I performed calculations on metal-metal multiple bond. For the Cr-Cr system, one electron on σ*-anti-bonding, two on σ-bonding, one electron on π*-anti-bonding, two on π-bonding, the bond order for Cr-Cr is 0. However, for the Fe-Fe system, one electron on σ*-anti-bonding, two on σ-bonding, one electron on π*-anti-bonding, two on π-bonding, the bond order for Fe-Fe is 1. Why? Who can tell me on what condition the half bond could exist, please?