LOCAL APPROACH TO VARIATIONAL METHOD FOR ELECTRON CORRELATION IN NARROW BAND SYSTEMS

Using the local approach, we have performed a third order perturbation calculation to investigate the electron correlation effect in extended Hubbard model with both positive and negative intraatomic and interatomic correlation energies. We found the significant third order correction over the second order terms. Consequently, all published results, especially the phase diagrams which were derived with the second order perturbation calculation are not reliable.