We present analytic gradients and derivative couplings for the simplest possible multireference configuration interaction method, CIS-1D, an electronic structure Ansatz that includes all single excitations and one lone double excitation on top of a Hartree–Fock reference state. We show that the resulting equations are numerically stable and require the evaluation of a similar number of integrals as compared to standard CIS theory; one can easily differentiate the required frontier orbitals (h and l) with minimal cost. The resulting algorithm has been implemented within the Q-Chem electronic structure package and should be immediately useful for understanding photochemistry with S₀–S₁ crossings.

中文翻译：

---

用于与所有单激发和一个双激发进行构型相互作用的解析梯度和导数耦合-通往非绝热动力学

我们介绍了最简单的多参考配置相互作用方法CIS-1D（一种电子结构Ansatz，其中包括在Hartree-Fock参考状态之上的所有单激励和一个单独的双激励）的解析梯度和微分耦合。我们证明了所得方程在数值上是稳定的，与标准CIS理论相比，需要对相似数量的积分进行评估；人们可以以最小的成本轻松区分所需的前沿轨道（h和l）。生成的算法已在Q-Chem电子结构包中实施，对于理解具有S ₀ – S ₁穿越的光化学应立即有用。