Multireference methods such as multistate complete active space second-order perturbation theory (MS-CASPT2) and multiconfiguration pair-density functional theory (MC-PDFT) can be very accurate, but they have the disadvantage that they are not black-box methods, and finding a good active space for the reference wave function often requires nonsystematic procedures based on intimate knowledge of the system under study. In this paper, we propose a scheme called the ABC scheme, which is a three-step calculation using three parameters, for automatic selection (without looking at the orbitals and without using any knowledge of the specific system at hand) of the active space (including both the size of the active space and the orbitals in the active space) for MS-CASPT2 or MC-PDFT calculations, and we report tests of the scheme on the first five excitation energies for a set of ten doublet radicals. The results show that the ABC scheme is very successful for both MS-CASPT2 and MC-PDFT for all ten systems considered here.

中文翻译：

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通过MS-CASPT2或MC-PDFT自动选择用于计算电子激发光谱的活动空间

多参考方法，例如多状态完全活动空间二阶摄动理论（MS-CASPT2）和多配置对密度泛函理论（MC-PDFT）可能非常准确，但是它们的缺点是它们不是黑盒方法，并且为参考波函数寻找良好的活动空间通常需要基于对所研究系统的深入了解的非系统程序。在本文中，我们提出了一种称为ABC方案的方案，该方案是使用三个参数进行的三步计算，用于自动选择（无需查看轨道并且无需使用手头的特定系统的任何知识）活动空间（包括用于MS-CASPT2或MC-PDFT计算的有效空间大小和有效空间轨道），并且我们报告了该方案在一组十个双重态自由基中的前五个激发能的测试结果。结果表明，对于此处考虑的所有十个系统，MS-CASPT2和MC-PDFT的ABC方案都非常成功。