Careful, there is an position on the question of grace and free will called Augustinian, which was expounded by Giovanni Berti in the 18th century that is different from the Thomistic system. The difference is that the Thomists hold to a physical “pressure” in regards to the efficacious grace whereas the Augustininas hold to a moral pressure. It’s true that both Augustinians and Thomists hold to an unconditioned predestination. But so do most Scotists, and even some Molinists (such as Cardinal Bellarmine).
As to how Thomism differs from Calvinism, this is hard question, because not every Calinist is the same, and not every Thomist is the same. It would overly simplistic to say that Calvinists believe in double predestination, in that God predestines some to heaven and some to hell. That’s not really true (at least not for the majority of Calvinists). Also, some Thomists, alas, like John of St. Thomas, hold to a position that comes dangerously close in my and many others’ opinon to double predestination, since he holds that there is a positive, and not merely permissive, unconditional decree of non-election for the reprobate.
For a modern Calvinist view of predestination, you can see R.C. Sproul discuss it in four parts.
Here’s the link to the first part (you can continue looking up for the other three parts).
For a Thomistic view of the controversy, I recommend reading Reginald Garrigou-Lagrange’s
Grace, book. A link can be found to it
here.
There’s a lot more I could say about this, but I think this should suffice.
Benedicat Deus,
Latinitas