# When is it necessary to consider size consistency?

For what kinds of reactions is it essential to account for size consistency? To my understanding for dissociation reactions $(\ce{AB -> A + B})$ it is important to consider. However, is it also necessary to consider for isodesmic reactions? How about for other addition or insertion reactions?

• Your terminology is, to some extent, rather obscure en.wikipedia.org/wiki/Isodesmic_reaction Dunno what "integration" is supposed to mean, insertion maybe? – Mithoron Mar 25 '18 at 18:23
• Yes, my apologies. I meant insertion reaction. – Dr.K Mar 25 '18 at 21:50
• @Mithoron Isodesmic is a fairly often-used term in my reading of the computational literature. Isodesmic reactions are commonly used as exemplars of systems where the electronic character of the products and reactants are similar, allowing benchmarking/comparison of methods while introducing the minimum of effects from variations in gross electronic structure. – hBy2Py Mar 25 '18 at 22:01
• @Mithoron It's a specific term of art for a particular kind of error inherent to certain kinds of calculations. See here. – hBy2Py Mar 26 '18 at 0:34
• The obvious answer is that you should always worry about size consistency, however, the only method that truly suffers from it is truncated configuration interaction. – Martin - マーチン Mar 26 '18 at 11:40