Christlmaier Evelin Martine, Kats Daniel, Alavi Ali, Usvyat Denis
Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany.
Max Planck Institute for Solid State Research, Heisenbergstraße 1, D-70569 Stuttgart, Germany.
J Chem Phys. 2022 Apr 21;156(15):154107. doi: 10.1063/5.0084040.
We present an embedded fragment approach for high-level quantum chemical calculations on local features in periodic systems. The fragment is defined as a set of localized orbitals (occupied and virtual) corresponding to a converged periodic Hartree-Fock solution. These orbitals serve as the basis for the in-fragment post-Hartree-Fock treatment. The embedding field for the fragment, consisting of the Coulomb and exchange potential from the rest of the crystal, is included in the fragment's one-electron Hamiltonian. As an application of the embedded fragment approach, we investigate the performance of full configuration interaction quantum Monte Carlo (FCIQMC) with the adaptive shift. As the orbital choice, we use the natural orbitals from the distinguishable cluster method with singles and doubles. FCIQMC is a stochastic approximation to the full CI method and can be routinely applied to much larger active spaces than the latter. This makes this method especially attractive in the context of open shell defects in crystals, where fragments of adequate size can be rather large. As a test case, we consider dissociation of a fluorine atom from a fluorographane surface. This process poses a challenge for high-level electronic structure models as both the static and dynamic correlations are essential here. Furthermore, the active space for an adequate fragment (32 electrons in 173 orbitals) is already quite large even for FCIQMC. Despite this, FCIQMC delivers accurate dissociation and total energies.
我们提出了一种嵌入式片段方法，用于对周期性系统中的局部特征进行高级量子化学计算。片段被定义为一组对应于收敛的周期性哈特里 - 福克解的定域轨道（占据和虚拟）。这些轨道作为片段内后哈特里 - 福克处理的基础。片段的嵌入场由晶体其余部分的库仑势和交换势组成，包含在片段的单电子哈密顿量中。作为嵌入式片段方法的一个应用，我们研究了具有自适应移位的全组态相互作用量子蒙特卡罗（FCIQMC）的性能。作为轨道选择，我们使用来自具有单双激发的可区分簇方法的自然轨道。FCIQMC是全CI方法的一种随机近似，并且可以常规地应用于比后者大得多的活性空间。这使得该方法在晶体中的开壳缺陷情况下特别有吸引力，在这种情况下，足够大小的片段可能相当大。作为一个测试案例，我们考虑氟原子从氟代石墨烯表面解离的过程。这个过程对高级电子结构模型提出了挑战，因为这里静态和动态相关性都是必不可少的。此外，即使对于FCIQMC，一个足够片段的活性空间（173个轨道中的32个电子）已经相当大。尽管如此，FCIQMC仍能提供准确的解离能和总能量。
