We present a study of ground state conformations of all-trans beta-carotene using Density Functional Theory (DFT). To reproduce the carotenoid spectrum, the DFT approach was combined with the Multi-Reference Configuration Interaction. Our results show that the global minimum corresponds to an asymmetric structure where the b-ionone rings are twisted with respect to the polyene chain. The next higher-lying conformer is more s-cis symmetric and is populated at room temperature (30%). We discuss the relation of these conformers to S* and show that our model readily explains the temperature dependence and the narrowing of the ground state bleach at long population times. (C) 2011 Elsevier B.V. All rights reserved.