@steem-atlas is not a witness itself so you cannot vote for it.

You can however set your witness proxy to @steem-atlas which means your witness voting power is given to @steem-atlas.

If you set your witness proxy to @steem-atlas you will receive the higher Delegator Reward rate of 0.45 STEEM / 1000 SP / day.

Votes are calculated dynamically every day depending on the number of posts to be voted, and the beneficiary rates and delegation levels.