Umbrella sampling does not generate PMF results

GROMACS version:2018.2-gcc910
GROMACS modification: No
Hi, All,

I found the following problem when I do umbrella sampling calculation.

When I do WHAM, it says it switched to exact iteration and the output in hist.xvg shows all the energies are zero.

What could be wrong? I am doing PMF calculation of pulling two slabs of polymers apart. I imagine the force could be very large.

Any comments or suggestions would be appreciated.

"Wrote histo.xvg
Getting initial potential by integration.
Initialized rapid wham stuff (contrib tolerance 1.05263e-08)
Evaluating only 4609 of 19000 expressions.

       1) Maximum change 2.864556e+02
     100) Maximum change 7.539510e-01
     200) Maximum change 7.172030e-01
     300) Maximum change 1.411430e+00

Switched to exact iteration in iteration 303
Converged in 304 iterations. Final maximum change 0
Wrote hist.xvg"

hist.xvg should have multiple histograms of positions within each window along the reaction coordinate, not energies. Do you get any overlap between these histograms? They should not be all zero, as some sampling had to occur somewhere.

My hist.xvg is actually the PMF output file. They have all zeros.
The histo.xvg shows the reaction coordinates. They have decent ovelap between those histograms.
The output is like this.

This file was created Fri Nov 20 11:48:34 2020

Created by:

:-) GROMACS - gmx wham, 2018.2 (-:

Executable: /share/apps/gromacs/2018.2/bin/gmx_mpi

Data prefix: /share/apps/gromacs/2018.2

Working dir: /home/**/umbrella_sampling2

Command line:

gmx_mpi wham -it tpr-files.dat -if pull-files.dat -o hist -unit kT

gmx wham is part of G R O M A C S:

Good gRace! Old Maple Actually Chews Slate

@ title “Umbrella potential”
@ xaxis label “\xx\f{} (nm)”
@ yaxis label “E (kT)”
@TYPE xy
1.520579e+00 0.000000e+00
1.545180e+00 0.000000e+00
1.569781e+00 0.000000e+00
1.594382e+00 0.000000e+00
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1.643584e+00 0.000000e+00
1.668185e+00 0.000000e+00
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1.717387e+00 0.000000e+00
1.741988e+00 0.000000e+00
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