# Density/number in Real Units

Dear Lammpsusers,
I am simulating water flow confined between Graphene sheets. Using ave/spatial I have obtained the profile of Density/mass and Density/number whose values are as follow:

Density/mass Density/number
0 0
0.00456557 0.0021233
0.724465 0.0699176
0.718348 0.0733305
0.787507 0.0789899
0.776581 0.0779707
0.779293 0.0781798
0.792223 0.0794162
0.724277 0.0739281
0.730415 0.0705037
0.00451417 0.00211618
0 0
The Density/mass units look correct as g/cm3 as specified in the documentation. But what are the units for Density/number? As the lengths are as angstroms, is Density/number as 1/A3? But the data do not seem to be compatible.
Thanks for any help.
Best regards,
Fahim

Dear Lammpsusers,
I am simulating water flow confined between Graphene sheets. Using
ave/spatial I have obtained the profile of Density/mass and Density/number
whose values are as follow:

​fix ave/spatial has been declared obsolete over three years ago removed
from LAMMPS over two years ago.

axel.​

I think I would get same results if I replace ave/spatial with ave/chunk.
By the way, let’s address the main issue. What is the output unit for Density/number in Metal Units sets?

Sorry in Real Units sets.

I think I would get same results if I replace ave/spatial with ave/chunk.

​prove it!

By the way, let's address the main issue. What is the output unit for
Density/number in Metal Units sets?

​i don't care about the results from obsoleted code. with the current
implementation you *do* get number densities in 1/\AA^3.

Sorry in Real Units sets.

​same difference. both use angstrom as length unit.

axel.​

I replaced the ave/spatial with two commands of chunk/atom and ave/chunk; and got the same results.
Now, let’s address the main issue. The values of Density/mass and Density/number do not seem to be compatible.
=>
Density/mass (g/cm3) * 1e-24 = Density/mass (g/A3)
Density/mass (g/A3) * Avogadro constant (1/mol) / 18 (g/mol) = Density/number (1/A3)

Above relations should hold true; and based on these the values I obtained (see below) are not compatible.
Would you please investigate and comment? Thanks.

I replaced the ave/spatial with two commands of chunk/atom and ave/chunk;
and got the same results.
Now, let's address the main issue. The values of Density/mass and
Density/number do not seem to be compatible.
=>
Density/mass (g/cm3) * 1e-24 = Density/mass (g/A3)
Density/mass (g/A3) * Avogadro constant (1/mol) / 18 (g/mol) =
Density/number (1/A3)

​you don't seem to factor in here, that each water molecule has 3(!) atoms​.

...
Above relations should hold true; and based on these the values I obtained
(see below) are not compatible.

​number density is average number of atoms per bin divided by volume of the
bin and *that* number *does* hold true.
you know the count per bin from the output file, and if you don't trust
that, choose your bin spacing so that there is only one bin which
encompasses the total system and thus all atoms and you can check the count.

Would you please investigate and comment? Thanks.

is see no need to investigate further. PEBCAC, AFAICT.

​axel.​