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<DIV><FONT size=2 face=Arial>Hi, all,</FONT></DIV>
<DIV><FONT size=2 face=Arial> I was wondering why I got large
pressure after energy minimization. In MD, the pressure and pressure tensor
is defined like ( equation 4.151 in manual):</FONT></DIV>
<DIV><FONT size=2
face=Arial>
P=2*(E_kinetic + W)/(3*V), V is volume and W is
Virial.</FONT></DIV>
<DIV><FONT size=2 face=Arial>I run a simple system, fcc gold with LJ
potential to check the pressure after energy minimization. I
got:</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial> Pressure (bar)<BR>
8.75158e+03</FONT></DIV>
<DIV><FONT size=2 face=Arial><BR>Steepest Descents converged to Fmax < 1 in 1
steps<BR>Potential Energy = -1.4870625e+03<BR>Maximum
force = 2.2925207e-05 on atom 1606<BR>Norm of
force = 7.9803776e-06</FONT> </DIV>
<DIV> </DIV>
<DIV><FONT size=2 face=Arial>The force is very small and the energy minimization
finished in 1 steps, since all the atoms are at equilibrium. So I was wondering
why I got this huge pressure. And in a lot of simulations, I found huge
pressure after energy minimization. Even in free energy tutorial on Wiki, I
found large pressure as larege as 1 GPa. After energy minimization, the forces
are very small and no velocity, so the pressure should be close to
zero.</FONT></DIV>
<DIV><FONT size=2 face=Arial>Anyone has any idea about this?</FONT></DIV>
<DIV><FONT size=2 face=Arial>PS: Why the forces in traj.trr are all zeros?
</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>This is the mdp file:</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>; RUN CONTROL PARAMETERS
=<BR>integrator
=
steep<BR>nsteps
= 500<BR>; Output frequency for coords (x), velocities (v) and forces (f)
=<BR>nstxout
=
1<BR>nstvout
=
1<BR>nstfout
= 1<BR>; Output frequency for energies to log file and energy file
=<BR>nstlog
=
1<BR>nstenergy
= 1<BR>; ENERGY MINIMIZATION OPTIONS =<BR>; Force tolerance and initial
step-size
=<BR>emtol
=
1<BR>emstep
= 0.01<BR>; NEIGHBORSEARCHING PARAMETERS =<BR>; nblist update frequency
=<BR>nstlist
= 1<BR>; ns algorithm (simple or grid)
=<BR>ns_type
= simple<BR>; Periodic boundary conditions: xyz or none
=<BR>pbc
= xyz<BR>; nblist cut-off
=<BR>rlist
= 0.9<BR>; OPTIONS FOR ELECTROSTATICS AND VDW =<BR>; Method for doing
electrostatics
=<BR>coulombtype
=
PME<BR>rcoulomb
= 0.9<BR>; Dielectric constant (DC) for cut-off or DC of reaction field
=<BR>epsilon-r
= 1<BR>; Method for doing Van der Waals
=<BR>vdw-type
= Cut-off<BR>; cut-off lengths
=<BR>rvdw
= 0.9<BR></FONT></DIV>
<DIV><FONT size=2 face=Arial>Thanks in advance,</FONT></DIV>
<DIV><FONT size=2 face=Arial>Shuangxing Dai</DIV></FONT></BODY></HTML>