Teuteberg Thorsten L, Eckhoff Marco, Mata Ricardo A
Institut für Physikalische Chemie, Universität Göttingen, Tammannstraße 6, Göttingen D-37077, Germany.
J Chem Phys. 2019 Apr 21;150(15):154118. doi: 10.1063/1.5080427.
An additive quantum mechanics/molecular mechanics (QM/MM) model for the theoretical investigation of molecular crystals (AC-QM/MM) is presented. At the one-body level, a single molecule is chosen as the QM region. The MM region around it consists of a finite cluster of explicit MM atoms, represented by point charges and Lennard-Jones potentials, with additional background charges to mimic periodic electrostatics. Cluster charges are QM-derived and calculated self-consistently to ensure a polarizable embedding. We have also considered the extension to many-body QM corrections, calculating the interactions of a central molecule to neighboring units in the crystal. Full gradient expressions have been derived, also including symmetry information. The scheme allows for the calculation of molecular properties as well as unconstrained optimizations of the molecular geometry and cell parameters with respect to the lattice energy. Benchmarking the approach with the X23 reference set confirms the convergence pattern of the many-body extension although a comparison to plane-wave density functional theory reveals a systematic overestimation of cohesive energies by 6-16 kJ mol. While the scheme primarily aims to provide an inexpensive and flexible way to model a molecule in a crystal environment, it can also be used to reach highly accurate cohesive energies by the straightforward application of wave function correlated approaches. Calculations with local coupled cluster with singles, doubles, and perturbative triples, albeit limited to numerical gradients, show an impressive agreement with experimental estimates for small molecular crystals.
本文提出了一种用于分子晶体理论研究的加性量子力学/分子力学（QM/MM）模型（AC-QM/MM）。在单体水平上，选择单个分子作为量子力学区域。其周围的分子力学区域由有限的显式分子力学原子簇组成，用点电荷和 Lennard-Jones 势表示，并带有额外的背景电荷以模拟周期性静电作用。簇电荷由量子力学导出并自洽计算，以确保可极化嵌入。我们还考虑了向多体量子力学修正的扩展，计算中心分子与晶体中相邻单元的相互作用。已经推导了完整的梯度表达式，其中也包括对称性信息。该方案允许计算分子性质，以及针对晶格能量对分子几何结构和晶胞参数进行无约束优化。用 X23 参考集对该方法进行基准测试证实了多体扩展的收敛模式，尽管与平面波密度泛函理论的比较表明内聚能存在 6 - 16 kJ/mol 的系统高估。虽然该方案主要旨在提供一种在晶体环境中对分子进行建模的廉价且灵活的方法，但通过直接应用波函数相关方法，它也可用于获得高精度的内聚能。使用含单、双和微扰三重激发的局部耦合簇方法进行的计算，尽管仅限于数值梯度，但与小分子晶体的实验估计值显示出令人印象深刻的一致性。
