where 𝑆 ⃗ and 𝐼 ⃗ are electron and nuclear spin operators, respectively, 𝐵 ⃗ is the external magnetic field, 𝜇 is the Bohr magneton, 𝑔 is the g-tensor, and 𝐴 is the hyperfine tensor between the electron and 14 N nuclear spins. According to the Hamiltonian in Eq. ( ), the energy of the system depends linearly on the values of the 𝑔 -and 𝐴 -tensors. In the discussion below, we report the principal components of the tensors (i.e., gx, gy, and gz) obtained by the diagonalization of the respective computed tensors. We also report isotropic values, i.e., giso = (gx+gy+gz)/3 and Aiso = (Ax+Ay+Az)/3, g-value shifts (i.e., chemical shifts) from the free electron g-value, i.e., ∆gx = (gxge)/ge, where ge = 2.00232, as well as the difference from the experimental value, i.e., dgx = (gxgi exp )/gx exp (gx exp = 2.00955, gy exp = 2.00632, and gz exp = 2.00240). Table shows the experimental values for g-and A-tensors of TEMPO and compares them with the computed values using ωB97X-D/6-311+G(2df,p) optimized structure (no pressure, standard optimization) and using the structure optimized with X-HCFF at a pressure of 0.1 GPa. The gtensors were computed at the CCSD/6-311G(2df,p) level of theory and A-tensors (coupling between the electron and 14 N nuclear spins) were computed using DFT (with the TPSS0 functional and 6-31++G* basis; the results with ωB97X-D and a larger basis set are given in the SI). As one can see, the calculation results of the g-and A-values for X-HCFF/0.1 GPa and standard optimization (no X-HCFF) are identical within the precision used, and the agreement between theory and experiment is quite good as well. The computed gx, gy, and gz values near ambient condition are 2.01020, 2.00643 and 2.00229, and the corresponding the g-value shifts from the free electron g-value (∆gi = (gi -ge)/ge i = x, y, and z) are ∆gx = 3938 ppm, ∆gy = 2051 ppm, and ∆gz = -14 ppm. These calculated g-shifts are comparable to the previous study. The difference from the experimental values at ambient pressure (dgi = (gi -gi exp )/gi exp where i = x, y, and z) are dgx = 325 ppm, dgy = 53 ppm, and dgz = -55 ppm. These small discrepancies between the theoretical and experimental g-tensor values are within the anticipated error bars due to insufficient treatment of correlation and a relatively small basis set. The computed A-values also agree reasonably well with the experiment. With TPSS0/6-31++G*, we obtained Ax exp = 16.6 MHz, Ay exp = 17.4 MHz, and Az exp = 85.9 MHz. The difference from the experimental values (e.g., dAx = (Ax-A exp )/A exp ) are -13.7, -9.2 and -12.4 %. These small differences are in good agreement with the previous computational study at ambient pressure (< 11 %). In the SI, we report additional results