8.1.3. Run multiple simulations from one input script
This can be done in several ways. See the documentation for individual commands for more details on how these examples work.
If “multiple simulations” means to continue a previous simulation for more timesteps, then you simply use the run command multiple times. For example, this script
units lj atom_style atomic read_data data.lj run 10000 run 10000 run 10000 run 10000 run 10000
would run 5 successive simulations of the same system for a total of 50,000 timesteps.
If you wish to run totally different simulations, one after the other, the clear command can be used in between them to re-initialize LAMMPS. For example, this script
units lj atom_style atomic read_data data.lj run 10000 clear units lj atom_style atomic read_data data.lj.new run 10000
would run 2 independent simulations, one after the other.
For large numbers of independent simulations, you can use variables and the next and jump commands to loop over the same input script multiple times with different settings. For example, this script, named in.polymer
variable d index run1 run2 run3 run4 run5 run6 run7 run8 shell cd $d read_data data.polymer run 10000 shell cd .. clear next d jump in.polymer
would run 8 simulations in different directories, using a data.polymer file in each directory. The same concept could be used to run the same system at 8 different temperatures, using a temperature variable and storing the output in different log and dump files, for example
variable a loop 8 variable t index 0.8 0.85 0.9 0.95 1.0 1.05 1.1 1.15 log log.$a read data.polymer velocity all create $t 352839 fix 1 all nvt $t $t 100.0 dump 1 all atom 1000 dump.$a run 100000 clear next t next a jump in.polymer
All of the above examples work whether you are running on 1 or multiple processors, but assumed you are running LAMMPS on a single partition of processors. LAMMPS can be run on multiple partitions via the -partition command-line switch.
In the last 2 examples, if LAMMPS were run on 3 partitions, the same scripts could be used if the “index” and “loop” variables were replaced with universe-style variables, as described in the variable command. Also, the “next t” and “next a” commands would need to be replaced with a single “next a t” command. With these modifications, the 8 simulations of each script would run on the 3 partitions one after the other until all were finished. Initially, 3 simulations would be started simultaneously, one on each partition. When one finished, that partition would then start the fourth simulation, and so forth, until all 8 were completed.