Highly excited vibration–rotation states of floppy triatomic molecules by a localized representation method: The HCN/HNC molecule.

All rovibrational levels of HCN/HNC up to ∼16 000 cm-1, relative to the HCN minimum, for J=0, 1, 2, have been calculated accurately. All internal degrees of freedom are included in these calculations, performed on the realistic, empirical potential surface by Murrel et al. [J. Mol. Spectrosc. 93, 307 (1982)]. Body-fixed mass-scaled Jacobi coordinates are employed, together with the discrete variable representation of the large amplitude motion (LAM) angular coordinate, and a 2-D distributed Gaussian basis for the radial degrees of freedom. The successive diagonalization–truncation procedure results in a compact matrix representation of the full rovibrational Hamiltonian, allowing accurate and efficient determination of a large number (>350 for J=2, p=0 case) of highly excited LAM rovibrational states of HCN/HNC. This approach is suitable for a broad class of floppy, isomerizing triatomic molecules and van der Waals complexes. In addition to energy levels and wave functions, expectation values of Jacobi coordinates, 〈R〉, 〈r〉, and 〈θ〉, are calculated for most states. The majority of calculated J=1,2 levels lie above the top of the isomerization barrier, and are delocalized to a varying degree over both local minima. Rotation appears to lower the energy threshold for extensive delocalization; for the states with J=1, or 2, it is ∼460–480 cm-1 below that for J=0 states. Moreover, increasing rotational excitation affects significantly the degree of localization of a given state. [ABSTRACT FROM AUTHOR]