Knowledge Bank

A new method for calculating rovibrational states of weakly-bound complexes from a given multi-dimensional potential surface is described and tested. This ``iterative secular equation"" (ISE) method generates eigenvalues and wave functions for a few states at a time, which makes it particularly useful for simulating Van der Waals spectra, and for use in least-squares fitting to such spectra to determine potential energy surfaces. The ISE method is in principle exact for bound states, and it yields reliable estimates of the energies and widths of metastable levels. The range of applicability of the ISE method is demonstrated by application to complexes such as $He-C2H2$, He-HF, Ar-HCN, and Ar-HCl. The ISE method is also shown to be more efficient than certain other coupled-channel or basis-set methods, for many problems.