We present an implementation and benchmark of new approximations in multireference algebraic diagrammatic construction theory for simulations of neutral electronic excitations and UV/vis spectra of strongly correlated molecular systems (MR-ADC). Following our work on the first-order MR-ADC approximation [J. Chem. Phys.2018, 149, 204113], we report the strict and extended second-order MR-ADC methods (MR-ADC(2) and MR-ADC(2)-X) that combine the description of static and dynamic electron correlation in the ground and excited electronic states without relying on state-averaged reference wave functions. We present an extensive benchmark of the new MR-ADC methods for excited states in several small molecules, including the carbon dimer, ethylene, and butadiene. Our results demonstrate that, for weakly correlated electronic states, the MR-ADC(2) and MR-ADC(2)-X methods outperform the third-order single-reference ADC approximation and are competitive with the results from equation-of-motion coupled cluster theory. For states with multireference character, the performance of the MR-ADC methods is similar to that of an N-electron valence perturbation theory. In contrast to conventional multireference perturbation theories, the MR-ADC methods have many attractive features, such as a straightforward and efficient calculation of excited-state properties and a direct access to excitations outside of the frontier (active) orbitals.

中文翻译：

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激发态的多参考代数图解构造理论：扩展二阶实现和基准

我们提出了多参考代数图解构造理论中新近似值的实现和基准，用于模拟中性电子激发和强相关分子系统 (MR-ADC) 的紫外/可见光谱。继我们对一阶 MR-ADC 近似的研究之后 [ J. Chem. 物理。2018年149, 204113]，我们报告了严格和扩展的二阶 MR-ADC 方法（MR-ADC(2) 和 MR-ADC(2)-X），它们结合了地面和激发电子中静态和动态电子相关性的描述。状态而不依赖于状态平均参考波函数。我们为几种小分子（包括碳二聚体、乙烯和丁二烯）中的激发态提供了新的 MR-ADC 方法的广泛基准。我们的结果表明，对于弱相关电子态，MR-ADC(2) 和 MR-ADC(2)-X 方法优于三阶单参考 ADC 近似，并与运动方程的结果竞争耦合聚类理论。对于具有多参考特征的状态，MR-ADC 方法的性能类似于 N 电子价微扰理论的性能。