The electronically excited states of the Si(100) surface and acetylene, benzene, and 9,10-phenanthrenequinone adsorbed on Si(100) are studied with time-dependent density functional theory. The computational cost of these calculations can be reduced through truncation of the single excitation space. This allows larger cluster models of the surface in conjunction with large adsorbates to be studied. On clean Si(100), the low-lying excitations correspond to transitions between the π orbitals of the silicon-silicon dimers. These excitations are predicted to occur in the range 0.4-2 eV. When organic molecules are adsorbed on the surface, surface f molecule, molecule f surface, and electronic excitations localized within the adsorbate are also observed at higher energies. For acetylene and benzene, the remaining ππ* excitations are found to lie at lower energies than in the corresponding gas-phase species. Even though the aromaticity of 9,10-phenanthrenequinone is retained, significant shifts in the ππ* excitations of the aromatic rings are predicted. This is in part due to structural changes that occur upon adsorption.
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