In order to theoretically study on the magnetic coupling interaction in the Mn-Mn dimer, quantum chemical calculations on the equilibrium Mn-Mn distance Re, total energy ET, binding energies Be and exchange interaction energies EðSÞ under dierent spin multiplets were performed at CASPT2 level of theory. It is found that the calculated Re, ET, and Be values are correlated with the spin multiplet, and with increasing the total spin S, Re increases, but ET and Be exhibit a decrease trend. The exchange interaction energies EðSÞ "deviate significantly from the Lande interval rule". This unusual magnetic phenomenon is primarily attributed to the biquadratic jðSa SbÞ2 term contribution in spin Hamiltonian for the Mn-Mn dimer.