Quantum Monte Carlo (QMC) is a stochastic method that has been particularly successful for ground-state electronic structure calculations but mostly unexplored for the computation of excited-state energies. Here, we show that within a Jastrow-free QMC protocol relying on a deterministic and systematic construction of nodal surfaces using selected configuration interaction (sCI) expansions, one is able to obtain accurate excitation energies at the fixed-node diffusion Monte Carlo (FN-DMC) level. This evidences that the fixed-node errors in the ground and excited states obtained with sCI wave functions cancel out to a large extent. Our procedure is tested on two small organic molecules (water and formaldehyde) for which we report all-electron FN-DMC calculations. For both the singlet and triplet manifolds, accurate vertical excitation energies are obtained with relatively compact multideterminant expansions built with small (typically double-ζ) basis sets.

中文翻译：

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使用选定的配置相互作用节点从扩散蒙特卡洛获得的激发能

量子蒙特卡洛（QMC）是一种随机方法，在基态电子结构计算中特别成功，但在激发态能量的计算中却很少被探索。在这里，我们证明了在无Jastrow QMC协议的基础上，它使用选定的配置相互作用（sCI）展开依赖于节点表面的确定性和系统构造，从而能够在固定节点扩散蒙特卡洛（FN- DMC）级别。这证明了用sCI波函数获得的基态和激发态的固定节点误差在很大程度上抵消了。我们的程序在两个小有机分子（水和甲醛）上进行了测试，我们在其中报告了全电子FN-DMC计算。对于单线态和三线态歧管，ζ）基集。