Ramirez Francisco F, Bustamente Carlos M, González Lebrero Mariano C, Scherlis Damián A
Departamento de Química Inorgánica, Analítica y Química Física/INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pab. II, Buenos Aires C1428EHA, Argentina.
J Chem Theory Comput. 2020 May 12;16(5):2930-2940. doi: 10.1021/acs.jctc.9b01122. Epub 2020 Apr 20.
In the context of electron dynamics simulations, when the charge density of a molecule is subject to a perturbation in the form of a short electric field pulse, density fluctuations develop in time. In the absence of dissipation, these oscillations continue indefinitely, reflecting the resonances of the electronic system; as a matter of fact, from the Fourier transform of the time dependent dipole arising from them, the absorption spectrum of the molecule can be calculated. Since these oscillations are the result of the electrons moving through the molecular structrure, it seems plausible that they carry information on the transport properties of the system. This is the idea explored in the present article for the case of conjugated polymers. Specifically, we depart from a nonequilibrium state with the charge concentrated on the ends of the molecule, and estimate the currents flowing back and forth during the evolution of electron dynamics simulations. These show that the charge oscillates between the sides of the polymer with the predominance of a frequency that is coincident with one of the main bands in the absorption spectrum, which can be ascribed to a charge transfer transition. Thus, from the charge transfer band frequency appearing in the absorption spectrum, the molecular conductance of a conjugated molecule can be calculated. Also interestingly, we find that, while a perturbation excites all resonances of an electronic system, the form in which this perturbation is applied can be manipulated to determine the relative manifestation of the response. The electric field pulse excites all resonances according to the transition dipole moment and is then appropriate to produce the absorption spectrum. A charge separated initial state, however, specifically stimulates the charge transfer mode and is then suitable to calculate transport properties. This allows us to propose a simple approach to obtain molecular conductances and tunneling decay constants in agreement with results from much more demanding electronic structure techniques.
在电子动力学模拟的背景下，当分子的电荷密度受到短电场脉冲形式的扰动时，密度涨落会随时间发展。在没有耗散的情况下，这些振荡会无限持续，反映了电子系统的共振；事实上，从由它们产生的随时间变化的偶极矩的傅里叶变换中，可以计算出分子的吸收光谱。由于这些振荡是电子在分子结构中移动的结果，那么它们携带有关系统输运性质的信息似乎是合理的。本文针对共轭聚合物的情况探讨了这一观点。具体而言，我们从电荷集中在分子末端的非平衡态出发，并估计在电子动力学模拟演化过程中来回流动的电流。这些结果表明，电荷在聚合物的两侧之间振荡，其主要频率与吸收光谱中的一个主带重合，这可以归因于电荷转移跃迁。因此，根据吸收光谱中出现的电荷转移带频率，可以计算共轭分子的分子电导。同样有趣的是，我们发现，虽然扰动会激发电子系统的所有共振，但可以操纵这种扰动的施加形式来确定响应的相对表现形式。电场脉冲根据跃迁偶极矩激发所有共振，因此适合产生吸收光谱。然而，电荷分离的初始状态会特别激发电荷转移模式，因此适合计算输运性质。这使我们能够提出一种简单的方法来获得与要求更高的电子结构技术结果一致的分子电导和隧穿衰减常数。
