This is an old revision of the document!
http://www6.atomicorp.com/channels/atomic/centos/6/i386/RPMS/
Atomic offers RPM packages not available in the official repos.
Check for version and architecture first (64-bit shown here):
rpm -Uvh http://www6.atomicorp.com/channels/atomic/centos/6/i386/RPMS/atomic-release-1.0-14.el6.art.noarch.rpm
Disable the repository unless specifically enabled:
vim /etc/yum.repos.d/atomic.repo enabled=0