Next, we investigate the pressure dependence. We first discuss the effect of pressure on the structure, focusing on the NO bond length as the most relevant parameter. The value of NO bond length from the X-ray crystallography 32 is 1.282 Å. The ωB97X-D/6-311+G(2df,p) computed value is 1.270 Å (the effect of the basis on the computed structures is discussed in the SI). Figure shows the N-O distance as a function of pressure. As one can see, the distance decreases linearly In order to assess whether these pressure-induced changes would be visible in the EPR spectra, we simulated 9 GHz (X-band) and 230 GHz EPR spectra using the computed g-and A-values with EasySpin software. We performed the simulations with a rotational correlation time (τc) of 2 ns by aiming to study the stability of spin-label EPR under hydrostatic pressure. The rotational correlation time represents the influence of the molecular motions on EPR spectrum. In particular, the correlation time of τc = 2 ns is typical for the nitroxide radical attached to a protein in an aqueous solution. the pressures of 0, 0.5, 1, and 15 GPa. By taking into account the convention of EPR spectroscopy, which utilizes the magnetic field modulation technique, the spectrum was plotted as a derivative of the absorption spectrum. As can be seen, the EPR spectrum spans from 318 to 323 mT, corresponding to the width of 5 mT. The EPR spectra consist of three lines because of the hyperfine coupling of the TEMPO spin. Note that the effects of the anisotropic g-factor are very small (see EPR spectrum is less influenced by the motional averaging effects. Overall, the shape of the EPR spectrum is determined by the anisotropic g-values. Thus, the result suggests that the EPR spectral analysis at 230 GHz will be less affected by the pressure effects of the spin label, therefore, the EPR analysis at 230 GHz EPR spectroscopy may be suitable for studying molecular dynamics under hydrostatic pressures without accounting for the changes in magnetic parameters. In summary, we carried out a computational study of the hydrostatic pressure effects on the gand A-tensors of the TEMPO molecule. We observed the linear dependence on the g-and A-tensors under pressures in the range of 0 -15 GPa. The shifts of the g-values are in the range of 0 to 37 ppm by increasing the pressure from 0 to 15 GPa whereas the shifts of the A-tensor are in the range of 1 to 7 %. We also determined the rates of the pressure dependence for x-, y-, and zcomponents of both g-and A-tensors. The observed linear dependence on the g-and A-tensors shows a potential application of the nitroxide radical as a pressure sensor. Furthermore, we examined the effect of the pressure-dependence on EPR spectra under a typical condition where the nitroxide radical is used as a spin label. The simulation of EPR spectra with the obtained gand A-tensors showed that the pressure-dependent changes in the EPR spectrum are more pronounced in 9 GHz (X-band) EPR spectroscopy since the 9 GHz spectrum depends on the Atensor. On the other hand, the changes on the 230 GHz EPR spectra in the range of 0 to 15 GPa are likely to be too small to detect. Therefore, the calculations suggest that a high-frequency EPR spectroscopy, such as 230 GHz EPR, will allow one to avoid spectral changes due to the pressure dependence on the spin label and to investigate structures of target molecules under hydrostatic pressures. The present investigation illustrates the utility of the presented computational protocol for determining the pressure dependence of various spin parameters of molecular spins under extreme environments such as high and low pressures, strong electric fields, and high magnetic fields.