# Adjustment of the density and pressure

Good morning every body

Being a beginner, i wonder if it is any way to get the desired density,pressure by Lammps.

for example with copper at room pressure and temperature the density is 8.96 g/cm3.
Working in NVT if i adjust my system to get the appropriate density i lose the pressure and vice versa.

Best Regards

Good morning every body

Being a beginner, i wonder if it is any way to get the desired
density,pressure by Lammps.

for example with copper at room pressure and temperature the density is
8.96 g/cm3.
Working in NVT if i adjust my system to get the appropriate density i lose
the pressure and vice versa.

​please note that this question has no relation to LAMMPS at all. you would
get the same behavior with any other MD software using the same settings
and parameters.

parameterization for classical MD and how well (or not) potentials
​represent experimental properties. it is worth reading the paper(s)
discussing the parameter set(s) you are using as well.

axel.

Thank you for your prompt answer, thankful for your all useful suggestions that help us to improve our scientific levels.

First we get a good results using Matlab program of classical molecular dynamics calculations many characteristics of copper (so we are mastering the use of all the parameters about this matter). We would like to make a system more complex so is very hard to achieve it by matlab.

So to get the desired density/pressure by Lammps in NVT we couldn’t.

for example with copper at room T= 300 K the density is 8.96 g/cm3.

Weight = (Number of atoms)( atomic weight) = 204863,546= 130 142 g/mol
Volume = (Lattice fcc*Region block)^3= (28,906)^3= 24 152 a^3
Weight/Volume=5,38 g/(mol.a^3)=8,95 g/Cm^3

The pairwise potential used is Lennard-Jones : The same is used with matlab program gives a good results.
Depth of the potential well : 39.52
Finite distance: 2.3377
Rcut: 6.99

Normally in NVT group if i adjust my system to get the appropriate density i lose the pressure which should be approximately 1 atm.

that help us to improve our scientific levels.

First we get a good results using Matlab program of classical molecular
dynamics calculations many characteristics of copper (so we are mastering
system more complex so is very hard to achieve it by matlab.

So to get the desired density/pressure by Lammps in NVT we couldn't.

for example with copper at room T= 300 K the density is 8.96 g/cm3.

Weight = (Number of atoms)*( atomic weight) = 2048*63,546= 130 142 g/mol
Volume = (Lattice fcc*Region block)^3= (28,906)^3= 24 152 a^3
Weight/Volume=5,38 g/(mol.a^3)=8,95 g/Cm^3

The pairwise potential used is Lennard-Jones : The same is used with matlab
program gives a good results.
Depth of the potential well : 39.52
Finite distance: 2.3377
Rcut: 6.99

so is what you are saying, that LAMMPS with the lj/cut potential is
wrong and your matlab code is correct?
do you have any other proof for that claim outside that your matlab
implementation matches your expectations in this specific case?

Normally in NVT group if i adjust my system to get the appropriate density i
lose the pressure which should be approximately 1 atm.

so what pressure *do* do get?

axel.