A similar situation is revealed on the CO stretching vibrational frequency reduction, a similar situation is revealed, see Fig. . Generally, ν decreases as the Eads decreases, from an initial value of 2131 cm -1 for CO in vacuum. Linear trends are found accounting for all values with an R of 0.855, but when decomposing by crystallographic structures, only bcc and hcp retain high R values of 0.816 and 0.870, while fcc cases have an R of 0.552. As before, the two trends are clearly distinguishable depending on whether CO is perpendicular or parallel to the surface. With an R of 0.765, the parallel adsorption mode decreases the CO stretching vibrational frequency in a much more accentuated way than the perpendicular mode does, with an R of 0.719, reinforcing the idea that the bonding mechanism and bonding structure play a major role than the crystallographic phase itself.