A novel protocol to compute and analyze NMR chemical shifts for extended paramagnetic solids, accounting comprehensively for Fermi-contact (FC), pseudocontact (PC), and orbital shifts, is reported and applied to the important lithium ion battery cathode materials LiFePO₄ and LiCoPO₄. Using an EPR-parameter-based ansatz, the approach combines periodic (hybrid) DFT computation of hyperfine and orbital-shielding tensors with an incremental cluster model for g- and zero-field-splitting (ZFS) D-tensors. The cluster model allows the use of advanced multireference wave function methods (such as CASSCF or NEVPT2). Application of this protocol shows that the ⁷Li shifts in the high-voltage cathode material LiCoPO₄ are dominated by spin–orbit-induced PC contributions, in contrast with previous assumptions, fundamentally changing interpretations of the shifts in terms of covalency. PC contributions are smaller for the ⁷Li shifts of the related LiFePO₄, where FC and orbital shifts dominate. The ³¹P shifts of both materials finally are almost pure FC shifts. Nevertheless, large ZFS contributions can give rise to non-Curie temperature dependences for both ⁷Li and ³¹P shifts.

中文翻译：

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应用于LiFePO ₄和LiCoPO _4的顺磁性固体中NMR化学位移的量子化学方法

报告了一种新颖的协议，用于计算和分析扩展的顺磁性固体的NMR化学位移，全面考虑了费米接触（FC），伪接触（PC）和轨道位移，并将其应用于重要的锂离子电池正极材料LiFePO ₄和LiCoPO ₄。该方法使用基于EPR参数的ansatz，将超精细和轨道屏蔽张量的周期性（混合）DFT计算与用于g和零场分裂（ZFS）D张量的增量簇模型结合在一起。群集模型允许使用高级多参考波函数方法（例如CASSCF或NEVPT2）。该协议的应用表明，高压阴极材料LiCoPO ₄的⁷ Li位移与以前的假设相反，自旋轨道引起的PC贡献占主导地位，从根本上改变了对共价变化的解释。对于相关的LiFePO ₄的⁷ Li位移，PC贡献较小，其中FC和轨道位移占主导。两种材料的³¹ P位移最终几乎都是纯FC位移。尽管如此，对于⁷ Li和³¹ P位移，较大的ZFS贡献都可能引起非居里温度依赖性。