PERTURBATION-THEORY FOR ELECTRONIC EXCITED-STATES - THE LOW-LYING RYDBERG STATES OF WATER

The recently developed Lambda-transformed perturbation theory offers a rigorous solution of the intruder state problem and can thus be employed for systematic construction of convergent perturbation series. The method is applied to the vertical excitation energies of the low lying Rydberg states of the water molecule and compared to results obtained by standard techniques for the calculation of excited electronic states. The values calculated by Lambda-transformed perturbation theory are found to be of about the same quality as the best theoretical results available. It is shown that for practical applications the computational effort for a Lambda-transformed perturbation series is only slightly greater than the cost for the original series. (C) 1995 American Institut of Physics.