Direct calculation of overtones: Application to the CD3H molecule

We report a comparison of two methods designed for directly determining high lying energy levels in a polyatomic molecule. The Bloch operator, in the distorted wave version proposed by Jolicard [Chem. Phys. 115, 57 (1987)], allows one to compute a few specific eigenstates, as specified by some zero-order description. The second method makes use of a spectral transform, in order to open up an energy window in a very dense spectrum. Test calculations have been carried out on the nine-dimensional CD3H molecular system, for different C–H stretch overtone levels. They show that the Bloch formulation rapidly converges the target eigenstate, provided that the intramolecular coupling is not too strong. In the strong coupling regime, the spectral transform has been shown to successfully display all the eigenstates located in the energy window of interest, but requires more computational effort.