A method to predict the D tensor in the molecular frame with multiconfigurational wave functions in large active space was proposed, and the spin properties of the lowest triplets of aromatic molecules were examined with full-π active space; such calculations were challenging because the size of active space grows exponentially with the number of π electrons. In this method, the exponential growth of complexity is resolved by the density matrix renormalization group (DMRG) algorithm. From the D tensor, we can directly determine the direction of the magnetic axes and the ZFS parameters, D- and E-values, of the phenomenological spin Hamiltonian with their signs, which are not usually obtained in ESR experiments. The method using the DMRG-CASSCF wave function can give correct results even when the sign of D- and E-values is sensitive to the accuracy of the prediction of the D tensor and existing methods fail to predict the correct magnetic axes.

中文翻译：

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芳香族分子三重激发态的零场分裂张量：价态全π完全活性空间自洽场研究

提出了大活性空间中多构型波函数分子框架中D张量的预测方法，并利用全π活性空间考察了芳香族分子最低三重态的自旋性质；这种计算具有挑战性，因为活性空间的大小随着 π 电子的数量呈指数增长。在该方法中，复杂度的指数增长通过密度矩阵重整化群（DMRG）算法来解决。从D张量，我们可以直接确定磁轴的方向以及唯象自旋哈密顿量的 ZFS 参数D和E值及其符号，这些在 ESR 实验中通常无法获得。即使当D值和E值的符号对D张量的预测精度敏感并且现有方法无法预测正确的磁轴时，使用 DMRG-CASSCF 波函数的方法也可以给出正确的结果。