Dressed Second-order Epstein–Nesbet Perturbation Theory and Consequences of Orbital Delocalization for the BSSE Correction in Dimer Systems (in Honor of J.-P. Malrieu)

The dressed diagonal approximation to the self-consistent – size consistent CI, corrected for off-diagonal Fock matrix elements in localized orbitals is developed and applied to the ammoniac dimer system. A quite correct correlation energy can be obtained for this system, with a significantly reduced dependence of the results on the choice of the localization procedure. When calculating an interaction energy, the choice of the monomer orbitals and the application of the Boys–Bernardi counterpoise procedure shows in this case an unusual behavior: the correlation energy does not increase with the size of the atomic basis sets. Nevertheless a reasonable potential curve can be obtained.