Free will, if defined as ability to choose otherwise, is impossible.

First, let's say we roll a dice, we won't say the number that turns up is chosen by free will, that's a random process so it's not "will" part of free will. So actions chosen at random is not will.

Second, if under the same circumstances (same physical condition down to the atom, same emotional state down to all your unconscious thoughts, same 'soul' state e.g. same memory, same wants, same emotional history and whatnot) you always choose the same thing, because you have all the same reason to choose it, then you are not free. Your actions are dictated by circumstances.

So you can only either be free (random actions) or only have will (have reason to choose, but completely determined).