Re-assignment of atom IDs problem for post-processing

Hi all,

Having faced the issure of re-assignment of atom IDs for the LIGGGHTS post-processing. As I want to record the position or velocity of some certain particles using
the following commands:
---------------------------------------------------
...
variable x1 equal x[512]
variable y1 equal y[512]
variable z1 equal z[512]
fix pos all print 1000 "${time} ${x1} ${y1} ${z1}" file post/pos_512.txt
....
_____________________________

here 512 is the atom ID (in Paraview as PointID) at time = 0.

Unfortunately, the recorded positions in "pos_512.txt" are not correct after plotting the data , because the atom ID of a single sphere is changing over time as shown in the attached image 1 and 2. This is a big issue for the post-processing of certain atoms.

So I wonder, how to turn off the atom IDs renumbering so that certain quantities of a single atom/sphere can be written to a file?

One extra question: for a single atom/sphere, the position and velocity can be accessed by:
x[i], y[i], z[i], vx[i], vy[i], vz[i]

for the compute style quantity, such as:
compute disp all displace/atom
how can we access the displacement of a single atom?

thanks for your help

Best regards,

David