DENSITY-FUNCTIONAL STUDY OF THE NO DIMER USING GGA AND LAP FUNCTIONALS

The nature of the ON-NO bonding in the NO dimer still remains a challe nge for currently available theoretical and experimental methods. Most of the theoretical studies reported so far predict a singlet cis grou nd state. However, the fully optimized geometry of the NO dimer may fa vor a triplet ground stale, depending on the approximate method used. In this work we explore in detail the electronic structure of the full y optimized trans- and cis-NO dimer including a vibrational analysis i n different electronic states, using several exchange-correlation func tionals within the Kohn-Sham DFT method. The recently developed LAP ex change-correlation schemes that use the Laplacian of the density and t he self-consistent kinetic energy density, improves significantly the results. The N-N bond distance is in better agreement with the experim ental results, and the triplet/singlet gap is smaller, however, still predicting a triplet ground state. The nature of the electronic ground state is discussed in detail. We explored the possibility that in suc h a system the singlet ground state may be well approximated as a brok en spin symmetry state with localized magnetic moments on each NO mono mer aligned antiferromagnetically. A KS broken symmetry solution was t hus obtained. However, the energy lowering due to the symmetry breakin g was not sufficient to reverse the triplet/singlet energy ordering. T he LAP functional avoids symmetry breaking out to larger distances tha n does the generalized gradient approximation (GGA). Although progress has been steady, the existing exchange-correlation functionals are no t yet able to describe fully all aspects of this demanding system. (C) 1998 American Institute of Physics. [S0021-9606(98)00625 -4].