Phase spacing in Umbrella sampling

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GROMACS version: 2018.1
GROMACS modification: Yes/No
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Q. How to select frames or conformations for umbrella samplig?
I have run umbrella sampling after steered MD simulation for RNA system by taking equidistant of 0.2 nm between two consecutive .gro files for 5 windows and did WHAM analysis for same but I didn’t get overlapped histogram. I have also tried with different time steps as well different distance spacing but I’m facing same problem with them also. I can provide files (summary_distance.dat, profile.xvg, histo.xvg and .gro files) if required.
I have used the following command for WHAM analysis:
gmx wham -it tpr-files.dat -if pullf-files.dat -o -hist -unit kCal
Query: So my question is- how can we get overlapped histogram from umbrella sampling and what is the basis of selecting the conformation for umbrella sampling for to get overlapped histogram.

Thanks and Regards…
Kailash Jangid

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What is the length of your reaction coordinate? 5 snapshots of 0.2 nm would be just about or might even be shorter that that of a non-bonded cut-off. Consider longer reaction coordinate first.
As for the non-overlapped histograms, you should see how much force you are applying to constrain each snapshot. Too large forces will impose high restrictions, whereas low forces lead to large fluctuation of your molecules during sampling.
Normally window distance of 0.1-0.2 nm should work.
Maybe start by performing a good SMD benchmarking as explained here.

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