...Anselm's notion of a self-existent or self-explanatory being is rather obscure. For example, Anselm takes it to be an implication of divine self-existence that (i) God's existence is not explained by anything else, (ii) God's existence is explained by his essence, and hence that (iii) God is a necessary being. Unhappily, (i) and (ii) are incompatible unless God is identical with his essence. Anselm accepts the doctrine that God is identical with his essence; among traditional theologians such as Anselm this doctrine is commonly thought to be an implication of divine simplicity. But as we have argued, it is a category mistake to suppose that God, a substance, is identical with his essence, a quality. Moreover, necessarily, any quality of a concrete entity [of any sort] inheres in that concrete entity. But God's essence is a quality of God, and God is a concrete entity. So, God's essence inheres in God. Since it is impossible for a concrete entity to inhere in itself, it follows that God cannot inhere in himself. Because God's essence inheres in God, but God does not, God and his essence are diverse. For all of these reasons, God and his essence cannot be identical. Hence, (i) and (ii) are incompatible. Thus, if God's existence is explained by his essence, then strictly speaking God's existence is explained by something else. However, God's existence being explained by his essence seems compatible with God's being maximally great. There is no reason to accept without qualification Anselm's assumption that God's existence cannot depend upon something else.
Valid? Sure. Sound? 'Unsound' just doesn't do it justice. I would like to know what the point of having an essence is when it just inheres in a substance along with all the substance's other properties/qualities etc. Fond/convinced as I am of the usefulness of the formal distinction, I don't think I would posit it as obtaining between God and the divine essence.