[gmx-users] Can Ryckaert-Bellemans(RB) function be used to calculate improper dihedrals?

[CG]

In Gromacs manual 4.2.12, it lists a few of functions calculating proper and
improper dihedrals. I am wondering if the RB function can be treated as a
universal function to calculate both proper and improper dihedrals? 

This question came up in my mind because I used the Perl script 'amb2gmx.pl'
to convert AMBER format prmtop file to topol.top file in GROMACS format. By
checking the outcome topol.top, all the dihedrals are addressed to function
type 3, which is the Ryckaert-Bellemans dihedrals. This is somewhat
confusing since my original AMBER files show there are plenty of improper
dihedrals there but the topol.top file only contains one type of dihedral.
Then I read the amb2gmx.pl in detail and found it seems like all kinds of
dihedrals are mixed and converted to Ryckaert-Bellemans dihedrals. 

I think I just need to know more about the concept of RB function. Can
anyone give some hints? Thanks. 

--C.G.

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