Calculating High-Pressure Vibrational Frequencies Analytically with the Extended Hydrostatic Compression Force Field Approach

We report the implementation of the analytical Hessian for the mechanochemical eXtended Hydrostatic Compression Force Field (X-HCFF) method in the Q-Chem program package. To verify the implementation, the analytical Hessian was compared with finite difference calculations. Additionally, we calculated the pressure dependency of all IR and Raman active vibrational modes of the Buckminsterfullerene and compared the results with experimental and theoretical data. Our implementation paves the way for analyses of geometric points on a pressure-deformed potential energy surface and provides a straightforward model to calculate the vibrational properties of molecules under high pressure.