Quantum chemical excited state calculations on pigment-protein complexes require thorough geometry re-optimization of experimental crystal structures

Calculations of vertical excited states of a strongly coupled chlorophyll pair and a carotenoid-chlorophyll complex stemming from the light harvesting complex II of green plants employing the experimentally determined crystal structures and quantum chemically re-optimized model complexes demonstrate the need for preceding re-optimizations of the experimental structures at quantum chemical level. While in the case of the chlorophyll dimers, the re-optimization step is crucial for a correct description of the coupling of the excited states, in carotenoid-chlorophyll complexes the S-1 excitation energies of carotenoids depend strongly on the structure, in particular on the correct bond lengths alternation pattern of its conjugated double bond chain, which is not sufficiently accurately reproduced by experimental structures.