Potential energy curves for electronic states of the sodium dimer with multireference coupled cluster calculations

Abstract

Accurate potential energy curves (PECs) were obtained for the 26 lowest lying electronic states of the sodium dimer. The current approach is based on the first principle size-extensive multireference coupled cluster method formulated in the (2,0) sector of the Fock space (FS). The (2,0) sector of FS provides the description of the states obtained by the attachment of two electrons to the reference system. This makes it possible to adopt the doubly ionised sodium dimer as a reference with its very concrete advantage in the calculations of the PECs, i.e. it dissociates into closed shell fragments (i.e. sodium cations) hence the restricted Hartree–Fock reference can be used in the whole range of interatomic distances. This results in the correct description of the interatomic potentials for all bond lengths. The theoretical PECs stay very close to the experimental curves wherever the latter are available and the computed spectroscopic parameters reproduce the experiment with very good accuracy. Relativistic corrections included at the second-order Douglass–Kroll level have a non-negligible effect on the accuracy of computed excitation and dissociation energies.