Potential energy curves of the NaH molecule and its cation with the Fock space coupled cluster method

The Fock space coupled cluster theory provides a description of the states obtained by attachment of one ((1,0) sector) or two ((2,0) sector) electrons to the reference system. If the reference is assumed to be a doubly ionized cation then the results relate to a cation or a neutral molecule, respectively. In the current work the above scheme is applied to extensive ab initio calculations of the potential energy curves (PECs), and the spectroscopic constants of NaH and its cation for the eight lowest lying states, adopting as a reference system the doubly ionized structure, i.e., NaH2+. Such a computational strategy relies on the fact that the closed shell reference (NaH2+) dissociates into the closed shell fragments. This is advantageous since the restricted Hartree-Fock function can be used as the reference in the whole range of interatomic distances. This scheme offers a first principle size-extensive method without any model or effective potential parameters for the description of the bond breaking processes. The computed PECs and spectroscopic constants stay very close to the experimental values, with accuracy exceeding that of other theoretical approaches (in most cases) including those based on the effective core potentials.