Multidimensional Potential Energy Surfaces Resolved at the RASPT2 Level for Accurate Photoinduced Isomerization Dynamics of Azobenzene

We have used state-of-the-art ab initio restricted active RASPT2 computations using a 16 orbitals, 18 electrons active space to produce an extended three-dimensional map of the potential energy surfaces (PESs) of the ground and first nπ* excited states of azobenzene along CNNC torsion and the two CNN bending angles, which are the most relevant coordinates for the trans–cis photoisomerization process. Through comparison with fully unconstrained optimizations performed at the same level of theory, we show that the three selected coordinates suffice to correctly describe the photoisomerization mechanism and the S1–S0 crossing seam. We also provide a map of the nonadiabatic coupling between the two states in the region where they get closer in energy. Eventually, we show that treating the two CNN bending angles as independent coordinates is fundamental to break the symmetry and couple the two electronic states. The accuracy of the S0 and S1 PESs and couplings was validated with semiclassical dynamics simulations in the reduced space of the scanned coordinates, showing results in good agreement with published full-coordinate dynamics.

本研究采用当前最先进的从头算限制性活性空间RASPT2（restricted active RASPT2）计算方法，以16个轨道、18个电子的活性空间为基础，构建了偶氮苯基态与第一nπ*激发态势能面（potential energy surfaces, PESs）的扩展三维图谱。该图谱沿着CNNC扭转角与两个CNN弯曲角这三个与反式-顺式光异构化过程最相关的坐标维度展开。通过与相同理论级别下的全无约束几何优化结果进行对比，本研究证实所选的三个坐标足以准确描述光异构化机制以及S1-S0交叉缝。本研究还提供了两态在能量趋近区域内的非绝热耦合图谱。最终研究表明，将两个CNN弯曲角作为独立坐标进行处理，是打破体系对称性并耦合两个电子态的关键前提。本研究通过在扫描坐标的简化空间中进行半经典动力学模拟，验证了基态S0、第一激发态S1的势能面以及非绝热耦合的准确性，模拟结果与已发表的全坐标动力学结果吻合良好。