多组分CASSCF的再认识：质量精确的质子密度需要大的活性空间
Multicomponent CASSCF Revisited: Large Active Spaces are Needed for Qualitatively Accurate Protonic Densities
摘要：
多组分方法试图处理选定的原子核，通常是质子，完全量子力学和等效的化学系统的电子。在这样的方法中，众所周知，由于忽略了电子-质子相关性，电子-质子相互作用的Hartree-Fock（HF）描述灾难性地失败，导致定性地不正确的质子性质。在单组分量子化学中，HF的定性失败通常表明需要多参考方法，如完全活性空间自洽场（CASSCF）。虽然多组分CASSCF方法在近20年前就已经实现，但它只能对非常小的活性空间（~105个多组分构型）进行计算。因此，为了扩展多组分CASSCF方法的适用范围，本研究推导并实现了一种新的两步多组分CASSCF方法，该方法使用多组分热浴配置相互作用的配置相互作用步骤，使计算具有非常大的活性空间（48个轨道中多达16个电子）。我们发现，需要大的电子活性空间，以获得定性准确的质子密度的HCN和FHF-分子。此外，这里实现的多组分CASSCF方法应该有进一步的应用双阱质子势和系统，本质上是电子多参考。
英文摘要：
Multicomponent methods seek to treat select nuclei, typically protons, fully quantum mechanically and equivalent to the electrons of a chemical system. In such methods, it is well known that due to the neglect of electron-proton correlation, a Hartree-Fock (HF) description of the electron-proton interaction catastrophically fails leading to qualitatively incorrect protonic properties. In single-component quantum chemistry, the qualitative failure of HF is normally indicative of the need for multireference methods such as complete active space self-consistent field (CASSCF). While a multicomponent CASSCF method was implemented nearly twenty years ago, it is only able to perform calculations with very small active spaces (~105 multicomponent configurations). Therefore, in order to extend the realm of applicability of the multicomponent CASSCF method, this study derives and implements a new two-step multicomponent CASSCF method that uses multicomponent heat-bath configuration interaction for the configuration interaction step, enabling calculations with very large active spaces (up to 16 electrons in 48 orbitals). We find that large electronic active spaces are needed to obtain qualitatively accurate protonic densities for the HCN and FHF- molecules. Additionally, the multicomponent CASSCF method implemented here should have further applications for double-well protonic potentials and systems that are inherently electronically multireference.
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