Coulombic couplings between pigments in the major light-harvesting complex LHC II calculated by the transition density cube method

We present ab initio transition density cube (TDC) calculations of the coulombic couplings between chlorophyll and carotenoid pigments in the major light-harvesting complex of photosystem II (LHC II) based on the 2.72 Å structure [Liu et al., Nature 428(2004) 287–292]. A comparison with couplings calculated by the ideal dipole approximation (IDA) demonstrate that for inter-pigment distances of less than ∼25 Å the IDA-values can deviate by up to one order of magnitude from the exact values calculated by the TDC-method. The largest deviations are observed for interactions involving Qx states because of a significant multipole character of the corresponding Qx transitions.