Boyn Jan-Niklas, Mazziotti David A
The Department of Chemistry, The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA.
J Chem Phys. 2021 Apr 7;154(13):134103. doi: 10.1063/5.0045007.
The accurate description of biradical systems, and in particular the resolution of their singlet-triplet gaps, has long posed a major challenge to the development of electronic structure theories. Biradicaloid singlet ground states are often marked by strong correlation and, hence, may not be accurately treated by mainstream, single-reference methods such as density functional theory or coupled cluster theory. The anti-Hermitian contracted Schrödinger equation (ACSE), whose fundamental quantity is the two-electron reduced density matrix rather than the N-electron wave function, has previously been shown to account for both dynamic and strong correlations when seeded with a strongly correlated guess from a complete active space (CAS) calculation. Here, we develop a spin-averaged implementation of the ACSE, allowing it to treat higher multiplicity states from the CAS input without additional state preparation. We apply the spin-averaged ACSE to calculate the singlet-triplet gaps in a set of small main group biradicaloids, as well as the organic four-electron biradicals trimethylenemethane and cyclobutadiene, and naphthalene, benchmarking the results against other state-of-the-art methods reported in the literature.
对双自由基体系的精确描述，尤其是其单重态-三重态能隙的解析，长期以来一直是电子结构理论发展面临的重大挑战。类双自由基单重态基态通常具有很强的相关性，因此，主流的单参考方法如密度泛函理论或耦合簇理论可能无法对其进行准确处理。反厄米特收缩薛定谔方程（ACSE），其基本量是双电子约化密度矩阵而非N电子波函数，先前已表明，当从完全活性空间（CAS）计算中引入强相关猜测时，它能够同时考虑动态相关和强相关。在此，我们开发了ACSE的自旋平均实现方式，使其能够从CAS输入处理更高多重度的态，而无需额外的态制备。我们应用自旋平均ACSE来计算一组小的主族类双自由基以及有机四电子双自由基三甲叉甲烷、环丁二烯和萘中的单重态-三重态能隙，并将结果与文献中报道的其他先进方法进行基准比较。
