0 CH5 (+) levels are compared with those obtained from other theoretical methods and with experimental ground state combination differences of Asvany et al. [Science, 347, 1346 (2015)]. If the assignment of Asvany et al. is correct, there are important differences between the levels we compute and those observed. We propose a different assignment of the experimental levels that reduces the maximum error from 34 to 2 cm(-1). The new assignment can only be correct if states of both parities exist in the experiment. Although, ro-vibrational levels of CH5 (+) cannot be associated with individual vibrational states, they do occur in blocks separated by gaps.