The accurate computation of ionization potentials (IPs), within 0.10 eV, is one of the most challenging problems in modern computational chemistry. The extended Koopmans’ theorem (EKT) provides a systematic direct approach to compute IPs from any level of theory. In this study, the EKT approach is integrated with the coupled-cluster singles and doubles with perturbative triples [CCSD(T)] method for the first time. For efficiency, the density-fitting (DF) approximation is employed for electron repulsion integrals. Further, the EKT-CCSD(T) method is applied to a set of 23 molecules, denoted as IP23, for comparison with the experimental ionization potentials. For the IP23 set, the EKT-CCSD(T) method, along with the aug-cc-pV5Z basis set, provides a mean absolute error of 0.05 eV. Hence, our results demonstrate that direct computations of IPs at high-accuracy levels can be achieved with the EKT-CCSD(T) method. We believe that the present study may open new avenues in IP computations.

中文翻译：

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结合CCSD（T）方法的扩展Koopmans定理的垂直电离势的最新计算

在0.10 eV以内的电离势（IPs）的精确计算是现代计算化学中最具挑战性的问题之一。扩展的考夫曼定理（EKT）提供了一种系统的直接方法，可以从任何理论水平上计算IP。在这项研究中，EKT方法首次与微扰三元组[CCSD（T）]方法与耦合集群单打和双打集成在一起。为了提高效率，将密度拟合（DF）近似值用于电子斥力积分。此外，将EKT-CCSD（T）方法应用于一组23个分子（表示为IP23），以与实验电离势进行比较。对于IP23集，EKT-CCSD（T）方法与aug-cc-pV5Z基本集一起提供了0.05 eV的平均绝对误差。因此，我们的结果表明，使用EKT-CCSD（T）方法可以实现高精度IP的直接计算。我们认为，本研究可能会为IP计算开辟新的途径。