Classifications of all kinds are there to meet a need. For some purposes it makes perfect sense and is completely accurate to simply combine much of the world as 'theist' and all the rest as 'atheist'. In other contexts it makes much more sense to separate the theists into Hindi, Sikh, Christian and so on. So it seems to be a legitimate question whether 'atheist' can also be divided like that. And I think they can, but the demarcations are fewer. Many, many atheists use science as the foundation of their world-view, but by no means all. And even those that do will differ dramatically in the extent of their scientific understanding, from so superficial that it cannot really be called understanding at all, through to a very through understanding of the method and one or more subjects. Then they will differ in attitude: some atheists have a passive approach and are content to let anyone else believe what they like, others can't resist trying to prove everyone who is a theist is wrong. So I get back to my original point: yes, you can split either of these groups up or combine them. You do so based on what you want to discuss.