Department of Chemistry, Duke University, Durham, North Carolina 27708, USA.
Department of Chemistry and Department of Physics, Duke University, Durham, North Carolina 27708, USA.
J Chem Phys. 2013 Dec 14;139(22):224105. doi: 10.1063/1.4834875.
Double, Rydberg, and charge transfer (CT) excitations have been great challenges for time-dependent density functional theory (TDDFT). Starting from an (N ± 2)-electron single-determinant reference, we investigate excitations for the N-electron system through the pairing matrix fluctuation, which contains information on two-electron addition/removal processes. We adopt the particle-particle random phase approximation (pp-RPA) and the particle-particle Tamm-Dancoff approximation (pp-TDA) to approximate the pairing matrix fluctuation and then determine excitation energies by the differences of two-electron addition/removal energies. This approach captures all types of interesting excitations: single and double excitations are described accurately, Rydberg excitations are in good agreement with experimental data and CT excitations display correct 1/R dependence. Furthermore, the pp-RPA and the pp-TDA have a computational cost similar to TDDFT and consequently are promising for practical calculations.
双激发态、里德堡激发和电荷转移激发一直是时间依赖密度泛函理论（TDDFT）的巨大挑战。本文从（N ± 2）电子单行列式参考态出发，通过配对矩阵涨落研究 N 电子体系的激发，其中包含了关于双电子添加/去除过程的信息。我们采用粒子-粒子随机相位近似（pp-RPA）和粒子-粒子 Tamm-Dancoff 近似（pp-TDA）来近似配对矩阵涨落，然后通过双电子添加/去除能的差值来确定激发能。该方法可以捕捉到所有类型的有趣激发：单激发和双激发态的描述非常准确，里德堡激发与实验数据吻合良好，电荷转移激发显示出正确的 1/R 依赖性。此外，pp-RPA 和 pp-TDA 的计算成本与 TDDFT 相似，因此在实际计算中具有很大的应用潜力。
