But that be the case, how can we say God chose to create rather than say he had to create (not on the basis of external influence but rather by an internal determined nature)?
Because creating is not some imperative or end of God’s nature. His nature is perfectly fulfilled whether he creates or not. A human being acts to fulfill his nature. An automaton acting in a programmed way is it fulfilling its nature. St. Thomas argued that God necessarily must know and will himself. It cannot be otherwise, and God is his own end. But he does not necessarily will other things. Or to try to put it another way, the general, natural tendencies I have (or a bird has, or a rock has) are all tendencies that in some way are directed to fulfilling that creature’s nature. We tend towards fulfillment. God’s nature is already fulfilled, and so has (and so his nature has…) no tendency towards not creating, or tendency to create, or tendency to create this world instead of that world. Creation is not an end of God’s nature.
Now when it comes to mankind or other creatures I want to be clear that I don’t think having natural tendencies (towards needing to eat, for example) behind our choices rules out free choice. What’s important is that we have in a sense the capacity to choose between different options and that we are agents in making choices (that is, the choice follows from intrinsic principles in us, such as our knowledge and the movement of the will towards what the intellect grasps as a good), that if in principle we (following our knowledge) grasped some different end as more good we would choose that instead. The choice stems from these intrinsic principles and are not just because we’re being moved like puppets by external principles (or for no reason at all).
[And my toddler just woke up, but I was nearing the end anyway.]
But my point was to illustrate what I wrote in a previous post, that in this consideration God’s choice to act as he has seems may be seen as the examplar case of a free choice.