Unfortunately, there's no way to request Atmos to compute a checksum for existing objects.  However, you can use the checksum feature when uploading objects and that checksum will be validated by Atmos and can be viewed at a later time through the object's metadata.
We tried to locate the checksum value on object's metadata resides on EMC storage without luck.   Thus, we tried to leverage on the Atmos-php method to ReadObject from EMC storage to perform the checksum value calculation as below:     Unfortunately, this is not an efficient/acceptable way especially on huge file. As this will incur bandwidth to download the object to perform such checksum calculation.   Please advise is there a way to perform this checksum calculation at EMC storage end without download it back locally? Thank you.   Jason Cwik 
Atmos-php EMC Storage Object Checksum Value