LAMMPS WWW Site - LAMMPS Documentation - LAMMPS Commands

if command

Syntax:

if value1 operator value2 then command1 else command2 

Examples:

if ${steps} > 1000 then exit
if $x <= $y then "print X is smaller = $x" else "print Y is smaller = $y"
if ${eng} > 0.0 then "timestep 0.005"
if ${eng} > ${eng_previous} then "jump file1" else "jump file2" 

Description:

This command provides an in-then-else test capability within an input script. Two values are numerically compared to each other and the result is TRUE or FALSE. Note that as in the examples above, either of the values can be variables, as defined by the variable command, so that when they are evaluated when substituted for in the if command, a user-defined computation will be performed which can depend on the current state of the simulation.

If the result of the if test is TRUE, then command1 is executed. This can be any valid LAMMPS input script command. If the command is more than 1 word, it should be enclosed in double quotes, so that it will be treated as a single argument, as in the examples above.

The if command can contain an optional "else" clause. If it does and the result of the if test is FALSE, then command2 is executed.

Note that by jumping to a label in the same input script, the if command can be used to break out of a loop. See the variable delete for info on how to delete the associated loop variable, so that it can be re-used later in the input script.

Note that if either command1 or command2 is a bogus LAMMPS command, such as "exit" in the first example, then executing the command will cause LAMMPS to halt.

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 thru 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: none

Related commands:

variable

Default: none