The anharmonic constants for a symmetric top

The purpose of this Letter is to validate the formulae of Pliva for the anharmonic constants of a symmetric top molecule. Special attention is paid to molecules with high-order principal axes. Calculations have been performed on molecules with principal axes of order four (SF5Cl) and six (C6H6). Values for the anharmonic constants of C6H6 are compared with the earlier values of Maslen, Handy, Amos and Jayatilaka.