Finding state of a material using molecular dynamics method: Liquid

 In this blog, I will be showing how to check whether a given material at a given temperature is solid or not. For this, I am using Lead(Pb) as the material and the temperature to be around 1250K.

Input script

units metal 

boundary p p p

atom_style atomic #<What style of atoms is to be used in the simulation>

#================================================

region forbox block 0 49.508 0 49.508 0 49.508 units box

create_box 1 forbox 

#================================================

lattice fcc 4.9508

create_atoms 1 box basis 1 1 basis 2 1 basis 3 1 basis 4 1 units box 

#================================================

mass 1 207.2 #<Mass of atom type 1 is 207.2 [mass units grams/mole]> 

pair_style eam/alloy

pair_coeff * * pb.eam.alloy Pb

#=================================================

group pb type 1 

minimize 1e-12 1e-25 10000 10000 #<Minimize the energy using a conjugate gradient step. 

#=================================================

# Provide an initial maxwellian distribution of velocity corresponding to temperature 5K

velocity all create 2500 198728 dist gaussian

#Perform an NVE integration with this initial position and velocity distribution

timestep 0.001 

compute rdfs all rdf 100 1 1 

fix frdf all ave/time 1 10000 10000 c_rdfs[1] c_rdfs[2] c_rdfs[3] file rdf_liquid.txt mode vector

# The above command averages the RDFs over time.

fix 1 all nve 

thermo_style custom step time cpu temp pe ke etotal press vol#<Print the thermodynamic information >

thermo 1000

run 10000 #<Run for 3 ps (Since it seems to reach constant P and T in that time>

unfix 1

#End

#=====================================================

Visit LAMMPS Manual for a detailed explanation of commands used.

Result

Explanation

After running the simulation, the RDF values will be stored in "rdf_liquid.txt" file. With the values, you can plot the curve. Rdf-Radial Distribution Function physically gives the probability of finding an atom at a certain point. In solid the probability of finding an atom at equal interval is somewhat low. So we get a curve like this. The plot will be similar for any liquid.

Hope the post is useful for you. For further updates, keep visiting the blog!