What is the ruling on shaking hands with an atheist and smiling at him after a discussion with him?

The majority of jurists are of the opinion that it is disliked (makrooh) to shake hands with disbelievers, including atheists, due to their denial of God's existence. Shaking hands with disbelievers and Magians is disliked. This dislike is removed, however, in cases of need or a recognized benefit, such as hoping for their conversion to Islam or removing their misconceptions.