Spectroscopic properties of 34 electronic states of zirconium dimer (Zr2)

State-averaged complete active space multiconfiguration self-consistent field followed by multireference singles + doubles configuration interaction calculations are carried out on low-lying electronic states of zirconium dimer (Zr2). In addition relativistic configuration interaction calculations are carried out including spin–orbit effects. Thirty-four bound electronic states are found for Zr2. Three very low-lying states of 7∑+u, 3∑+u, and 1∑+g symmetries are found as candidates for the ground state. Higher-order correlation effects and extension of basis sets tend to favor the 1∑+g state as the ground state of Zr2 exhibiting approximately a quadruple bond (bond order =3.6). Complete potential energy curves of seven states of Zr2 are presented. The nature of low-lying electronic states is analyzed through Mulliken populations, natural orbitals and their occupations, spin–orbit effects and weights of various configurations.