In photosynthesis, light is captured by antenna proteins. These proteins transfer the excitation energy with almost 100% quantum efficiency to the reaction centers, where charge separation takes place. The time scale and pathways of this transfer are controlled by the protein scaffold, which holds the pigments at optimal geometry and tunes their excitation energies (site energies). The detailed understanding of the tuning of site energies by the protein has been an unsolved problem since the first high-resolution crystal structure of a light-harvesting antenna appeared >30 years ago [Fenna RE, Matthews BW (1975) Nature 258:573-577].Here, we present a combined quantum chemical/electrostatic approach to compute site energies that considers the whole protein in atomic detail and provides the missing link between crystallography and spectroscopy. The calculation of site energies of the Fenna-Matthews-Olson protein results in optical spectra that are in quantitative agreement with experiment and reveals an unexpectedly strong influence of the backbone of two ␣-helices. The electric field from the latter defines the direction of excitation energy flow in the Fenna-Matthews-Olson protein, whereas the effects of amino acid side chains, hitherto thought to be crucial, largely compensate each other. This result challenges the current view of how energy flow is regulated in pigment-protein complexes and demonstrates that attention has to be paid to the backbone architecture.energy transfer ͉ light-harvesting ͉ optical spectra ͉ photosynthesis ͉ structure-based simulation P hotosynthesis is the fundamental biological process in which solar energy is converted into biomass. The first step is the capture of light by arrays of protein-bound dye molecules (pigments). These pigment-protein complexes (PPCs) are therefore termed light-harvesting complexes or antenna proteins (1). They transfer the excitation energy with high quantum yield to specialized PPCs, the reaction centers, where the energy is used to trigger the chemical modification of substrates. To guide the excitation energy flow in a certain direction there has to be an energy sink, that is, the pigments in the target region are required to absorb at lower energies than the initially excited chromophores. A complication of this simple picture arises from long-range electrostatic interactions between the local excitations (excitonic couplings), which are a prerequisite for energy transfer. These couplings cause the excited states of the PPC (exciton states) to be delocalized, that is, their electronic wave functions contain contributions of several pigments in the complex. Directed energy transport results from energetic relaxation transferring population between exciton states of different spatial extents. The latter depend crucially on excitonic couplings and site energies, so that the elucidation of energy-transfer mechanisms on the basis of spectroscopic data (2-4) and crystal structures (5-7) requires knowledge of both these quantities (8, 9), which are not direct...

In this paper we report first-principles calculations on the ground-state electronic structure of two infinite one-dimensional systems: ͑a͒ a chain of carbon atoms and ͑b͒ a chain of alternating boron and nitrogen atoms. Meanfield results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by second-order Mo "ller-Plesset perturbation theory and the coupled-cluster approach. The calculations were performed using 6-31G** basis sets, including the d-type polarization functions. Both at the Hartree-Fock ͑HF͒ and the correlated levels, we find that the infinite carbon chain exhibits bond alternation with alternating single and triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In addition, we also performed density-functional-theory-based local-density-approximation ͑LDA͒ calculations on the infinite carbon chain using the same basis set. Our LDA results, in contradiction to our HF and correlated results, predict a very small bond alternation. Based upon our LDA results for the carbon chain, which are in agreement with an earlier LDA calculation ͓E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488 ͑1998͔͒, we conclude that the LDA significantly underestimates Peierls distortion. This emphasizes that the inclusion of many-particle effects is very important for the correct description of Peierls distortion in onedimensional systems.

Results of Wannier orbital-based Hartree–Fock and various correlated ab initio calculations using 6–31G** basis sets are reported for the two boron–nitrogen polymer systems polyaminoborane [BNH4]∞ and polyiminoborane [BNH2]∞. At the Hartree–Fock level the calculated equilibrium geometries, cohesive energies, polymerization energies, and band structures are virtually identical with those obtained from the standard Bloch orbital-based approach. Electron correlation effects on the investigated ground state properties are discussed within Mo/ller–Plesset second-order perturbation theory and coupled-cluster singles, doubles, and triples theory. For polyaminoborane no bond alternation is found in contrast to previous studies. Correlation corrections to the band structures are considered in second-order Mo/ller–Plesset perturbation theory with third-order localization diagrams included. They lead to a decrease of the fundamental gap of polyaminoborane and polyiminoborane by 40% and 51%, respectively, and reduce the band dispersions.

Correlated ab initio electronic structure calculations are reported for the polymers lithium hydride chain [LiH] ∞ and beryllium hydride [Be 2 H 4 ] ∞ . First, employing a Wannier-function-based approach, the systems are studied at the Hartree-Fock level, by considering chains, simulating the infinite polymers. Subsequently, for the model system [LiH] ∞ , the correlation effects are computed by considering virtual excitations from the occupied HartreeFock Wannier functions of the infinite chain into the complementary space of localized unoccupied orbitals, employing a full-configuration-interaction scheme. For [Be 2 H 4 ] ∞ , however, the electron correlation contributions to its ground state energy are calculated by considering finite clusters of increasing size modelling the system. Methods such as Møller-Plesset second-order perturbation theory and coupled-cluster singles, doubles and triples level of theory were employed. Equilibrium geometry, cohesive energy and polymerization energy are presented for both polymers, and the rapid convergence of electron correlation effects, when based upon a localized orbital scheme, is demonstrated.