The Second-Order Energy Contribution from the Spin-Orbit Interaction Operator to the Potential Energy Curve of Cr2

The second-order Rayleigh-Schrodinger energy correction to the Born-Oppenheimer poten-tial energy due to the spin-orbit interaction can be expressed as a linear response function evalu-ated at zero frequency. We have calculated this energy contribution to the Cr2 singlet ground state X ' Z i potential energy function for a multiconfiguration self-consistent field (MCSCF) wave function. The calculations show that the effect of spin-orbit interaction is small and of the same magnitude for the whole potential energy curve. The Cr2 molecule with its hexuple bond is a cause cCl6bre in electronic struc-ture calculations. No method so far has been able to give a satisfactory descrip-tion of its binding [l, 21. Much of the problems encountered in these calculations are related to the hexuple bond of the Cr, molecule with the entailed problem of describing the electron correlation. Moreover, the presence of several close-lying excited states and the unusually shallow potential energy curve of the ground state make it plausible that finer effects such as relativistic and nonadiabatic cou-plings may be important for the binding energy and the bond length. For Cr