Van den Heuvel Willem, Calvello Simone, Soncini Alessandro
School of Chemistry, The University of Melbourne, VIC 3010, Australia.
Phys Chem Chem Phys. 2016 Jun 21;18(23):15807-14. doi: 10.1039/c6cp02325h. Epub 2016 May 27.
A successful and commonly used ab initio method for the calculation of crystal field levels and magnetic anisotropy of lanthanide complexes consists of spin-adapted state-averaged CASSCF calculations followed by state interaction with spin-orbit coupling (SI-SO). Based on two observations valid for Ln(iii) complexes, namely: (i) CASSCF 4f orbitals are expected to change very little when optimized for different states belonging to the 4f electronic configuration, (ii) due to strong spin-orbit coupling the total spin is not a good quantum number, we show here via a straightforward analysis and direct calculation that the CASSCF/SI-SO method can be simplified to a single configuration-averaged HF calculation and one complete active space CI diagonalization, including spin-orbit coupling, on determinant basis. Besides its conceptual simplicity, this approach has the advantage that all spin states of the 4f(n) configuration are automatically included in the SO coupling, thereby overcoming one of the computational limitations of the existing CASSCF/SI-SO approach. As an example, we consider three isostructural complexes [Ln(acac)3(H2O)2], Ln = Dy(3+), Ho(3+), Er(3+), and find that the proposed simplified method yields crystal field levels and magnetic g-tensors that are in very good agreement with those obtained with CASSCF/SI-SO.
一种用于计算镧系配合物晶体场能级和磁各向异性的成功且常用的从头算方法，包括自旋适配的态平均CASSCF计算，随后进行自旋轨道耦合的态相互作用（SI-SO）。基于对Ln(iii)配合物有效的两个观察结果，即：(i) 当针对属于4f电子构型的不同态进行优化时，CASSCF 4f轨道预计变化很小；(ii) 由于强自旋轨道耦合，总自旋不是一个好的量子数，我们在此通过直接分析和直接计算表明，CASSCF/SI-SO方法可以简化为单一构型平均的HF计算以及在行列式基上包含自旋轨道耦合的一次完全活性空间CI对角化。除了概念上的简单性之外，这种方法的优点是4f(n)构型的所有自旋态都自动包含在SO耦合中，从而克服了现有CASSCF/SI-SO方法的一个计算限制。作为一个例子，我们考虑三种同构配合物[Ln(acac)3(H2O)2]，Ln = Dy(3+)、Ho(3+)、Er(3+)，并发现所提出的简化方法产生与用CASSCF/SI-SO获得的结果非常一致的晶体场能级和磁g张量。
