Last but not less, Pauli repulsion (EPAULI) compensates for all the stabilizing effects, standing for 84-97% of the whole destabilizing energy. Pauli repulsion is related to the volume exclusion effects when molecular fragments are brought into close interaction. Due to the latter, EPAULI is sensitive to the number of methyl groups, molecular volume, oxidation state, and relative adsorption orientation in methylarsenicals. For instance, EPAULI is higher for anionic PhosO-aMMA V complex (EPAULI11.8 eV) compared to neutral PhosO-MMA III (EPAULI3.4 eV), which is due to the anionic nature, relatively higher molecular volume, and short-range P-O bonding in the pentavalent case (Table ).