Dear LAMMPS users:

I got question on the “fix langervin ”. My intension is to apply langervin thermal scale while keeping the total force unchanged. As manual instructed, I use “zero yes” as below:

fix 1 sub_g langevin 300 300 100 123456 zero yes

The group applied with langervin has only one atom type, thus the total force is supposed to be unchanged after applying “fix langervin”. However, after running, I found the total force on the group significantly changed. I tried to adjust the damping parameter but unable to help. Any suggestions? Thanks!

Thanks & Best Regards!

Dai Ling

Scientist

Institue of High Performance Computing, A*STAR

Singapore

Tel: +65-64191428

However,

after running, I found the total force on the group significantly changed.

How did you determine this, on a timestep by timestep basis?

Steve

Also: Please post the line from your input script containing fix ... nve?

(You said you applied langevin dynamics on only one atom, and then

zero the total force. This is a very strange test. Is the atom

moving?)

Cheers

Andrew

Hi:

Thanks for your kind comments.

Actually the ones applied with langervin has one type of atom (containing thousands of atoms actually), not one atom only.

I run two simulations with nve, one with langervin & one without. I output the total force information (all the force, including those from langervin setting) every 100 steps. As instructed in the manual, I zero out the total force that applied from the langervin setting. Thus, the total force values from the two runs shall be equal. However, it is not the case, there is always 5-20% differences between them. This is question that I raise.

Thanks for attention!

Input scripts:

# thermal equil

fix t_scale_1 therm_sub_g langevin 300 300 301 123456 zero yes

#Run sliding

fix 1 all nve

run 10000

Again I ask, how are you computing the total force?

Steve

Hi:

Thanks for your kind comments.

Actually the ones applied with langervin has one type of atom (containing thousands of atoms actually), not one atom only.

I run two simulations with nve, one with langervin & one without. I output the total force information (all the force, including those from langervin setting) every 100 steps. As instructed in the manual, I zero out the total force that applied from the langervin setting. Thus, the total force values from the two runs shall be equal. However, it is not the case, there is always 5-20% differences between them. This is question that I raise.

those two different simulations will *always* diverge

(usually in an exponential fashion). that is a consequence

of MD being a chaotic system and using floating point math

which is not associative and thus will always result in small

differences in the per atom total force, which will nucleate

the exponential divergence quickly. thus you can only expect

the equivalence of the two different simulations for a small

number of steps.

axel.