LAGRANGE-MULTIPLIER METHOD IN CORRELATED-ELECTRON SYSTEMS - EXACT DIAGONALIZATION STUDY

We investigate the Lagrange-multiplier method which is frequently used in strongly correlated-electron systems to impose the constraints. We assume that the constraint operators commute with the Hamiltonian. It may be implicitly believed that the Lagrange multipliers are determin ed uniquely due to the variational conditions. We discuss whether this is true or not by using the exact diagonalization method. For the mea n-field Hamiltonian which does not commute with the constraint operato rs, the Lagrange multipliers may be determined uniquely.