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 emplo yed for systematic construction of convergent perturbation series. The method is applied to the vertical excitation energies of the low lyin g Rydberg states of the water molecule and compared to results obtaine d by standard techniques for the calculation of excited electronic sta tes. The values calculated by Lambda-transformed perturbation theory a re found to be of about the same quality as the best theoretical resul ts available. It is shown that for practical applications the computat ional effort for a Lambda-transformed perturbation series is only slig htly greater than the cost for the original series. (C) 1995 American Institut of Physics.