Doubt to save the center of mass of previous timestep

LAMMPS LAMMPS General Discussion
2

That is quite tricky business, but here is a possible way to do what you want:

reset_timestep  0
compute         com mygroup com
fix             com all vector 1 c_com[1] c_com[2] c_com[3]
variable        step equal (step<1)+step
variable        cx equal f_com[v_step][1]
variable        cy equal f_com[v_step][2]
variable        cz equal f_com[v_step][3]

Here the variables cx, cy, and cz will contain the values of the com compute of the previous step, exept for the first step.

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