Objection 1. It seems that God has not free-will. For Jerome says, in a homily on the prodigal son [Ep. 146, ad Damas.]; "God alone is He who is not liable to sin, nor can be liable: all others, as having free-will, can be inclined to either side."
Objection 2. Further, free-will is the faculty of the reason and will, by which good and evil are chosen. But God does not will evil, as has been said (9). Therefore there is not free-will in God.
On the contrary, Ambrose says (De Fide ii, 3): "The Holy Spirit divideth unto each one as He will, namely, according to the free choice of the will, not in obedience to necessity."
I answer that, We have free-will with respect to what we will not of necessity, nor be natural instinct. For our will to be happy does not appertain to free-will, but to natural instinct. Hence other animals, that are moved to act by natural instinct, are not said to be moved by free-will. Since then God necessarily wills His own goodness, but other things not necessarily, as shown above (Article 3), He has free will with respect to what He does not necessarily will.
Reply to Objection 1. Jerome seems to deny free-will to God not simply, but only as regards the inclination to sin.
Reply to Objection 2. Since the evil of sin consists in turning away from the divine goodness, by which God wills all things, as above shown (De Fide ii, 3), it is manifestly impossible for Him to will the evil of sin; yet He can make choice of one of two opposites, inasmuch as He can will a thing to be, or not to be. In the same way we ourselves, without sin, can will to sit down, and not will to sit down.
Source: Summa Theologica (New Advent)