In order to theoretically study on the magnetic coupling interaction in the Mn-Mn dimer, quantum chemical calculations on the equilibrium Mn-Mn distance R-e, total energy E-T, binding energies B-c and exchange interaction energies E(S) under different spin multiplets were performed at CASPT2 level of theory. It is found that the calculated R-c, E-T, and B-c values are correlated with the spin multiplet, and with increasing the total spin S, R-c increases, but E-T and B-c exhibit a decrease trend. The exchange interaction energies E(S) deviate significantly from the 'Lande interval rule'. This unusual magnetic phenomenon is primarily attributed to the biquadratic j(S-a . S-b)(2) term contribution in spin Hamiltonian for the Mn-Mn dimer. (C) 2004 Elsevier B.V. All rights reserved.