Theoretical calculations on electronic excitation energies and absorption cross sections of the H3 defect center in diamond were performed. We constructed model clusters with up to 180 atoms to imitate the local environment of the defect center and conducted first-principle calculations. TD-DFT predicted most accurately vertical excitation energy, whereas TD-HF and CIS provided rough estimations for one- and two-photon absorption cross sections. It was found that a model cluster with a diameter larger than similar to 1.0 nm is required, while relatively low-cost level of theory and basis set are sufficient for characterizing the excitation properties. (c) 2009 Elsevier B.V. All rights reserved.