next command — LAMMPS documentation

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# next command

## Syntax

    next variables

- variables = one or more variable names

## Examples

    next x
    next a t x myTemp

## Description

This command is used with variables defined by the variable command. It assigns the next value to the variable from the list of values defined for that variable by the variable command. Thus when that variable is subsequently substituted for in an input script command, the new value is used.

See the variable command for info on how to define and use different kinds of variables in LAMMPS input scripts. If a variable name is a single lower-case character from “a” to “z”, it can be used in an input script command as $a or $z. If it is multiple letters, it can be used as ${myTemp}.

If multiple variables are used as arguments to the next command, then all must be of the same variable style: index, loop, file, universe, or uloop. An exception is that universe- and uloop-style variables can be mixed in the same next command.

All the variables specified with the next command are incremented by one value from their respective list of values. A file-style variable reads the next line from its associated file. An atomfile-style variable reads the next set of lines (one per atom) from its associated file. String- or atom- or equal- or world-style variables cannot be used with the next command, since they only store a single value.

When any of the variables in the next command has no more values, a flag is set that causes the input script to skip the next jump command encountered. This enables a loop containing a next command to exit. As explained in the variable command, the variable that has exhausted its values is also deleted. This allows it to be used and re-defined later in the input script. File-style and atomfile-style variables are exhausted when the end-of-file is reached.

When the next command is used with index- or loop-style variables, the next value is assigned to the variable for all processors. When the next command is used with file-style variables, the next line is read from its file and the string assigned to the variable. When the next command is used with atomfile-style variables, the next set of per-atom values is read from its file and assigned to the variable.

When the next command is used with universe- or uloop-style variables, all universe- or uloop-style variables must be listed in the next command. This is because of the manner in which the incrementing is done, using a single lock file for all variables. The next value (for each variable) is assigned to whichever processor partition executes the command first. All processors in the partition are assigned the same value(s). Running LAMMPS on multiple partitions of processors via the -partition command-line switch. Universe- and uloop-style variables are incremented using the files “tmp.lammps.variable” and “tmp.lammps.variable.lock” which you will see in your directory during and after such a LAMMPS run.

Here is an example of running a series of simulations using the next command with an index-style variable. If this input script is named in.polymer, 8 simulations would be run using data files from directories run1 through run8.

    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

If the variable “d” were of style universe, and the same in.polymer input script were run on 3 partitions of processors, then the first 3 simulations would begin, one on each set of processors. Whichever partition finished first, it would assign variable “d” the fourth value and run another simulation, and so forth until all 8 simulations were finished.

Jump and next commands can also be nested to enable multi-level loops. For example, this script will run 15 simulations in a double loop.

    variable i loop 3
      variable j loop 5
      clear
      ...
      read_data data.polymer.$i$j
      print Running simulation $i.$j
      run 10000
      next j
      jump in.script
    next i
    jump in.script

Here is an example of a double loop which uses the if and jump commands to break out of the inner loop when a condition is met, then continues iterating through the outer loop.

    label       loopa
    variable    a loop 5
      label     loopb
      variable  b loop 5
      print     "A,B = $a,$b"
      run       10000
      if        $b > 2 then "jump in.script break"
      next      b
      jump      in.script loopb
    label       break
    variable    b delete

    next        a
    jump        in.script loopa

## Restrictions

As described above.

## Related commands

jump, include, shell, variable,

## Default

none