[lammps-users] possible reasons for increasing energy during NVE process

Hi, all

I am simulating a single polymer chain with all kinds of elements in it. In order to calculate the property of this chain, I need to stablize the chain at certain temperature. So I apply NVT and then NVE to the system. Strangely, I find the energy of the system keeps increasing dramatically during NVE process.

Same thing happens here when I simulation a carbon nanotube with some bumps on it. When I try to stablize the system at certain temperature and apply NVT and then NVE to the system. The energy of the system keeps increasing dramatically during NVE process.

I check the structures and they are all fine, without any bond breaking or overlapping problem. I am wondering what might be the possible reason for this during the NVE process.

The command for this is as follows:

fix 1 all nvt 300 300 10 drag 1
run 100000
unfix 1
write_restart nvt1.MLD

fix 2 all nve
run 100000
unfix 2
write_restart nve1.MLD

The temperature during the NVE increases from 300K to around 900K or higher depending on the simulation time and the energy also increases from around 600 to above 1000 and there is no sign showing they will stop at a certain value.

Thanks

2010-06-18

2010/6/18 justinliujun <[email protected]>:

Hi, all

I am simulating a single polymer chain with all kinds of elements in it. In
order to calculate the property of this chain, I need to stablize the
chain at certain temperature. So I apply NVT and then NVE to the system.
Strangely, I find the energy of the system keeps increasing dramatically
during NVE process.

Same thing happens here when I simulation a carbon nanotube with some bumps
on it. When I try to stablize the system at certain temperature and apply
NVT and then NVE to the system. The energy of the system keeps increasing
dramatically during NVE process.

I check the structures and they are all fine, without any bond breaking or
overlapping problem. I am wondering what might be the possible reason for
this during the NVE process.

the most common reason for that is
using too aggressive a time step.

have you checked at all how well your
simulation setup conserves energy?

also the time step is determined by
the fastest motion, i.e. the higher the
temperature, the shorter the maximally
possible time step.

cheers,
    axel.