It's not "Achaemened" it's Achaemenid. The "-id" ending is taken from the construction of Greek patronymic adjectives, which generally end -ίδης in the "hybrid" declension (c.f. the opening line of the Iliad, μῆνιν ἄειδε θεὰ Πηληιάδεω Ἀχιλῆος)