[gmx-users] plus and minus signs in rtp files

[Joan Clark Nicolas]

Dear gmx users,
I am triying to add a new residue to the aminoacids.rtp file and I found
some plus and minus signs that I don't know what to do with. For example,
for the ASP, we have these:

[ bonds ]
(...)
    -C     N
(...)
[ impropers ]
    -C    CA     N     H
    CA    +N     C     O

Can you tell me what this signs mean and what rules I have to follow to put
them?

Thank you very much!

*Joan Clark i Nicolas*