No, a positive belief that there is no god is not necessary to be an atheist. The atheist simply believes that there is no evidence that god exists. It is impossible to provide evidence that something does not exist. The burden of proof is on those that wish to prove that it does exist. Without any evidence, God does not exist in the same way that unicorns and dragons do not exist. But there are people who are atheist, and then there are Atheists.