Hi all ,

I encountered a problem when I tried to use the command fix addforce to apply the sample loading .I received the error information “ERROR: Expected floating point parameter in input script or data file”.Here is my partial input files.

label loop variable n loop 5000 variable A equal -0.005*($n-1) variable B equal -0.005*$n variable C equal 0.005*($n-1) variable D equal 0.005*$n variable f1 equal ramp($A,$B) variable f2 equal ramp($C,$D) fix 4 left addforce v_f1 NULL NULL fix 5 right addforce v_f2 NULL NULL run 500000 unfix 4 unfix 5 fix 6 left addforce $B NULL NULL fix 7 right addforce $D NULL NULL run 500000 unfix 6 unfix 7 next n jump in.line_little420 loop variable n delete

Someone told me that I should replace NULL with a floating point parameter , because lammps regards the NULL to a integer. Then I replaced the NULL with 0.0.

Namely，the changed input files as follows:

label loop variable n loop 5000 variable A equal -0.005*($n-1) variable B equal -0.005*$n variable C equal 0.005*($n-1) variable D equal 0.005*$n variable f1 equal ramp($A,$B) variable f2 equal ramp($C,$D) fix 4 left addforce v_f1 0.0 0.0 fix 5 right addforce v_f2 0.0 0.0 run 500000 unfix 4 unfix 5 fix 6 left addforce $B 0.0 0.0 fix 7 right addforce $D 0.0 0.0 run 500000 unfix 6 unfix 7 next n jump in.line_little420 loop variable n delete

To my happiness, There is no error any more .But In my simulation ,I could not add force in the y and z direction.That is to say, I must keep the y and z direction relaxed.If I replaced the NULL with 0.0,the simulation is not that I want to get .So what I want to know is whether there exists another way to solving the problem rather than what I did .Can anone give me some suggestions or feedback? I would really appreciate your kindly help.