Many atheists are also naturalists but do not realize it. Worldview naturalism is a science based non-supernatural view that denies the existence of anything science does not currently hold to be true, such as souls, supernatural afterlives, ghosts, gods, demons, and anything else supernatural.
Many atheists agree with naturalism (Richard Dawkins, Sam Harris, Daniel Dennett, and Christopher Hitchens did too.) - but it is not a word that is used often enough, even though it is the proper word for those who hold a scientific worldview, whereas "atheist" only covers the stance that one is not convinced of god claims. For more information on naturalism check out naturalism.org which is ran by Tom W. Clark - a man that was talking about determinism twelve years before Sam Harris popularized the free will debate.