Benchmarking Calculations of Excitonic Couplings between Bacteriochlorophylls

Abstract

Excitonic couplings between (bacterio)chlorophyll molecules are necessary for simulating energy transport in photosynthetic complexes. Many techniques for calculating the couplings are in use, from the simple (but inaccurate) point-dipole approximation to fully quantum-chemical methods. We compared several approximations to determine their range of applicability, noting that the propagation of experimental uncertainties poses a fundamental limit on the achievable accuracy. In particular, the uncertainty in crystallographic coordinates yields an uncertainty of about 20% in the calculated couplings. Because quantum-chemical corrections are smaller than 20% in most biologically relevant cases, their considerable computational cost is rarely justified. We therefore recommend the electrostatic TrEsp method across the entire range of molecular separations and orientations because its cost is minimal and it generally agrees with quantum-chemical calculations to better than the geometric uncertainty. Understanding these uncertainties can guard against striving for unrealistic precision; at the same time, detailed benchmarks can allow important qualitative questions-which do not depend on the precise values of the simulation parameters-to be addressed with greater confidence about the conclusions.

Importance and Nature of Short-Range Excitonic Interactions in Light Harvesting Complexes and Organic Semiconductors

“…The phytyl chain of BChlA and ChlA was substituted with a methyl group since its effects on the excited state properties are not large enough to justify the additional computational cost of including it. 60 A plausible structure of the disordered phase of zinc phthalocyanine (ZnPc) was obtained from a molecular dynamics simulation performed with the GROMACS 5.0.5 software 61 using a force field parametrized for ZnPc. 62 A sample of 90 molecules starting from the experimental crystalline phase 63 was first equilibrated at 300 K (for 5 ns), then heated (over 5 ns) and equilibrated at 700 K (for 30 ns) under NVT conditions using a Nose-Hoover thermostat.…”

“…Considering that the laser pulses used to excite the system in pumpprobe experiments usually have a duration of 10-50 fs, 68 resulting in an excitation bandwidth of 0.08-0.4 eV, it seems likely that S2 plays an important role in the ultrafast dynamics of the lowest excitons in these systems 69,70 and it may influence the coupling between S1 at close separations. 60 The importance of considering S2 has been recognized for phthalocyanines 5 and porphyrins 71 although often only S1 of BChlA is considered in studies of excitons in FMO 8 and LH2. 9 Therefore, with the exception of peridinin (where ΔE12 = 1.1 eV and ΔE23 = 0.3 eV), all intermolecular excitonic couplings J11, J22, J12 and J21 were considered.…”

Ultrafast charge-transfer in organic photovoltaic interfaces: geometrical and functionalization effects

“…1b within the framework of the quantum-classical hybrid dynamics employing the Ehrenfest dynamics 31,32 . Other theoretical approaches using quantum-chemistry methods, such as MS-CASPT2, fragment excitation-difference, and configuration interaction, would be more suitable to be used as the exciton-charge coupling is explicitly included at a considerable computational cost [35][36][37] . This initial electronic structure is then evolved on time t> 0 over several femtoseconds within the time-dependent density functional formalism 33 .…”

Delocalized excitons in natural light-harvesting complexes

“…Arguably, a definition of atomic charges that is physically accurate and adequate for electrostatic interaction is the one based on electrostatic potential (ESP) fitting. The calculation of the Coulomb coupling with transition ESP charges (TrEsp) was developed by Renger and coworkers (Madjet et al, 2006), and represents now a widely used method (Kenny and Kassal, 2016;Olbrich and Kleinekathöfer, 2010;van der Vegte et al, 2015).…”