Performance of the effective-characteristic-polynomial pi 2 method for diatomic molecules: Basis-set dependencies and vibrational levels

The performance of the recently introduced nu upsilon lambda lambda) is inv estigated for some diatomic molecules. For this end, ground state energies are calculated at the MP4 level for various basis sets of increasing size. With negligible extra effort, the pi 2, F4, and [212] estimators are obtain ed, together with information on the reliability of the basic perturbation series. (((1) under bar)). The results are compared to more expensive CCSD( T) results. Also, electronic energy hypersurfaces are calculated at these l evels. As a further possibility to test the performance of the method, vibr ational frequencies and other spectroscopic constants of diatomic molecules are calculated by fitting different analytic functions to the hypersurface s obtained by different methods and compared to experimental data.