[lammps-users] Simulated annealing

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1

Dear users,

I would like to ask you a question about how to perform annealing for equilibration of a hydrocarbon system using LAMMPS.

For doing volume annealing, I am confused about which of the following commands I should use (say, I want to expand the box)

1) fix 1 all deform 100 x scale 1.5 y scale 1.5 z scale 1.5
or
2) fix 2 all npt temp 600.0 600.0 100.0 iso 1.0 0.5 1000.0

Any help is greatly appreciated.

Thanks.

Farshad Saberi-Movahed

Graduate Assistant
Mechanical Engineering Department
University of Alabama at Birmingham

2

You can do either. Fix deform will expand the box regardless
of pressure. Fix npt will expand/contract the box in response
to pressure.

Steve

3

Steve,

Thanks for you response. I tried both of them and I have come up with some issues.

1) By fix deform, I could truly reach the desired volume, but as you said the pressure increased drastically and uncontrollably to 150,000 atm.

2) By fix npt, whatever value I used for Pstart and Pstop parameters in "iso" style, the volume decreased from the initial value of 8000 A^3 to around 1000 A^3 while I wanted to reach 4000 A^3.

Any help would be greatly appreciated.

Farshad

4

Steve,

Thanks for you response. I tried both of them and I have come up with some issues.

1) By fix deform, I could truly reach the desired volume, but as you said the pressure increased drastically and uncontrollably to 150,000 atm.

2) By fix npt, whatever value I used for Pstart and Pstop parameters in "iso" style, the volume decreased from the initial value of 8000 A^3 to around 1000 A^3 while I wanted to reach 4000 A^3.

Any help would be greatly appreciated.

very obviously, something in your input does not
agree with what you expect to be the density of
the system. most likely you either use wrong potential
parameters (or settings for keywords that affect parameters,
e.g. units) or you miscalculated the density of your system.

axel.