The electronic properties of the fucoxanthin chromophore were studied by using MNDO-PSDCI,36 –40 (link) SAC-CI41 (link),42 and TD-DFT43 (link)–45 (link) molecular orbital theory. All calculations were based on a ground-state B3LYP/6-31G(d) minimized geometry.46 The MNDO-PSDCI calculations included the eight highest energy occupied π orbitals, the three highest energy occupied σ orbitals, the eight lowest energy unoccupied π orbitals and the lowest energy unoccupied σ orbital. Full single and double CI was carried out within the π system and full single CI within the σ system. In some calculations, quadruple configurations involving the π orbitals were introduced by using a coupled cluster perturbative approximation. The one photon spectra and the output from the molecular orbital calculations were analyzed by using MathScriptor, which also generated Figure 5 and Figure 6. The MNDO-PSDCI and MathScriptor programs are available by contacting RR Birge (rbirge@uconn.edu). The TD-DFT calculations used various functionals ranging from low-correlation PBE1PBE to the high correlation SVWN (or LSDA) functionals.44 ,45 (link) The SAC-CI calculations were carried out using the D95 basis set and included energy-selected single and double CI with the highest energy unoccupied and the lowest energy unoccupied orbitals.41 (link),42 The ground-state minimizations, SAC-CI and TD-DFT calculations were carried out within the Gaussian 03 program.46