[lammps-users] npt box length changing independly

Dear all,

When the command “fix npt” is used, one has to set the p-style parameters. With p−style = xyz or xy or yz or xz or aniso, some coupled 3 axis or 2 axis or no couple pressure can be achieved. Whatever those styles have been chosen, the 3 box length change with a same ratio if set with same pressure. For example begin box (10 10 10) will turn to ( 3 3 3) under pressure 2 or ( 8 4 2) to (4 2 1).

  1. How can I let the 3 box length change independently (maybe the scale of three axis will change) under the same pressure because to ellipse the end to end interaction is different to that of side to side? I feel the length of orientation axis should to longer than other two lengths. For example begin box (10 10 10) will turn to ( 3 3 5) under pressure 2.

  2. As the example in manual page 273 “fix 2 all npt/asphere 300.0 300.0 100.0 xz 5.0 5.0 NULL NULL 5.0 5.0 , is it mean that pressure (X)= pressure (Z)= 5 while pressure (Y)=0? When I checked the log file, it looks that the length of z box is not changed. What kind of system is the example want to describe?

With style aniso, the 3 box lengths should change independently. You can verify this by starting with a random configuration for the ellipsoids. Then, play with the potential parameters and initial configuration for your input script to understand why there is symmetry for your specific case. Hope that helps.

  • Mike