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 challenge for currently available theoretical and experimental methods. Most of the theoretical studies reported so far predict a singlet cis ground state. However, the fully optimized geometry of the NO dimer may favor a triplet ground stale, depending on the approximate method used. In this work we explore in detail the electronic structure of the fully optimized trans- and cis-NO dimer including a vibrational analysis in different electronic states, using several exchange-correlation functionals within the Kohn-Sham DFT method. The recently developed LAP exchange-correlation schemes that use the Laplacian of the density and the self-consistent kinetic energy density, improves significantly the results. The N-N bond distance is in better agreement with the experimental 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 such a system the singlet ground state may be well approximated as a broken spin symmetry state with localized magnetic moments on each NO monomer aligned antiferromagnetically. A KS broken symmetry solution was thus obtained. However, the energy lowering due to the symmetry breaking was not sufficient to reverse the triplet/singlet energy ordering. The LAP functional avoids symmetry breaking out to larger distances than does the generalized gradient approximation (GGA). Although progress has been steady, the existing exchange-correlation functionals are not yet able to describe fully all aspects of this demanding system. (C) 1998 American Institute of Physics. [S0021-9606(98)00625 -4].