Hello,
I am attempting to construct the free energy profile for rotation of a
short peptide hairpin. For this purpose, I am using [
angle_restraints_z ]. I will focus this post on the simplified test
system that I have been using to characterize the problem that I am
experiencing: single peptide in vacuo using the sd integrator. The
free energy profile for rotation here should be flat (After entropic
debiasing).
However, I do not find a flat PMF. I find that the PMF (generated by
WHAM) has local maxima at every center of restraint. This indicates
over-sampling at distances away from each center of restraint and if I
input a lower force constant to WHAM (400 kJ/mol in place of the 500
kJ/mol that I used for sampling), I can indeed get a flat PMF. So it
seems to me that there is some problem with either i) the sampling or
with ii) the post-sampling analysis of sampled positions. I have ruled
out some obvious possibilities for option ii: a) The stored angles
accurately reproduce the sampled angles as checked by comparison to
g_energy; b) The modified version of WHAM that I am using
(angle_restraints_z is non-harmonic) successfully produces a flat PMF
with fake data generated based on a MC generation program that I
wrote. Further, the local-maxima-at-centers-of-restraint problem is
not alleviated by increasing the window overlap.
Importantly, I'm using a highly unusual virtual atom architecture to
get at this angle to the z-axis because I was unable to define a COM
group for this purpose. I do end up defining the angle between two
COMs, but I do it via i) generation of a number of [ virtual_sites2 ] :
[ virtual_sites2 ]
;sites from funct a1
288 52 239 1 0.5
Where I eventually utilize virtual atom 288 as one atom in the
definition of angle_restraints_z. Note that atoms 52 and 239 are not virtual.
[ angle_restraints_z ]
138 288 1 0 500 1
I have created a test system in which I take a water molecule and
place a virtual atom between the two hydrogens and do similar [
angle_restraints_z ] on the angle between the oxygen and the new
virtual atom -- here I do not see the effect.
I have also tested not using the virtual site in my peptide
simulation, using instead:
[ angle_restraints_z ]
138 239 1 0 500 1
and in this case I also do not see the effect.
And ideas about what could be going on or test systems that I might try?
My best guess is that by putting the restraint on a virtual_sites2
atom, I am not doing exactly what I think I am doing, but I have no
idea what the difference might be.
Thank you,
Chris.
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