Dear gmx-users,

I have a problem concerning energy conservation when using user potentials 
(tables).
I'm using gromacs 4.5.4
I simulate a simple Lennard-Jones(6-12) +Fene polymer melt (1600 chains a 10 
beads in a 26x26x26 periodic box).

I tried different vdwtypes (cutoff always 3.24):
The cut-off version does not conserve energy -- okay.
The shift and switch versions conserve energy -- fine.

Now I wanted to do the same with user tables:
Simple Lennard-Jones table gives really the same results as the cut-off version 
-- good.

But if I use a table with shifted Lennard-Jones potential it is not comparable 
to the shift version
and the energy is not conserved -- ?

I use a shift function as written in the manual (chapter 4.1.5) -- there must 
be a factor alpha added in the constants A and B --
(r1=0).

The parameters are the same for shift version and shifted user version.

Has someone an idea why the shifted user potential doesn't work in this way?

Here is the mdp:


integrator              = md-vv
dt                      = 0.0035
nsteps                  = 1000
nstxout                 = 1
nstvout                 = 1
nstfout                 = 1
nstlog                  = 1
ns_type                 = grid
pbc                     = xyz
rvdw                    = 3.24
rlist                   = 3.6
tcoupl                  = no
tc-grps                 = System
tau_t                   = 2.0
ref_t                   = 127.2717
vdwtype                 = user;Shift
rcoulomb                = 3.6;2.24;1.12
coulombtype             = Cut-off
rvdw-switch             = 0.0

energygrps              = bead
energygrp_table         = bead bead


Thanks in advance
Anja



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