Khedkar Abhishek, Roemelt Michael
Lehrstuhl für Theoretische Chemie , Ruhr-Universität Bochum , D-44780 Bochum , Germany.
Max-Planck Institut für Kohlenforschung , Kaiser-Wilhelm Platz 1 , D-45470 Mülheim an der Ruhr , Germany.
J Chem Theory Comput. 2019 Jun 11;15(6):3522-3536. doi: 10.1021/acs.jctc.8b01293. Epub 2019 May 24.
Efficient and robust approximations to the full configuration interaction (full-CI) method such as the density matrix renormalization group (DMRG) and the full-CI quantum Monte Carlo (FCIQMC) algorithm allow for multiconfigurational self-consistent field (MC-SCF) calculations of molecules with many strongly correlated electrons. This opens up the possibility to treat large and complex systems that were previously untractable, but at the same time it calls for an efficient and reliable active space selection as the choice of how many electrons and orbitals enter the active space is critical for any multireference calculation. In this work we propose an Active Space Selection based on 1st order perturbation theory (ASS1ST) that follows a "bottom-up" strategy and utilizes a set of quasi-natural orbitals together with sensible thresholds for their occupation numbers. The required quasi-natural orbitals are generated by diagonalizing the virtual and internal part of the one-electron reduced density matrix that is obtained from strongly contracted n-electron valence perturbation theory (SC-NEVPT) on top of a minimal active space calculation. Self-consistent results can be obtained when the proposed selection scheme is applied iteratively. Initial applications on four chemically relevant benchmark systems indicate the capabilities of ASS1ST. Eventually, the strengths and limitations are critically discussed.
对完全组态相互作用（full-CI）方法的高效且稳健的近似，如密度矩阵重整化群（DMRG）和完全组态相互作用量子蒙特卡罗（FCIQMC）算法，使得对具有许多强关联电子的分子进行多组态自洽场（MC-SCF）计算成为可能。这为处理以前难以处理的大型复杂系统开辟了道路，但与此同时，它需要一种高效且可靠的活性空间选择，因为对于任何多参考计算而言，选择多少电子和轨道进入活性空间至关重要。在这项工作中，我们提出了一种基于一阶微扰理论的活性空间选择方法（ASS1ST），该方法遵循“自下而上”的策略，并利用一组准自然轨道以及对其占据数的合理阈值。所需的准自然轨道是通过对单电子约化密度矩阵的虚拟部分和内部部分进行对角化生成的，该单电子约化密度矩阵是在最小活性空间计算之上，通过强收缩n电子价微扰理论（SC-NEVPT）获得的。当迭代应用所提出的选择方案时，可以获得自洽结果。对四个化学相关基准系统的初步应用表明了ASS1ST的能力。最后，对其优点和局限性进行了批判性讨论。
