The theme that Augustine so stressed, about the movement from external law to the inner law written on ones mind and heart, is also found in many places in the writings of St. Thomas. Here is a link to an articlewhich, after discussing the views of St. Ignatius Loyola,has a helpful summary of Aquinass view on the relationship between written law and inward law. Here are two passages from St. Thomass commentaries on St. Pauls Epistles:

Where the Spirit of the Lord is, there is freedom (2 Cor 3:17) and A law is not given for a just person (1 Tim 1:9) Some people mistakenly take these words to mean that spiritual people are not obliged by the precepts of the divine law. But this is false. Gods precepts are the rule for the human will. There is no human being, nor even any angel, whose will does not have to be regulated and directed by the divine law. It is impossible, then, for any human being not to be subject to Gods precepts.The statement, A law is not given for a just person, should be interpreted thus: A law is not given for the sake of righteous people, who are moved by an inner habit to do the things Gods law commands, but for the sake of the unrighteous, even though the righteous also are bound by it.Similarly, the statement, Where the Spirit of the Lord is, there is freedom, should be understood thus: A free person is one who exists for his own sake [Liber est causa suiAristotle] while a slave is someone who exists for the sake of his lord. Whoever acts from himself, therefore, is free, while whoever acts because moved by another is not acting freely. Anyone, therefore, who avoids evil not because it is evil but because of a command of the Lord, is not free, while one who avoids evil because it is evil is free. This is what the Holy Spirit does: he perfects the mind inwardly by a good habit so that one avoids evil out of love, as if a divine law were commanding it. He is therefore said to be free, not in the sense that he is not subject to the divine law, but because by a good habit he is inclined to do what the divine law ordains. (Aquinas on 2 Cor 3:17)

Commenting on Rom 2:14, Since Gentiles, who do not have the law, naturally do what the law requires, not having the law, they are the law for themselves, Aquinas has this:

... He shows their dignity in that although they do not have the Jewish law, they are the law for themselves insofar as they perform the function of the law in their own regard by instructing themselves and leading themselves to the good; because, as the Philosopher says, Law, emanating from a certain wisdom and prudence, has compulsive force (Ethics, Bk. 10, ch. 91179b). Which is why it is said that a law is not made for a just person (1 Tim 1:9), that is, it is not compelled by an external law, but is set down for the unrighteous, who need to be forced from without.And the highest level of dignity among human beings is this: that they are led to the good not by others but by themselves. The second level is that of people who are led by another but without compulsion. The third is that of people who need to be compelled in order to become good. The fourth level is that of people who cannot be directed to the good even by compulsion.(Aquinas, In Epist. ad Romanos, ch. 2, lect. 3; Marietti edition, 216-217)