[gmx-users] LJ simulation

Manoj justkewlguy at yahoo.com
Thu Jan 6 22:25:41 CET 2005


I am simulating a simple LJ fluid at a particular
state point (NVT run) and I am using the option
'vdwtype = Shift'. Now, whether I specify
'rvdw_switch' or not drastically changes the dynamics
I get. The pressure obtained in both the cases is
different but still not accurate and its closer to
accurate if rvdw_switch is specified. On the other
hand, when I use the option 'vdwtype=cutoff' along
with 'DispCorr = EnerPres' (without specifying
rvdw_switch), the pressure I get is pretty accurate
and compares very well with the Monte Carlo result I
already have.

My questions are:

1. What is 'rvdw_switch' fundamentally and what does
it actually do?
2. When Shift potentials are used, there is no long
range correction, right? The log file shows Dispersion
correction to be 0.0
3. Does gromacs code actually correct the shifted
potential back to true potentials and if it does then
why are the results so different for cut-off and Shift
4. If rvdw_switch is so important, what is the optimum
value for it?
5. Is there any standard ideal run input file which
can be used for pure LJ simulation for all kinds;
cut-off, cut-off and shifted ?

The values I am using are as follows:

There are no colulomb interations in my simulation.

My box dimensions are (2.43nm)^3

vdwtype             =  Shift
coulombtype         =  Shift
DispCorr            =  EnerPres
rvdw                =  1.0	
rvdw_switch         =  0.8
rcoulomb            =  1.0
rlist               =  1.1

Any insights or comments appreciated.


Gromacs User

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