Configuration interaction method nonrelativistic Hamiltonian

An alternative approach to the perturbation theory in treating many-electron systems is the configuration-interaction (Cl) method which is based on the variational principle. Nonrelativistic Cl techniques have been used extensively in atomic and molecular calculations. The generalization to relativistic configuration-interaction (RCI) calculations, however, presents theoretical as well as technical challenges. The problem originates from the many-electron Dirac Hamiltonian commonly used in RCI calculations ... 

The lack of correlation is the actual source of aU errors. In particular, a Slater determinant incorporates exchange correlation, i.e., the motion of two electrons with parallel spins is correlated (the so-called Fermi correlation). Unfortunately, the motion of electrons with opposite spins remains uncorrelated. It is common to define correlation energy, corr> as the difference between the exact nonrelativistic energy of the system, eo, and the Hartree-Fock energy, Eo, obtained in the limit that the basis set approaches completeness Ecorr = fo - o- The simplest manner to understand the inclusion of the correlation effects is through the method of configuration interaction (Cl). The basic idea is to diagonalize the N-electron Hamiltonian in a basis of N-electron functions we represent the exact wave function as a linear combination of N-electron trial functions and use the linear variational method. 

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