On the nature of the bonding in Cu2

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Abstract
The ground state of Cu2 is found to arise from the 2S(4s13d10) +2S(4s13d10) limit and to involve a 4s–4s sigma bond pair. The dominant bond pair correlations are left–right and angular, with the former lengthening the bond and the latter contracting the bond, so that at the two‐electron MCSCF level the Re is slightly longer (0.02a0) than at the SCF level. Correlation of the 3d electrons shortens the bond by 0.19a0, leading to a final bond length of 4.35a0, which is 0.15a0 longer than experiment. This error is of the same magnitude as twice the relativistic contraction of the 4s orbital of the 2S state of the Cu atom (0.13a0) and most of the remaining error in Re is thought to be due to this relativistic contraction.