In the full optimized reaction space and valence-complete active space self-consistent field (vCAS) methods, a set of active orbitals is defined as the union of the valence orbitals on the atoms, all possible configurations involving the active orbitals are generated, and the orbitals and configuration coefficients are self-consistently optimized. Such wave functions have tremendous flexibility, which makes these methods incredibly powerful but can also lead to inconsistencies in the description of the electronic structure of molecules. In this paper, the problems that can arise in vCAS calculations are illustrated by calculations on the BH and BF molecules. BH is well described by the full vCAS wave function, which accounts for molecular dissociation and 2s–2p near-degeneracy in the boron atom. The same is not true for the full vCAS wave function for BF. There is mixing of core and active orbitals at short internuclear distances and swapping of core and active orbitals at large internuclear distances. In addition, the virtual 2π orbitals, which were included in the active space to account for the 2s–2p near degeneracy effect, are used instead to describe radial correlation of the electrons in the F2pπ-like pairs. Although the above changes lead to lower vCAS energies, they lead to higher vCAS+1+2 energies as well as irregularities and/or discontinuities in the potential energy curves. All of the above problems can be addressed by using the spin-coupled generalized valence bond-inspired vCAS wave function for BF, which includes only a subset of the atomic valence orbitals in the active space.

中文翻译：

---

一个警示故事：高炉基态（X1Σ+）的化合价-CASSCF描述中的问题。

在完全优化的反应空间和价完整的有源空间自洽场（vCAS）方法中，一组有源轨道定义为原子上的价轨道的并集，生成了所有与有源轨道有关的可能构型，并且轨道和配置系数是自洽优化的。这种波函数具有极大的灵活性，这使这些方法异常强大，但也可能导致分子电子结构描述不一致。本文通过对BH和BF分子的计算说明了vCAS计算中可能出现的问题。完整的vCAS波函数很好地描述了BH，这说明了分子解离和2 s –2 p硼原子的近简并。对于BF的完整vCAS波功能，情况并非如此。核间距离短时，核心轨道和活动轨道混合，而核间距离大时，存在核心和活动轨道的交换。另外，包含在活动空间中的虚拟2π轨道用来解释2 s –2 p的近简并效应，用来描述F2 p中电子的径向相关性似π对。尽管上述变化导致较低的vCAS能量，但它们导致较高的vCAS + 1 + 2能量以及势能曲线中的不规则和/或不连续性。上述所有问题都可以通过使用自旋耦合的，由BF激发的广义价键激发的vCAS波函数解决，该函数仅包含活动空间中原子价轨道的一个子集。