A combined approach of first-principles density-functional calculations and
the systematic cluster-expansion scheme is presented. The dipole, quadrupole,
and Coulomb matrix elements obtained from ab initio calculations are used as an
input to the microscopic many-body theory of the excitonic optical response. To
demonstrate the hybrid approach for a nontrivial semiconductor system, the
near-bandgap excitonic optical absorption of rutile TiO2 is computed.
Comparison with experiments yields strong evidence that the observed
near-bandgap features are due to a dipole-forbidden but quadrupole-allowed
1s-exciton state.Comment: 14 pages, 4 figure
