  Contrary to previous studies that classify Na2IrO3 as a realization of the
Heisenberg-Kitaev model with dominant spin-orbit coupling, we show that this
system represents a highly unusual case in which the electronic structure is
dominated by the formation of quasi-molecular orbitals (QMOs), with substantial
quenching of the orbital moments. The QMOs consist of six atomic orbitals on an
Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of
such QMOs in solids invokes very different physics compared to the models
considered previously. Employing density functional theory calculations and
model considerations we find that both the insulating behavior and the
experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow
from the QMO model.
