核-电子轨道耦合团簇方法中的电子-电子-质子三重激发和二级近似
Triple electron-electron-proton excitations and second-order approximations in nuclear-electronic orbital coupled cluster methods
摘要：
核量子效应的精确描述，如零点能量，是重要的建模范围广泛的化学和生物过程。在核电子轨道方法中，通过用分子轨道技术以量子力学方式处理电子和选定的原子核（通常是质子），以计算效率高的方式纳入了这种效应。在这里，我们实现和测试的近地天体耦合集群方法，明确包括三重电子-质子激发，其中两个电子和一个质子被同时激发。我们的计算表明，这种近地天体-CCSD（eep）方法提供了高度准确的质子密度和质子亲合能，优于任何以前研究过的近地天体方法。这些例子突出了电子-电子-质子三重激发对于精确描述核量子效应的重要性。此外，我们还实现和测试了二阶近似的单双耦合团簇（NEO-CC 2）方法，以及它的缩放反旋（SOS）版本。NEO-SOS $'$ -CC 2方法对电子-质子相关能以及电子-电子相关能中的反自旋和同自旋分量进行定标，在所研究的性质方面，其精度几乎与NEO-CCSD（eep）方法相同。由于其低计算成本，这种方法将使大分子系统的化学和光化学应用范围广泛。这项工作为近地天体框架内的广泛开发和应用奠定了基础。
英文摘要：
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-proton excitations, where two electrons and one proton are excited simultaneously. Our calculations show that this NEO-CCSD(eep) 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-CCSD(eep) 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 wide range of developments and applications within the NEO framework.
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