Calculation of rovibrational spectra of water by means of particles-on-concentric-spheres models. II. Excited states of stretching vibrations

It is shown that adiabatic separation of high-frequency stretching modes from bending and overall rotational motions in triatomic molecules XY2 leads naturally to the particles-on-a-sphere (POS) model treated previously [J. Chem. Phys. 81, 3400 (1984)]. Solution of the rovibrational problem using a further approximation in which stretching motions are treated as uncoupled modes is then investigated in detail. It is shown that, for states with a significantly larger number of quanta in one bond than the other, the POS model in this approximation yields energy levels that are essentially identical with those for the particles-on-concentric-spheres (POCS) model, where the latter is obtained using a different decoupling of the basic set of differential equations.