In answer to your question, none.  Atheists are atheists because there is no evidence of a god.  If irrefutable evidence emerged, all rational atheists would immediately become theists.  Simple.  Their lack of a belief in a god is based on the same principles that you don't believe in unicorns (I hope).  The positive beliefs of atheists, like scientists, are based on the facts and evidence before them.  Existing beliefs are considered in the light of new evidence and good evidence leads to changed beliefs.  The default theist position (but probably with regards to religious issues only) is the opposite of this: new evidence is considered in the light of existing beliefs and supporting (or non-contradicting) evidence is believed and and conflicting evidence is ignored.