Various more sophisticated active spaces were developed, focusing on the phosphorus atoms, in an attempt to improve calculated shifts by incorporating the phosphorus valence orbitals, however only minor improvements to calculated 31 P shifts were achieved (Table ). The calculation in closest agreement with experiment was a restricted active space configuration interaction spin-orbit (RASCI-SO) calculation using the orbitals generated by the above minimal CASSCF-SO calculations, where the phosphorus 1s, 2s, 3s and 3p orbitals were placed in RAS1, the Ln 4f orbitals defined RAS2, and the 13 lowest-energy virtual orbitals comprised RAS3: this is just about the computational limit of CASSCF/RASSCF methods. Nonetheless, these calculations deliver P shifts that are still far from experiment, highlighting the difficulty of capturing the details of electronic structure that pNMR shifts are so sensitive to. It can also be noted that solution P NMR spectroscopy is particularly sensitive to factors such as temperature as well as various stereoelectronic effects, which are often not captured in the calculation of P chemical shifts when starting from the crystal structure. Therefore, small differences in structure both in solution and solid-state can lead to significant changes in chemical shifts.