A case study of density functional theory and domain-based local pair natural orbital coupled cluster for vibrational effects on EPR hyperfine coupling constants: vibrational perturbation theory versus <i>ab initio</i> molecular dynamics
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Local approximations of high-level <i>ab initio</i> methods make superior accuracy in the computation of molecular properties accessible by drastically decreasing computational times. As a consequence, these methods become applicable not only for large systems but also in schemes for which large numbers of calculations are necessary. In this work, we apply a recently developed open-shell implementation of the domain-based pair natural orbital coupled cluster singles doubles (DLPNO-CCSD) approach for the computation of vibrational corrections to the isotropic values of electron paramagnetic resonance (EPR) hyperfine coupling constants. We assess density functional theory (DFT) and DLPNO-CCSD approaches using two common but very different schemes: (1) vibrational perturbation theory based on equilibrium geometries, and (2) explicit canonical ensemble averages using configuration snapshots sampled from revPBE0-D3(0) <i>ab initio</i> molecular dynamics simulations. Both approaches are found to yield very similar results for the spin probe 2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl (HMI) and are both feasible for systems of around 30 atoms. However, the numerical stability required for higher derivatives can become a limitation for local correlation methods in the case of vibrational perturbation theory.
高精度从头算（ab initio）方法的局域近似方案，可通过大幅缩减计算耗时，使分子性质计算获得更优异的精度表现。正因如此，这类方法不仅可应用于大型分子体系，还能适用于需要开展大量计算的研究场景。本研究采用近期开发的域配对自然轨道耦合簇单双激发（domain-based pair natural orbital coupled cluster singles doubles, DLPNO-CCSD）开壳层实现方案，用于计算电子顺磁共振（electron paramagnetic resonance, EPR）超精细耦合常数各向同性值的振动修正项。我们采用两种常见却差异显著的方案对密度泛函理论（density functional theory, DFT）与DLPNO-CCSD方法进行评估：（1）基于平衡几何结构的振动微扰理论；（2）通过revPBE0-D3(0)从头算分子动力学模拟采样得到的构型快照，开展显式正则系综平均计算。研究发现，对于自旋探针2,2,3,4,5,5-六甲基全氢咪唑-1-氧基（2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, HMI），两种方案所得结果高度一致，且二者均可适用于约30个原子的分子体系。不过，在振动微扰理论的应用场景中，高阶导数计算所需的数值稳定性，可能会成为局域相关方法的应用限制。
