Variational calculation of the rotational constants for acetylene and its isotopic derivatives

A method is presented for calculation of rotational constants for a linear molecule by treating the vibrations variationally, but treating the vibration–rotation coupling perturbatively. The method is applied to the potential energy function for acetylene given by L. Halonen, M. S. Child, and S. Carter [Mol. Phys. 47, 1097 (1982)]. It is found that even though the vibrational energy levels calculated from this potential are in excellent agreement with experiment, the calculated rotational constants are poorly predicted. The calculated ΔB’s are systematically inaccurate by about 5% for C2H2 and 8% for C2D2. This indicates that the potential energy function is inaccurate even near equilibrium.