The computation of frequencies should always be carried out with accint at least 4.0 to get results that make sense; many molecules, in particular those than contain metals, require higher precision, which is why the default is 6.0. The reason is that frequencies are computed by numerical differentiation of gradients computed in slightly displaced geometries. Obviously the noise in the gradients due to numerical integration errors should be small compared to the difference in gradients across the different geometries and since the latter are very close, a high numerical accuracy is required [106-107].
