EXCITON is a Gaussian orbital code for calculation of various spectroscopies of materials. It is capable of HartreeFock and DFT calculations on small unit cells and is interfaced to CRYSTAL09 for calculation of spectra for large unit cells. It uses the tetrahedron method for Brillouin zone integration and can be used for bulk systems, slabs and molecules. In particular it has been used for density of states and core and valence optical spectroscopy calculations and it is easily adapted to other spectroscopies. It makes full use of crystal symmetry in calculating matrix elements of operators.
Examples of total and atomprojected densities of states bulk materials (not to same scale)
Since CRYSTAL09 can perform all electron calculations, EXCITON can calculate core level optical absorption spectra including the optical matrix element whereas pseudopotential methods rely on a joint density of states calculation.
O1s core level spectra of wurtzite ZnO for the radiation electric vector parallel to three axes

The figure below shows examples of valence state dielectric functions computed using EXCITON for bulk Si, anatase TiO_{2} (with the electric vector parallel or perpendicular to the crystal c axis), a Si(001) slab with a c(4x2) dimer reconstruction with the electric vector perpendicular or parallel to the dimer rows and bulk SrTiO_{3}.
Examples of dielectric functions of bulk materials and slabs (not to same scale)
This work is sponsored by the Irish Higher Education Authority under the PRTLIV grant and by Science Foundation Ireland under grant number RFP/11/PHY/3047.
Optical spectra shown here are obtained from singleparticle hybrid density functional theory wave functions. They do not contain electronhole attraction and scattering which is included in the BetheSalpeter equation (BSE) approach to optical excitations and which results in some bound excitations in matter, called excitons. Following our earlier work with BSE calculations in a Gaussian orbital basis, a BSE module is being developed for the latest version of EXCITON. 