for the HOMO-3. These two MAD also suggest that there is a quantifiable difference in helicality between the HOMO-2 and HOMO-3. This difference is again not clear from the isosurface plotted at the top of Figure , but is noticeable in the graph of the cumulated sum. For spiro-[2.2]pentadiene (bottom of Figure ), the angular twist of the HOMO and HOMO-1 increases/decreases almost linearly. This is apparent both from the cumulated sum and the MAD (see Table ), which is very close to 1. For both molecules, their MOs hardly deviate from a cylindrical helix when close to the spiro-carbons and, consequently, both sets of MOs are similar to each other in this region. Moving away from the spiro-carbon, the dimethyl-spiro [4.4]nonatetraene starts to resemble a rectilinear ⇡-MO whereas the helical character persists in the HOMO and HOMO-1 of dimethyl-spiro[2.2]pentadiene.