Dear Lammps Users,
I want to compute the temperature profile in a system including gas molecules confined between two solid walls. I want to subtract the velocity of the center of mass for each chunk, and going through the LAMMPS documentation I noticed that “compute temp/chunk” does what I want.
To find out how this compute works I used it on a system without any stream velocity and compared the results with the output of fix ave/chunk as follows:
%-----------------------------------------------------------LAMMPS input
variable N_bin equal 10
variable Bin_Length equal 1/${N_bin}
compute layers Argon chunk/atom bin/1d y lower ${Bin_Length} units reduced
compute myChunk Argon temp/chunk layers temp com yes
fix 1 Argon ave/time 100 100 10000 c_myChunk file temp_c.txt mode vector
fix 2 Argon ave/chunk 100 100 10000 layers temp file temp_fix.txt
%----------------------------------------------------------------------------------------------
First of all, according to the LAMMPS documentation for compute temp/chunk: “This compute calculates a global array and the number of rows in the array=the number of chunks Nchunk, as calculated by the specified, compute chunk/atom”. The thing is in my case the output of fix ave/time (as you can see in the following) has always 6 rows and changing the number of bins in compute chunk/atom does not have any effect on it. I was expecting since I specified Nbin=10, temp_c.txt should have 10 rows, am I wrong here?
#-------------------------------------compute temp/chunk+ fix ave/time output
Time-averaged data for fix 1
TimeStep Number-of-rows
Row c_myChunk
1510000 6
1 289.143
2 306.101
3 284.518
4 3.147
5 -3.76361
6 -5.95348
-------------------------------------------------------------------------------------- Secondly, I compared the results for temperature computed by "**compute temp/chunk+ fix ave/time"** with the one computed by **fix ave/chunk (**you can see it in the following). Since there is no stream velocity here and I am using the same Nfreq for both fixes, I was expecting that the output temperatures should be in the same range. But as you can see the results from **compute temp/chunk+ fix ave/time** are very weird and I am getting negative values for temperature! ---------------------------------------fix ave/chunk output
Chunk-averaged data for fix ave 2 and group file
Timestep Number-of-chunks Total-count
Chunk Coord1 Ncount temp
1510000 10 1500
1 0.05 0 0
2 0.15 362.54 320.979
3 0.25 164.19 303.946
4 0.35 148.48 298.835
5 0.45 161.15 305.061
6 0.55 146.88 329.105
7 0.65 146.36 321.014
8 0.75 132.49 334.138
9 0.85 237.91 341.474
10 0.95 0 0
%-----------------------------------------------------------------------------------------
I highly appreciate if someone could tell me what I am doing wrong here.
Best regards,
Shahin Mohammadnejad