Using MIDUS Study data, I calculated mean Big 5 personality scores for atheists and others (sample size = 3,960).
Cohen's d .46
Cohen's d .60
Openness to experience
Cohen's d .85
*atheists and others are significantly different
Cohen's d is a measure of the gap between means. Compared to others, atheists are significantly less neurotic, less agreeable, and more open to experience. The differences for the latter two are large. It is not surprising to observe that atheists are disagreeable and open to new things.