Here, we propose a new method to analyze various electronic properties of molecules based on natural perturbation orbitals (NPOs). We applied the proposed method to chemical enhancement of the surface‐enhanced Raman scattering (SERS) intensity of M2–pyrazine–M2 (M = Cu, Ag, Au) complexes. The SERS intensity can be effectively decomposed into the contributions of four NPO pairs (1σ–1σ*, 2σ–2σ*, 1π–1π*, and 2π–2π*), so NPO analysis makes the SERS intensity much easier to understand than by conventional canonical molecular orbitals. Moreover, we analyzed the dependence of the density functional theory functional on the SERS intensity. For the Ag2–pyrazine–Ag2 complex, the BP86 functional overestimates the Raman intensity by about 23 times compared with coupled‐cluster singles and doubles level of theory, while the CAM‐B3LYP functional gives moderately accurate values. This overestimation arises from the inaccuracy of the energy derivative along the normal vibrational mode.

中文翻译：

---

M2-吡嗪-M2（M = Cu、Ag、Au）的表面增强拉曼散射：通过自然扰动轨道和密度泛函理论函数依赖分析

在这里，我们提出了一种基于自然微扰轨道 (NPO) 分析分子各种电子特性的新方法。我们将所提出的方法应用于 M2-吡嗪-M2（M = Cu、Ag、Au）配合物的表面增强拉曼散射（SERS）强度的化学增强。SERS 强度可以有效地分解为四个 NPO 对（1σ–1σ*、2σ–2σ*、1π–1π* 和 2π–2π*）的贡献，因此 NPO 分析使 SERS 强度比常规规范分子轨道。此外，我们分析了密度泛函理论函数对 SERS 强度的依赖性。对于 Ag2-pyrazine-Ag2 复合物，BP86 函数将拉曼强度高估了约 23 倍，与耦合簇单打和双打理论水平相比，而 CAM-B3LYP 函数给出了中等准确的值。这种高估源于沿正常振动模式的能量导数的不准确。