• effective exchange integrals (Jab) values;
  • hybrid DFT method;
  • magnetic effective density functional (MEDF) method;
  • spin correlation function;
  • spin projection


New schemes for effective exchange integrals (Jab) and approximate spin-projected energies for low spin states (EAPLS) were examined on simple two-site radical models. These schemes are based on Hubbard model and include ionic term in wavefunction. By estimation of expectation values of spin correlation function, those schemes could directly calculate Jab and EAPLS values without the approximation that on-site spins for high-spin (HS) state was equivalent to one of low-spin (LS) state. Calculated Jab values by new scheme were larger than conventional ones in short spin–spin distance, suggesting an importance of the on-site spins and ionic term. On the other hand, new EAPLS values were similar to conventional ones. Estimation methods for new Jab and EAPLS also were derived by using HS state energy, LS state energy, and 〈Ŝ2LS analytically. To correct dynamical correlation, magnetic effective density functional (MEDF) method was successfully applied to calculate new Jab and EAPLS. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004