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Critical nuclear charge and shape resonances for the two-electron systems

The hydrogen negative ion H$^-$ is the simplest two-electron system that exists in nature. This system is not only important in astrophysics but it also serves as an ideal ground to study electron-electron correlations. The peculiar balance of the correlations between the two electrons with the interaction of electron-nucleus in H$^-$ makes this system to have only two bound states, one being the ground state $1s^2\,^{1}\!S^e$ and the other the doubly-excited metastable state $2p^2\,^{3}\!P^e$ embedded below the hydrogen $n=2$ threshold. Here we report a calculation for the $2p^2\,^{3}\!P^e$ state of H$^-$ that yields the energy eigenvalue $E=-0.125\,355\,451\,242\,864\,058\,376\,012\,313\,25(2)$, in atomic units. Our result substantially improves the best available result by 16 orders of magnitude. We further study the critical nuclear charge $Z_{\rm cr}$, the minimum value of nuclear charge $Z$ that is required to bind a nucleus and two electrons. Our determination of $Z_{\rm cr}$ for the $2p^2\,^{3}\!P^e$ state of two-electron systems is $Z_{\rm cr}=0.994\,781\,292\,240\,366\,246\,3(1)$, corresponding to $1/Z_{\rm cr}= 1.005\,246\,085\,546\,985\,509\,4(1)$, which improves the best published value of $Z_{\rm cr}$ by about 10 orders of magnitude. We further investigate in a definitive way the unexplored regime of $Z < Z_{\rm cr}$ using the method of complex scaling and establish precise shape resonance poles for the state of $2p^2\,^{3}\!P^e$ in the complex energy plane.