Potential energy curves via double electron affinity calculations: example of NaLi molecule

The recently developed method designed to perform calculations for the double electron attached (DEA) states has been applied to study potential energy curves (PECs) for the ground and excited states of the NaLi molecule. The method is based on the multireference coupled cluster scheme formulated in the Fock space (FS) formalism for the (2,0) sector. To avoid the intruder state problems, the FS approach is realised within the framework of the intermediate Hamiltonian which generates an efficient size-extensive computational scheme. The DEA calculations can be applied to the double ionised system and then they provide results describing the neutral molecule. This is particularly advantageous in the studies of the PECs for the cases when the closed-shell molecule dissociates into open-shell fragments, but its double positive ion generates closed-shell parts. We used this strategy in the current work to generate the PECs for the ground and excited states of the NaLi molecule. In all cases, we were able to compute the smooth PECs for the whole range of interatomic distances from the equilibrium to the dissociation limit. In addition, spectroscopic characteristics of various electronic states are presented.