using pcseg-3 are used as the reference, the intensity errors decrease substantially when increasing the basis set size from pcseg-1 to pcseg-2, leaving no normal modes with absolute and relative errors that could impact the spectrum appreciably. However, this is not the case for acetone in vacuum (Fig. ), where the larger pcseg-4 basis set is used to calculate the reference values. Although the errors are also greatly reduced when increasing the basis set size, for pcseg-2, there are still a few normal modes with substantial absolute and relative differences. For IR, this applies for normal modes 4 (normalized absolute difference 0.