Parandekar Priya V, Hratchian Hrant P, Raghavachari Krishnan
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA.
J Chem Phys. 2008 Oct 14;129(14):145101. doi: 10.1063/1.2976570.
Hybrid QM:QM (quantum mechanics:quantum mechanics) and QM:MM (quantum mechanics:molecular mechanics) methods are widely used to calculate the electronic structure of large systems where a full quantum mechanical treatment at a desired high level of theory is computationally prohibitive. The ONIOM (our own N-layer integrated molecular orbital molecular mechanics) approximation is one of the more popular hybrid methods, where the total molecular system is divided into multiple layers, each treated at a different level of theory. In a previous publication, we developed a novel QM:QM electronic embedding scheme within the ONIOM framework, where the model system is embedded in the external Mulliken point charges of the surrounding low-level region to account for the polarization of the model system wave function. Therein, we derived and implemented a rigorous expression for the embedding energy as well as analytic gradients that depend on the derivatives of the external Mulliken point charges. In this work, we demonstrate the applicability of our QM:QM method with point charge embedding and assess its accuracy. We study two challenging systems--zinc metalloenzymes and silicon oxide cages--and demonstrate that electronic embedding shows significant improvement over mechanical embedding. We also develop a modified technique for the energy and analytic gradients using a generalized asymmetric Mulliken embedding method involving an unequal splitting of the Mulliken overlap populations to offer improvement in situations where the Mulliken charges may be deficient.
量子力学（QM:QM）和量子力学：分子力学（QM:MM）方法被广泛用于计算大型系统的电子结构，在这些系统中，以所需的高水平理论进行完全量子力学处理在计算上是不可行的。ONIOM（我们自己的N层集成分子轨道-分子力学）近似是较流行的混合方法之一，其中整个分子系统被划分为多个层次，每个层次采用不同的理论水平进行处理。在之前的一篇出版物中，我们在ONIOM框架内开发了一种新颖的QM:QM电子嵌入方案，其中模型系统被嵌入到周围低水平区域的外部穆利肯点电荷中，以考虑模型系统波函数的极化。在那里，我们推导并实现了一个关于嵌入能量以及依赖于外部穆利肯点电荷导数的解析梯度的严格表达式。在这项工作中，我们展示了我们带有点电荷嵌入的QM:QM方法的适用性并评估其准确性。我们研究了两个具有挑战性的系统——锌金属酶和氧化硅笼——并证明电子嵌入比机械嵌入有显著改进。我们还使用一种广义非对称穆利肯嵌入方法开发了一种用于能量和解析梯度的改进技术，该方法涉及穆利肯重叠布居的不等分，以在穆利肯电荷可能不足的情况下提供改进。
