[lammps-users] No fix, No move?

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1

Hi,

But I think the volume of the system is constraint by the fix NVE. Is this no business with the position/velocity update?
Another question: If I want to add some energy to the system in a period time and then let the system go, what can I do? It’s seemly that there is no ensemble I can use.

Thanks

在2008-10-23，“Vikas Varshney” <vv0210@…24…> 写道：

2

Hi,

But I think the volume of the system is constraint by the fix NVE. Is this no business with the position/velocity update?

there is no constraint. you just run. please have a deeper look
into one of the textbooks on molecular simulations.

Another question: If I want to add some energy to the system in a period time and then let the system go, what can I do? It's seemly that there is no ensemble I can use.

NVE ensemble doesn't change the energy, if you add a fix
that adds energy (fix heat, fix addforce etc.) then the
total energy will go up.

in fact, conservation of energy in NVE ensemble is a measure
of how well a simulation is set up (provided no energy is added,
etc.). with too large a time step, bad cutoffs, the energy
will go up, which would be unphysical.

axel.

3

If you want to add energy to the system, rescale
the velocities with the velocity command.

Steve

2008/10/22 Xiaopeng Huang <[email protected]...>:

4 
在2008-10-23，"Steve Plimpton" [[email protected]> 写道：
>If you want to add energy to the system, rescale
>the velocities with the velocity command.
>
>Steve
](mailto:[email protected]> 写道：>If you want to add energy to the system, rescale>the velocities with the velocity command.>>Steve)


 At the same time, should I turn off the NVE integration and turn it on after the rescaling? Or I should ask: Does fix NVE(1st fix) mean that the total energy of the system will be always the same as the initial energy? If I add energy to the system, will the total energy of fix NVE be the new one?
 
Thanks

>2008/10/22 Xiaopeng Huang
5

The velocity command is a one-time operation. It doesn't matter
what fixes are set. Fix NVE will generally conserve energy, but
there are various ways to change that.

Steve

2008/10/23 Xiaopeng Huang <[email protected]...>: