Determinism is true as far as science can tell. By determinism I mean human actions and thoughts are not above natural laws. Given sufficient knowledge of a person's past, he future is as predictable as the future positions of the planet earth.*
Free will is a relation between one's action and his desire, and is thus independent of determinism. If one's action is the outcome of his own desires, then he is free. In other words, whether one will be free or not in the future is as determined as in the past
Physics is not governed by mathematics. Mathematics is the art of saying the same thing in different ways. When a mathematical model contradicts a scientific observation, we revise the mathematical model; we do not punish the physical world for defying the law.
*If I'm not mistaken, Bertrand Russell's view is slightly different from materialism. The difference is similar to that between agnosticism and atheism. Russell holds that, although much of materialism is very likely to be true, everything one can say about the outside world is inferred from observations of his own mental events.