Pavošević Fabijan, Hammes-Schiffer Sharon
Center for Computational Quantum Physics, Flatiron Institute, 162 5th Ave., New York, New York 10010, USA.
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, USA.
J Chem Phys. 2022 Aug 21;157(7):074104. doi: 10.1063/5.0106173.
The accurate description of nuclear quantum effects, such as zero-point energy, is important for modeling a wide range of chemical and biological processes. Within the nuclear-electronic orbital (NEO) approach, such effects are incorporated in a computationally efficient way by treating electrons and select nuclei, typically protons, quantum mechanically with molecular orbital techniques. Herein, we implement and test a NEO coupled cluster method that explicitly includes the triple electron-electron-proton excitations, where two electrons and one proton are excited simultaneously, using automatic differentiation Our calculations show that this NEO-CCSDT method provides highly accurate proton densities and proton affinities, outperforming any previously studied NEO method. These examples highlight the importance of the triple electron-electron-proton excitations for an accurate description of nuclear quantum effects. Additionally, we also implement and test the second-order approximate coupled cluster with singles and doubles (NEO-CC2) method as well as its scaled-opposite-spin (SOS) versions. The NEO-SOS'-CC2 method, which scales the electron-proton correlation energy as well as the opposite-spin and same-spin components of the electron-electron correlation energy, achieves nearly the same accuracy as the NEO-CCSDT method for the properties studied. Because of its low computational cost, this method will enable a wide range of chemical and photochemical applications for large molecular systems. This work sets the stage for a variety of developments and applications within the NEO framework.
对核量子效应（如零点能量）进行准确描述，对于模拟广泛的化学和生物过程至关重要。在核 - 电子轨道（NEO）方法中，通过使用分子轨道技术对电子和选定的原子核（通常是质子）进行量子力学处理，以计算高效的方式纳入了此类效应。在此，我们实现并测试了一种NEO耦合簇方法，该方法明确包含三电子 - 电子 - 质子激发，即两个电子和一个质子同时被激发，采用自动微分技术。我们的计算表明，这种NEO - CCSDT方法提供了高度准确的质子密度和质子亲和势，优于任何先前研究过的NEO方法。这些例子突出了三电子 - 电子 - 质子激发对于准确描述核量子效应的重要性。此外，我们还实现并测试了含单双激发的二阶近似耦合簇（NEO - CC2）方法及其反自旋缩放（SOS）版本。NEO - SOS' - CC2方法对电子 - 质子相关能以及电子 - 电子相关能的反自旋和同自旋分量进行缩放，对于所研究的性质，其精度几乎与NEO - CCSDT方法相同。由于其计算成本低，该方法将能够应用于大分子系统的广泛化学和光化学应用。这项工作为NEO框架内的各种发展和应用奠定了基础。
