Fosso-Tande Jacob, Nguyen Truong-Son, Gidofalvi Gergely, DePrince A Eugene
Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306-4390, United States.
Department of Chemistry and Biochemistry, Gonzaga University , Spokane, Washington 99258-0089, United States.
J Chem Theory Comput. 2016 May 10;12(5):2260-71. doi: 10.1021/acs.jctc.6b00190. Epub 2016 Apr 27.
A large-scale implementation of the complete active space self-consistent field (CASSCF) method is presented. The active space is described using the variational two-electron reduced-density-matrix (v2RDM) approach, and the algorithm is applicable to much larger active spaces than can be treated using configuration-interaction-driven methods. Density fitting or Cholesky decomposition approximations to the electron repulsion integral tensor allow for the simultaneous optimization of large numbers of external orbitals. We have tested the implementation by evaluating singlet-triplet energy gaps in the linear polyacene series and two dinitrene biradical compounds. For the acene series, we report computations that involve active spaces consisting of as many as 50 electrons in 50 orbitals and the simultaneous optimization of 1892 orbitals. For the dinitrene compounds, we find that the singlet-triplet gaps obtained from v2RDM-driven CASSCF with partial three-electron N-representability conditions agree with those obtained from configuration-interaction-driven approaches to within one-third of 1 kcal mol(-1). When enforcing only the two-electron N-representability conditions, v2RDM-driven CASSCF yields less accurate singlet-triplet energy gaps in these systems, but the quality of the results is still far superior to those obtained from standard single-reference approaches.
本文介绍了完全活性空间自洽场（CASSCF）方法的大规模实现。活性空间采用变分双电子约化密度矩阵（v2RDM）方法描述，该算法适用于比用组态相互作用驱动方法所能处理的大得多的活性空间。对电子排斥积分张量采用密度拟合或Cholesky分解近似，可同时优化大量外部轨道。我们通过评估线性并苯系列和两种二氮杂戊二烯双自由基化合物的单重态-三重态能隙来测试该实现。对于并苯系列，我们报告了涉及在50个轨道中包含多达50个电子的活性空间以及1892个轨道同时优化的计算。对于二氮杂戊二烯化合物，我们发现，在部分三电子N可表示性条件下，由v2RDM驱动的CASSCF得到的单重态-三重态能隙与由组态相互作用驱动方法得到的能隙在1 kcal mol⁻¹的三分之一范围内一致。当仅施加双电子N可表示性条件时，v2RDM驱动的CASSCF在这些系统中产生的单重态-三重态能隙准确性较低，但结果质量仍远优于从标准单参考方法获得的结果。
