Hi,
I’m performing a isotherm of Ar in a box of 1000A^3
with the setting that I report below. I obtain a incorect results. (T zero P zero, etc.)
In forum I found many discussions about the use of ‘shake’, or other keywords, but at the end I didn’t find an univocal procedure to obtain a good results.
Peraphs LAMMPS is not a correct software to perform this type of simulation?
regards
Al
input
GCMC script with class2 ff for simulated isotherm Ar 87K
System
units real
atom_style full
dimension 3
boundary p p p
newton on on
Styles
pair_style lj/class2/coul/long 18.5
bond_style class2
angle_style class2
dihedral_style class2
improper_style class2
special_bonds lj 0 0 1 coul 0 0 1
kspace_style ewald 1.0e-07
Data file input
read_data box_10_Ar.data
Settings
dielectric 1.0
neighbor 3.0 bin
neigh_modify delay 0 every 1 check yes
run_style verlet
Probe
molecule ar ./ar.txt offset 0 0 0 0 0
Variable
variable itemp index 87.0
variable ftemp index 87.0
Run isotherm
compute_modify thermo_temp dynamic yes
fix 1 all gcmc 100 1000 1000 0 88891 87.0 0.0 1 mol ar pressure 0.90 fugacity_coeff 0.90
fix 2 all nvt temp 87.0 87.0 1.0
output
thermo_style custom step temp press pe ke density vol atoms
thermo 100
run
run 100000
LAMMPS does not abhor a vacuum. In fact it will run NVT dynamics on zero particles quite efficiently. However, no matter what you set the target temperature to be, it will stubbornly report the actual temperature as zero. If you would like to run a simulation with atoms, try increasing the pressure.
Thank you for the answer.
Your idea to increase the pressure is not a good idea if I want perform the isotherm of Ar gas phase at 87 K.
How can I increase the Temperature if the gcmc syntax is fixed to 87K?
Perhaps I must use only gcmc?
Regards
Al
---------- Messaggio inoltrato ----------
Thank you for the answer.
Your idea to increase the pressure is not a good idea if I want perform
the isotherm of Ar gas phase at 87 K.
How can I increase the Temperature if the gcmc syntax is fixed to 87K?
you cannot have a temperature, if you have no atoms in your system.
Perhaps I must use only gcmc?
perhaps you should not use gcmc at all? just use a fixed number of atoms
and then determine the averaged pressure?
perhaps you should start with making an estimate for how many atoms of a(n
ideal) gas would be present in your (rather small) volume at the given
pressure?
perhaps you also need to consider finite size effects.
axel.
An isotherm is a collection of points at different pressure.
In my system is present a free volume and an atom of argon.
If I want know many atoms are present in volume at definite pressure or fugacity, I must inset more atoms in a box, in indipendent mode, as in monte carlo procedure, and not manually.
In licterature to predict adsorbent property is used gran canonical monte carlo tecnique.
Lammps can perform this type of simulation??
An isotherm is a collection of points at different pressure.
no, an isotherm is a collection of points at the _same temperature_.
that is why it is called an *iso**therm*.
In my system is present a free volume and an atom of argon.
your thermo output speaks differently. you have one atom _initially_! but
then it keeps changing with obviously fix gcmc removing and inserting atoms.
If I want know many atoms are present in volume at definite pressure or
fugacity, I must inset more atoms in a box, in indipendent mode, as in
monte carlo procedure, and not manually.
so please explain, what is different from the point of thermodynamics, if
you keep the number of atoms fixed and determine the average pressure,
versus a simulation where you try to maintain a constant pressure through
inserting/removing atoms and then determine the average number of atoms.
this all at constant volume, of course.
also, please humor me and follow my advice to compute the estimated number
of ideal gas atoms for your given volume and pressure.
In licterature to predict adsorbent property is used gran canonical monte
carlo tecnique.
Lammps can perform this type of simulation??
yes, if it is performed correctly and with a meaningful system and
parameters.
axel.