Orbital spaces and density-functional theory

Only part of the correlation energy can be obtained from the wave function when expanded as a finite and linear combination of Slater determinants. For the remaining part, it was proposed by several authors to use density-functional theory. Thus, various methods coupling wave functions with density functionals were investigated in the past. In the present a class of such methods was developed, relying on a partition of the orbital space. Notably, for this coupled method special efforts were made to avoid double countings between the contributions of the wave function and the density functional. Moreover, the coupling was defined in order to allow a systematic improvement of the results. As a second step, the method was put to the test for spherically symmetric systems, especially in the case of near degeneracy. The numerical results obtained were discussed in order to improve the model. Finally, the wave function contribution that was retained relied on a coupledcluster formalism restricted to a small orbital space, whereas the density functional was chosen as a semilocal expression based on the physical picture of one-particle ionization potentials.