Rotational frequencies determined with ab initio molecular orbital theory can play an important role in guiding spectroscopic searches for new molecules and in corroborating the assignment of unidentified lines, from the laboratory and from space. In a systematic study of 22 levels of molecular orbital theory, CISD/6-311G** gave rotational frequencies to an accuracy of ±0.4 GHz when an empirical correction is applied to the results for C2H2, HCN, HNC, HCO+, N2H +, CO and N2. Larger errors can be expected when there are large vibrational effects on the rotational constants, as exemplified by COH+. Predicted J=0→1 rotational frequencies using these methods are 73.9±0.4 GHz for HCNH+, 78.6±0.4 GHz for HBO, 65.8±0.4 GHz for HBNH, and 72.1±0.4 GHz for HBF+. © 1984 American Institute of Physics.