An interesting case study for SCSD parameterization presents in molecules undergoing changes in their bonding. A HOMO-to-LUMO excitation should result in changes to the bonding strength and overall symmetry of a hypothetical π-system. The simplest example would be ethene -excitation π→π* would reduce the strength of the bond, and relaxing the planarizing influence of this bond, as anti-bonding is now present. Therefore, upon excitation, we could expect alteration in Ag and Au SCSD structural parameters (in D2h). This molecule is not ideal for this experiment for many reasons -with analysis reliant on H-atom positions, and the lifetime of ethene excitation approx. 50fs, B1u The B1g distortion measures several atoms and can therefore be more telling of structural change than an individual distance. While the Pt-Pt distance changes by 0.0081 -0.0127 Å, the corresponding movement of the other 28 atoms totals +0.04 Å -so easier to measure. This can be likened to observing the mechanical advantage of a drill-press, and the concerted movement of multiple atoms -independent of minor distortions. Of course, this is dependent strongly on the polymorph structure -the "Bu2" polymorph (XUVNAD and XUVNAD01) exhibits much stronger B2g distortion than "Bu1" polymorph (XUVNAD02 (off) and XUVNAD03 (on)) by 0.3 Å (6.97 -> 7.31 Å)though shows similar change on excitation (+0.07 Å). Each structure exhibits Eg(x) deformational character of around 1.8 Å.