Atom Schema or DTD

There is none. Stop looking!

Neither W3C Schema’s or Relax NG are capable of describing the the Atom specification so if you want a compliant validator you’ll need to write your own.

For a thread on the issue see:

No W3C Schema for Atom

I also ran into an interesting thread where James Clark puts forward Relax NG as a serious alternative to W3C Schema.

Relax NG vs W3C Schema

Having dealt with both I can say I prefer Relax NG. Its simpler.