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In previous work we have shown that the Density Matrix Renormalization Group(DMRG) enables near-exact calculations in active spaces much larger than arepossible with traditional Complete Active Space algorithms. Here, we implementorbital optimisation with the Density Matrix Renormalization Group to furtherallow the self-consistent improvement of the active orbitals, as is done in theComplete Active Space Self-Consistent Field (CASSCF) method. We use ourresulting DMRGCASSCF method to study the low-lying excited states of theall-trans polyenes up to C24H26 as well as \beta-carotene, correlating withnear-exact accuracy the optimised complete \pi-valence space with up to 24active electrons and orbitals, and analyse our results in the light of therecent discovery from Resonance Raman experiments of new optically dark statesin the spectrum.

Orbital Optimization in the Density Matrix Renormalization Group, with applications to polyenes and \beta-carotene