  Molecular contacts are generally poorly conducting because their energy
levels tend to lie far from the Fermi energy of the metal contact,
necessitating undesirably large gate and bias voltages in molecular electronics
applications. Molecular radicals are an exception because their partly filled
orbitals undergo Kondo screening, opening the way to electron passage even at
zero bias. While that phenomenon has been experimentally demonstrated for
several complex organic radicals, quantitative theoretical predictions have not
been attempted so far. It is therefore an open question whether and to what
extent an ab initio-based theory is able to make accurate predictions for Kondo
temperatures and conductance lineshapes. Choosing nitric oxide NO as a simple
and exemplary spin 1/2 molecular radical, we present calculations based on a
combination of density functional theory and numerical renormalization group
(DFT+NRG) predicting a zero bias spectral anomaly with a Kondo temperature of
15 K for NO/Au(111). A scanning tunneling spectroscopy study is subsequently
carried out to verify the prediction, and a striking zero bias Kondo anomaly is
confirmed, still quite visible at liquid nitrogen temperatures. Comparison
shows that the experimental Kondo temperature of about 43 K is larger than the
theoretical one, while the inverted Fano lineshape implies a strong source of
interference not included in the model. These discrepancies are not a surprise,
providing in fact an instructive measure of the approximations used in the
modeling, which supports and qualifies the viability of the DFT+NRG approach to
the prediction of conductance anomalies in larger molecular radicals.

Mesoscale and Nanoscale Physics;Strongly Correlated Electrons