ADIABATIC POTENTIAL CURVES AND NONADIABATIC COUPLING FUNCTIONS FOR THE 1ST 5 EXCITED 1-SIGMA-G+ STATES OF THE HYDROGEN MOLECULE

On closer examination, some of the spectroscopic assignments for the f ourth excited 1SIGMA(g)+ state of H-2 (4dsigma,P) and all of the ones for the fifth state (4s,O) prove to be wrong. To aid the re-evaluation of these experimental data we have performed (i) the first accurate a b initio calculation, over a wide range of internuclear distances, of the fifth and sixth electronic 1SIGMA(g)+ states, i.e., of their Born- Oppenheimer electronic energy curves and adiabatic corrections, and of their nonadiabatic couplings with the lower excited electronic 1SIGMA (g)+ states, (ii) an improved calculation of the first three excited e lectronic 1SIGMA(g)+ states and of their mutual couplings, and (iii) t he first calculation of relativistic corrections for the first two of these excited states, EF and GK, at the minima of their double-well po tential energy functions. We have found that the electronic energy cur ves of the pairs of singlet ndsigma and ns states (n = 3, 4), unlike t he triplet 3dsigma and 3s states, cross each other at internuclear dis tances which are much smaller than their equilibrium R values and with off-diagonal electronic energies that are at least 2 orders of magnit ude smaller than the diabatic coupling energies at the avoided crossin gs of the corresponding triplet states.