The built-in atomsel type

NEW IN VMD 1.8.6: The AtomSel class has been deprecated, in favor of a new built-in type called atomsel. The atomsel type functions much the same way as the old AtomSel class, but also provides methods similar to the Tcl interface for returning RMS fit matrices and applying those transformations to coordinates.

The new atomsel type is found in a new built-in module, also called atomsel. Use help(atomsel) to get the complete documentation.