In this post I said the following: "God is the immediate cause of the existence of everything that exists in the physical world, both substance and accident." I believe I misspoke somewhat.
To whit: "Therefore, as accidents and forms and the like non-subsisting things are to be said to co-exist rather than to exist, so they ought to be called rather "concreated" than "created" things; whereas, properly speaking, created things are subsisting beings" (ST, I, q. 45, a. 4c). The word translated in the Benzinger edition as "concreated" is translated as "co-created" in the Blackfriars edition. The Latin term being translated is "concreata" in the Leonine edition. My Latin isn't great, but I believe this is a participle of the verb "concreo," which, if I am not mistaken, means "to create with."
As such, I believe it is incorrect to say that God is the immediate cause of the existence of accidents, since accidents by nature participate in the existence of their subject. It would be more correct to say that God is the principle cause of the existence of everything that exists in the physical world, both substance and accident. The cause of the existence of accidents can be reduced to God, but comes through a mediating cause, viz. substance, rather than immediately. And this may play an important point in relation to an argument I make here regarding the principle cause of evil.
To wit: The effects of an intermediate cause may be reduced to the effects of the first and principle cause insofar as the intermediate cause is subordinate to the first cause. Thus the existence of any accident qua existing and, since that which exists is good insofar as it has existence, qua good is reducible to the causality of God. But insofar as an accident is evil, i.e. insofar as said accident lacks the fullness of being it should possess, it falls outside the order of the first and principle cause, viz. God, since insofar as it lacks the existence it should possess it falls outside the order of existence that the first and principle cause establishes. Thus, the existence of the accident insofar as it is evil cannot be reduced to the first and principle cause. It is rather reducible only to the intermediate cause.