Problems with calculating torsional ACF (gmx chi)

GROMACS version: 2016.4

Hi all,

I have a system of polymer chains and I want to calculate the torsional ACF to characterise the chain rigidity. When I use gmx chi -f .xtc -s .gro -corr I get the following error:

No dihedrals in your structure!

This makes sense because the model I use doesn’t define any dihedrals in the topology but surely there are still dihedrals there, I’m just not controlling them with some potential. Could anyone help me with this?


In general a tool needs information on the topology (topol.tpr) to recognize the dihedral angles or an index file with an group containing the dihedral angles.

An alternative way to calculate acf of dihedral angle is to generate an group for the torsional angles (gmx make_ndx or gmx select can help you) and then I think gmx angle can calculate acf.


1 Like