Department of Physics, University of Pretoria, 0002 Pretoria, South Africa.
Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, CZ-12116 Prague 2, Czech Republic.
J Chem Phys. 2022 Sep 7;157(9):095103. doi: 10.1063/5.0100977.
The accuracy of approximate methods for calculating linear optical spectra depends on many variables. In this study, we fix most of these parameters to typical values found in photosynthetic light-harvesting complexes of plants and determine the accuracy of approximate spectra with respect to exact calculation as a function of the energy gap and interpigment coupling in a pigment dimer. We use a spectral density with the first eight intramolecular modes of chlorophyll a and include inhomogeneous disorder for the calculation of spectra. We compare the accuracy of absorption, linear dichroism, and circular dichroism spectra calculated using the Full Cumulant Expansion (FCE), coherent time-dependent Redfield (ctR), and time-independent Redfield and modified Redfield methods. As a reference, we use spectra calculated with the exact stochastic path integral evaluation method. We find the FCE method to be the most accurate for the calculation of all spectra. The ctR method performs well for the qualitative calculation of absorption and linear dichroism spectra when the pigments are moderately coupled (∼15cm), but ctR spectra may differ significantly from exact spectra when strong interpigment coupling (>100cm) is present. The dependence of the quality of Redfield and modified Redfield spectra on molecular parameters is similar, and these methods almost always perform worse than ctR, especially when the interpigment coupling is strong or the excitonic energy gap is small (for a given coupling). The accuracy of approximate spectra is not affected by resonance with intramolecular modes for typical system-bath coupling and disorder values found in plant light-harvesting complexes.
近似方法计算线性光学谱的准确性取决于许多变量。在本研究中，我们将大多数参数固定为植物光合作用光捕获复合物中发现的典型值，并确定在色素二聚体的能隙和色素间耦合的情况下，近似谱相对于精确计算的准确性。我们使用具有叶绿素 a 的前八个分子内模式的光谱密度，并为光谱计算包含非均匀无序。我们比较使用全累积展开 (FCE)、相干时变 Redfield (ctR) 和时不变 Redfield 和修正 Redfield 方法计算的吸收、线性二色性和圆二色性光谱的准确性。作为参考，我们使用用精确随机路径积分评估方法计算的光谱。我们发现 FCE 方法对于所有光谱的计算最准确。当色素中度耦合（约 15cm）时，ctR 方法对于吸收和线性二色性光谱的定性计算表现良好，但当存在强色素间耦合（>100cm）时，ctR 光谱可能与精确光谱有很大差异。Redfield 和修正 Redfield 光谱的质量对分子参数的依赖性相似，这些方法几乎总是比 ctR 表现更差，尤其是当色素间耦合很强或激子能隙很小时（对于给定的耦合）。对于典型的系统-浴耦合和在植物光捕获复合物中发现的无序值，近似谱的准确性不受与分子内模式共振的影响。
