A Combined Selected Configuration Interaction and Many-Body Treatment of Static and Dynamical Correlation in Oligoacenes

We have combined our adaptive configuration interaction (ACI) [J. B. Schriber and F. A. Evangelista, J. Chem. Phys. 2016, 144, 161106] with a density-fitted implementation of the second-order perturbative multireference-driven similarity renormalization group (DSRG-MRPT2) [K. P. Hannon, C. Li, and F. A. Evangelista, J. Chem. Phys. 2016, 144, 204111]. We use ACI reference wave functions to recover static correlation for active spaces larger than the conventional limit of 18 orbitals. The dynamical correlation is computed using the DSRG-MRPT2 method to yield a complete treatment of electron correlation. We apply the resulting method, ACI-DSRG-MRPT2, to predict singlet–triplet gaps, metrics of open-shell character, and spin–spin correlation functions for the oligoacene series (2–7 rings). Our computations employ active spaces with as many as 30 electrons in 30 orbitals and up to 1350 basis functions, yielding gaps that are in good agreement with available experimental results. Large bases and reference relaxation lead to a significant reduction in the estimated radical character of the oligoacenes, with respect to previous valence-only treatments of correlation effects.我们将自适应组态相互作用（adaptive configuration interaction, ACI）[J. B. Schriber与F. A. Evangelista, 《化学物理学报》, 2016, 144, 161106]，与二阶微扰多参考驱动相似重整化群（DSRG-MRPT2）的密度拟合实现[K. P. Hannon、C. Li及F. A. Evangelista, 《化学物理学报》, 2016, 144, 204111]相结合。我们采用ACI参考波函数，针对尺寸超出传统18轨道极限的更大活性空间修正静态电子关联；随后借助DSRG-MRPT2方法计算动态电子关联，从而实现对电子关联效应的完整处理。我们将所得到的ACI-DSRG-MRPT2方法，应用于并苯系列（2至7个环）的单重态-三重态能隙、开壳层特征量化指标以及自旋自旋关联函数的预测。本计算采用的活性空间最多可包含30个电子与30个轨道，基函数数目可达1350个，所得能隙与已公开的实验结果吻合良好。相较于此前仅针对价态关联效应的处理方案，大基组与参考态弛豫显著降低了并苯体系的自由基特征估算值。