Is HeH− a stable system? Original Research Article

We present an ab initio study of the HeH− system. This hypothetical molecule is isolectronic with the He2 dimer, whose existence has been predicted since a long time and recently confirmed by experiment. We performed calculations on this system at several levels of accuracy, with extended basis sets containing up to 179 atomic basis functions. All the different methods predicted a well in the Born-Oppenheimer energy curve, with a minimum located at an internuclear separation of about 12 to 13 bohr. Its depth is extremely small: about 4 to 6 K, depending on the method used to compute the electronic wave function and on the atomic basis set. By solving numerically the Schrödinger equation for the nuclei, we obtained two vibrational levels, lying at about 0.8 K and 0.1 K below dissociation, respectively. While the precise value of their energies is uncertain, we believe that their presence, regardless the level of approximation used, strongly support the existence of this so far undetected species.