The Jacobi-Wilson method: A new approach to the description of polyatomic molecules

We present a new method adapted to the calculation of excited rovibrational states of semirigid molecules. It first relies on a description of the mol ecule in terms of polyspherical coordinates of Jacobi vectors, in order to obtain a compact expression for the kinetic energy operator (T) over cap (q ). This general description is then adapted to the molecule considered by d efining curvilinear normal modes from the corresponding zero order harmonic Hamiltonian (H) over cap (0)=(T) over cap (q(eq))+V-harm(q), the solutions of which are being used as the working basis set. The residual kinetic ter m DeltaT is treated mainly analytically in this basis, and displays no radi al contribution. Anharmonic coupling DeltaV(q) is handled by means of a pse udospectral scheme based on Gauss Hermite quadratures. This method is parti cularly adapted to direct iterative approaches which only require the actio n of (H) over cap on a vector, without the need of the associated matrix, t hus allowing ultralarge bases to be considered. An application to the excit ed vibrational states of the HFCO molecule is presented. It is shown in thi s example that energy levels can be trivially assigned from the leading exp ansion coefficient of the associated eigenvector. (C) 2001 American Institu te of Physics.