Abstract
The first-excited 1Σg+ state of the hydrogen molecule has a potential energy curve with two minima, labeled as E and F, respectively. In the transition from the E minimum to the F minimum, the mean interelectronic distance decreases as the nuclear distance increases, which is in contrast to the behavior of the ground state. Previously the F minimum has been identified as arising from the (2pσu)2 configuration. Our results suggest that it rather arises from the combination of the (1sσg)2 and (1sσu)2 configurations, which leads to the formation of an instant ionic pair, H− + H+, with the unshared electron pair resonating between the two hydrogen nuclei.