Stahl Terrence L, Sokolov Alexander Yu
Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, USA.
J Chem Phys. 2024 May 28;160(20). doi: 10.1063/5.0207684.
Algebraic diagrammatic construction (ADC) is a computationally efficient approach for simulating excited electronic states, absorption spectra, and electron correlation. Due to their origin in perturbation theory, the single-reference ADC methods may be susceptible to spin contamination when applied to molecules with unpaired electrons. In this work, we develop an approach to quantify spin contamination in the ADC calculations of electronic excitations and apply it to a variety of open-shell molecules starting with either the unrestricted (UHF) or restricted open-shell (ROHF) Hartree-Fock reference wavefunctions. Our results show that the accuracy of low-order ADC approximations [ADC(2) and ADC(3)] significantly decreases when the UHF reference spin contamination exceeds 0.05 a.u. Such strongly spin-contaminated molecules exhibit severe excited-state spin symmetry breaking that contributes to decreasing the quality of computed excitation energies and oscillator strengths. In a case study of phenyl radical, we demonstrate that spin contamination can significantly affect the simulated UV/Vis spectra, altering the relative energies, intensities, and order of electronic transitions. The results presented here motivate the development of spin-adapted ADC methods for open-shell molecules.
代数图形构造（ADC）是一种用于模拟激发电子态、吸收光谱和电子关联的计算效率高的方法。由于其起源于微扰理论，单参考ADC方法在应用于具有未成对电子的分子时可能易受自旋污染的影响。在这项工作中，我们开发了一种方法来量化电子激发的ADC计算中的自旋污染，并将其应用于各种开壳层分子，这些分子从无限制（UHF）或限制开壳层（ROHF）哈特里 - 福克参考波函数开始。我们的结果表明，当UHF参考自旋污染超过0.05 a.u.时，低阶ADC近似[ADC(2)和ADC(3)]的精度会显著降低。这种强自旋污染的分子表现出严重的激发态自旋对称性破缺，这导致计算的激发能和振子强度的质量下降。在苯自由基的案例研究中，我们证明自旋污染会显著影响模拟的紫外/可见光谱，改变电子跃迁的相对能量、强度和顺序。这里给出的结果推动了用于开壳层分子的自旋适应ADC方法的发展。
