Brorsen Kurt R
Department of Chemistry, University of Missouri, Columbia, Missouri 65203, United States.
J Chem Theory Comput. 2020 Apr 14;16(4):2379-2388. doi: 10.1021/acs.jctc.9b01273. Epub 2020 Mar 5.
Multicomponent quantum chemical methods seek to include nuclear quantum effects of select nuclei in quantum chemistry calculations by not invoking the Born-Oppenheimer approximation for these nuclei. In multicomponent methods, the inclusion of electron-proton correlation is essential for obtaining even qualitatively accurate protonic densities. However, most of the recently developed multicomponent methods have either used or obtained molecular orbitals from a single-reference mean-field wave function that neglects all electron-proton correlation that is analogous to using Hartree-Fock orbitals in a single-component framework. We examine the consequences of using Hartree-Fock orbitals in multicomponent calculations by developing the multicomponent heat-bath configuration interaction (HCI) method. Multicomponent HCI is a multicomponent selected configuration interaction (CI) technique that enables an accurate approximation of a complete active space or truncated CI wave function for systems with large active spaces. The multicomponent HCI method is shown to reproduce the ground-state protonic density of the HeHHe, HCN, and FHF systems when compared to reference grid-based calculations. For all three systems, the coefficient of the leading configuration in the wave function expansion is less than 0.95, indicating that all systems have multireference character. This is highly noteworthy as none of the systems have multireference character in a single-component framework and suggests that multireference character appears inherent to or at least more commonly in a multicomponent framework than a single-component framework. Even when natural orbitals are used rather than Hartree-Fock orbitals for the multicomponent HCI calculations, aspects of the multireference character remain for FHF and HCN. Consequences and implications of the multireference character of multicomponent quantum chemical systems are discussed.
多组分量子化学方法试图通过不对这些原子核调用玻恩 - 奥本海默近似，在量子化学计算中纳入选定原子核的核量子效应。在多组分方法中，包含电子 - 质子相关性对于获得哪怕是定性准确的质子密度都是至关重要的。然而，最近开发的大多数多组分方法要么使用单参考平均场波函数来获取分子轨道，要么从该波函数中得到分子轨道，而这种波函数忽略了所有类似于在单组分框架中使用哈特里 - 福克轨道时的电子 - 质子相关性。我们通过开发多组分热浴组态相互作用（HCI）方法来研究在多组分计算中使用哈特里 - 福克轨道的后果。多组分HCI是一种多组分选定组态相互作用（CI）技术，它能够对具有大活性空间的系统的完整活性空间或截断CI波函数进行精确近似。与基于参考网格的计算相比，多组分HCI方法被证明能够重现HeHHe、HCN和FHF系统的基态质子密度。对于所有这三个系统，波函数展开中主导组态的系数小于0.95，这表明所有系统都具有多参考特征。这是非常值得注意的，因为在单组分框架中这些系统都没有多参考特征，这表明多参考特征在多组分框架中似乎是固有的，或者至少比在单组分框架中更常见。即使在多组分HCI计算中使用自然轨道而非哈特里 - 福克轨道，FHF和HCN的多参考特征的某些方面仍然存在。本文讨论了多组分量子化学系统多参考特征的后果和影响。
