Zero conformational entropy for each residue based on C-alpha atoms

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GROMACS version:2018.1

Dear users,

I am trying to calculate the conformational entropy per residue based on C-alpha atoms but I get zero value. I couldn’t figure out what the problem is.

First I generated an index file having a group for each residue. Then I used the below commands to calculate the entropy of each residue based on C-alpha atoms.

echo 'C-alpha C-alpha_&_r_1' | gmx covar -s ref.pdb -f traj.pdb -n index.ndx
gmx anaeig -v eigenvec.trr -entropy

Unfortunately I get zero entropy for both Quasiharmonic approach and Schlitter's formula for all residues. Am I doing something wrong?

Thanks in advance