[PyMOL] caver plugin

[<sun...@al...>]

Dear pymol users,
 
Does anyone use the caver plugin of pymol to analyze a ion channel? I don't know how to specify selection for the starting of plugin. Should the starting point be at the center of the channel where there is empty and no atoms in the molecule? or should it be certain atom of the molecule? If it should be the latter, what is the correct syntax of the selection? For example, if I want to select CA atom of Try21 in chain A, should I input the following command?
 
chain A and resid 21 and CA 
But this is wrong and does not work.
Could someone give me some tutorial? Thanks.
 
Yeping Sun