Reduced effective Hamiltonian for global fitting of C2H2 ro-vibrational lines

An effective Hamiltonian describing globally all vibrational-rotational energy levels of C2H2 molecule in the ground electronic state has been written up to fourth order in Amat-Nielsen ordering scheme using symmetry arguments. This Hamiltonian contains all resonance interaction terms within polyad scheme discussed by M. E. Kellman [J. Chem. Phys. 95, 8671 (1991)]. D. M. Jonas et al [J. Chem. Phys. 99, 7350 (1993)]. M. Abbouti Temsamani and M. Herman [J. Chem. Phys. 103, 5931 (1995)]. The reduction of this effective Hamiltonian by means of unitary transformations has been performed.