**Magnetic coupling constants in three electrons three centers problems from effective hamiltonian theory and validation of broken symmentry-based approaches**

D. Reta, I. de P.R. Moreira, F. Illas.

**J. Chem. Theory Comput.,** 12 (2016) 3228.

In the most general case of three electrons in three symmetry unrelated centres with S_{1} = S_{2} = S_{3} = ^{1}⁄_{2} localized magnetic moments; the low energy spectrum consists of one quartet (Q) and two doublet (D_{1}, D_{2}) pure spin states. The energy splitting between these spin states can be described with the well-known Heisenberg-Dirac-Van Vleck (HDVV) model spin Hamiltonian, and their corresponding energy expressions are expressed in terms of the three different two-body magnetic coupling constants J_{12}, J_{23}, and J_{13}. However, the values of all three magnetic coupling constants cannot be extracted using the calculated energy of the three spin-adapted states since only two linearly independent energy differences between pure spin states exist. In the present work we investigate the 3 electrons in 3 centres problem by means of ab initio effective Hamiltonian theory using MRCI wave functions and validate the broken symmetry approach to extract the J_{12}, J_{23}, and J_{13} in the general non-symmetric case.