Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan.
Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan.
J Chem Theory Comput. 2020 Apr 14;16(4):2606-2616. doi: 10.1021/acs.jctc.9b01032. Epub 2020 Mar 10.
Electronic resonances are metastable ( + 1) electron states, in other words, discrete states embedded in an electronic continuum. While great progress has been made for certain types of resonances-for example, temporary anions created by attaching one excess electron to a closed shell neutral-resonances in general remain a great challenge of quantum chemistry because a successful description of the decay requires a balanced description of the bound and continuum aspect of the resonance. Here, a smoothed Voronoi complex absorbing potential (CAP) is combined with the XMS-CASPT2 method, which enables us to address the balance challenge by appropriate choice of the CAS space. To reduce the computational cost, the method is implemented in the projected scheme. In this pilot application, three temporary anions serve as benchmarks: the π* resonance state of formaldehyde; the π* and σ* resonance states of chloroethene as functions of the C-Cl bond dissociation coordinate; and the Π and Π resonance states of N. The convergence of the CAP/XMS-CASPT2 results has been systematically examined with respect to the size of the active space. Resonance parameters predicted by the CAP/XMS-CASPT2 method agree well with CAP/SAC-CI results (deviations of about 0.15 eV); however, as expected, CAP/XMS-CASPT2 has clear advantages in the bond dissociation region. The advantages of CAP/XMS-CASPT2 are further demonstrated in the calculations of Π and Π resonance states of N including their Σ and Δ parent states. Three of the involved states (Π, Σ, and Δ) possess multireference character, and CAP/XMS-CASPT2 can easily describe these states with a relatively modest active space.
电子共振态是亚稳态（+1）电子态，换句话说，是嵌入在电子连续体中的离散态。虽然某些类型的共振态（例如，通过将一个额外电子附加到闭合壳中性态上创建的临时阴离子）已经取得了很大进展，但共振态仍然是量子化学的一大挑战，因为成功描述衰减需要对共振的束缚和连续体方面进行平衡描述。在这里，平滑 Voronoi 复吸积势（CAP）与 XMS-CASPT2 方法相结合，通过适当选择 CAS 空间，使我们能够应对平衡挑战。为了降低计算成本，该方法在投影方案中实现。在这个试点应用中，三个临时阴离子作为基准：甲醛的π共振态；氯丙烯的π和σ*共振态作为 C-Cl 键离解坐标的函数；以及 N 的Π和Π共振态。CAP/XMS-CASPT2 结果的收敛性已针对活动空间的大小进行了系统检查。CAP/XMS-CASPT2 方法预测的共振参数与 CAP/SAC-CI 结果吻合良好（偏差约为 0.15 eV）；然而，正如预期的那样，在键离解区域，CAP/XMS-CASPT2 具有明显的优势。CAP/XMS-CASPT2 的优势在包括其Σ和Δ母态的 N 的Π和Π共振态的计算中进一步得到证明。所涉及的三个状态（Π、Σ和Δ）具有多参考特征，并且 CAP/XMS-CASPT2 可以用相对较小的活动空间轻松描述这些状态。
