Dipartimento di Chimica, Università di Ferrara, Via Borsari 46, I-44121 Ferrara, Italy.
J Chem Phys. 2012 Jul 21;137(3):034104. doi: 10.1063/1.4735018.
The use of multireference perturbation theory (MRPT) for the calculation of the magnetic coupling in binuclear complexes has shown to give poor results if applied on a minimal active space complete active space self-consistent field (CASSCF) wavefunction. In this work, we identify the origin of this problem in the starting CASSCF orbitals, which are exceedingly localized on the metal atoms. Focusing on the case of antiferromagnetic systems, it is shown that the form of the active orbitals has a dramatic effect on the relative description of the neutral and ionic structures. Finally, a simple and computational inexpensive strategy is proposed for the calculation of a set of magnetic orbitals describing in a more balanced way the neutral and ionic structures. The use of these orbitals, instead the CASSCF ones, in minimal active space MRPT2 calculations leads to a marked improvement of the J values, which become in reasonable agreement with those obtained with the expensive high level difference dedicated configuration interaction approach and with the experimental values.
多参考微扰理论（MRPT）在双核配合物的磁耦合计算中的应用表明，如果应用于最小活性空间完全自洽场（CASSCF）波函数，会得到较差的结果。在这项工作中，我们确定了这个问题的根源在于起始 CASSCF 轨道，它们在金属原子上极度局域。我们专注于反铁磁系统的情况，表明活性轨道的形式对中性和离子结构的相对描述有显著影响。最后，提出了一种简单且计算成本低廉的策略，用于计算一组磁轨道，以更平衡的方式描述中性和离子结构。在最小活性空间 MRPT2 计算中使用这些轨道，而不是 CASSCF 轨道，会导致 J 值的显著改善，使其与昂贵的高精度差专用组态相互作用方法和实验值更合理地一致。
