No, atheist is not another form of existentialism.
For those that think it is, please explain how atheism is another form of existentialism.
The term existentialism is also used to describe the branch of philosophy concerned with the fundamental nature of existence. In this sense, atheism would be described as materialism.
From my limited knowledge of a broad subject, I think Atheism is an extension of existentialism in that it deals with looking at one's surroundings and applying healthy cynicism. I however, do not think all atheists are exestential any more than I would assume all atheists are liberal (or relative potical party) individualism trumps any form of a cut and dry category.
Just my humble opinion.
Existentialism is a worldview that actually has a theistic and an atheistic side. Kierkegaard was considered by many to be a theistic existentialist. I like to think of atheism as a modifier of how I look at existentialism. When I was a theist, I viewed existentialism in the context of God wanting me to exercise my free will. Looking back, I realize that I was just taking baby steps away from religion and it was safe to call myself a theistic existentialist.
I would have originally considered atheism closer to Nihilism, but I don't think that anymore.