<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=windows-1252">
<META NAME="Generator" CONTENT="MS Exchange Server version 6.0.6249.1">
<TITLE>helices, again</TITLE>
</HEAD>
<BODY>
<!-- Converted from text/plain format -->
<P><FONT SIZE=2>Dear Gromacs Users!<BR>
<BR>
I apologize if experts are fed up with my questions, but I am a beginner<BR>
and would like to move ahead.<BR>
<BR>
I found a strange thing when I performed energy minimization. I created a<BR>
conf.gro file from the pdb input using pdb2gmx and also obtained topol.top<BR>
<BR>
Then I typed<BR>
<BR>
grompp -f emgas.mdp -c conf.gro -p topol.top<BR>
<BR>
(emgas.mdp file see below). The box vectors (bx, by, bz) in conf.out were<BR>
<BR>
Bx = 2.29, By = 4.74 Bz = 2.50<BR>
<BR>
As the result of EM, I obtained the confout.gro file. I compared conf.gro with confout.gro. The first column with real numbers (I assumed that they are<BR>
the x-coordinates) changed only by thousands of an nm (changes in the third<BR>
decimal). The difference of the third columns (z-coordinates) of conf.gro<BR>
and confout.gro was about 2Bz for the firs helix !!!. x and y coordinates<BR>
differed by about 0.001-0.005 nm for the first helix. When I moved to the<BR>
second one, then x changed in the two .gro's by up to 0.005 nm, now <BR>
y differed by "By" (it is not a joke with the word by, I meant the difference<BR>
was about By = 4.7 nm), and the z difference was reduced to 1Bz.<BR>
<BR>
Thus it seems to me that the coordinates were modified by 2Bz for<BR>
helix1, and by (1By + 1Bz) for helix2. Then I am not surprised that when<BR>
I performed an MD and checked the motion in ngmx, my double helix seemed to<BR>
have fallen apart. The very likely reason is a coordinate transformation<BR>
throughout the energy minimization. The minimization process itself probably works correctly, because the coordinate changes only by up to 0.01 nm, and the rude change is a consistent translation along the y and z axes. But what did cause it?<BR>
<BR>
For safety, I enclose the emgas.mdp file below:<BR>
<BR>
title = cpeptide<BR>
cpp = /lib/cpp<BR>
constraints = none<BR>
integrator = steep<BR>
dt = 0.002 ; ps<BR>
nsteps = 1000<BR>
nstlist = 10<BR>
ns_type = grid<BR>
rlist = 0.9<BR>
rcoulomb = 0.9<BR>
rvdw = 0.9<BR>
;<BR>
; Energy minimizing stuff<BR>
;<BR>
emtol = 100.0<BR>
emstep = 0.01<BR>
<BR>
<BR>
Thanks for any help<BR>
<BR>
Peter Nagy</FONT>
</P>
</BODY>
</HTML>