Cutoff in lj/cut

Hello all,

I have a question related to the cutoff arg in the lj/cut off command. My aim is to create a purely repulsive potential and I am confused as to whether the cutoff specification scales with the user specified sigma value

Suppose if I were to set sigma(the second arg) to 1,then setting the cutoff to 1.122 (2^1/6) would make it purely repulsive as 2^1/6*sigma corresponds to the potential minimum.

Will setting sigma to a higher value such as setting it to 2 require a higher cutoff such 1.122*2=2.244, or is the specification of 1.122 only required acting as a pure scaling factor?


This is a question that you should be able to figure out an answer for by yourself.

There are two way to figure it out:

  • through thinking and from a careful read of the documentation
  • empirically by crating some example inputs

Dear akohlmey,

I decided to ask this question after having read through the documentation where the cutoff is only specified as a global cutoff. I tried simulating an example of sigma = 1.5 and cutoff as 1.5*1.122= 1.683 but I was not able to fully make out if it was purely repulsive, and therefore I didn’t want to come to any conclusions as I am very new to lammps, and as a result I generally tend to be uninformed on many operating syntax in lammps.

I had generally wanted to assume that since a lot of parameters are reduced due to scaling in lj units, I felt the cutoff might be scaled too which is why I felt it would be appropriate to ask on this forum.

I would also like to apologize if this question was too inept of me
Thanking you,

LAMMPS does what the documentation says. No less, no more.