Which U.S. Presidents were atheists?

I'm assuming some were. Here's a summary of what we know about each of the Presidents. What do you think? Perhaps John Tyler:

There is not much evidence of Tyler's beliefs, though he's often listed as Episcopalian. Following his death a friend said, "He was a firm believer in the atonement of the son of God, and in the efficacy of his blood to wash away every stain of mortal sin. ... He was by faith and heirship a member of the Episcopal Church and never doubted divine revelation." However, no pastor came to his home to administer the last sacrament, and in her account of his last illness, Tyler's wife wrote nothing of faith, hope, Christ or eternity.