Finding state of a material using molecular dynamics method: Solid

 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 aluminium as the material and the temperature to be around 300K.

Input script

units metal

boundary p p p 

atom_style atomic 

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

region forbox block 0 39.86 0 39.86 0 39.86 units box

create_box 1 forbox 

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

lattice fcc 3.986

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

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

mass 1 26.982 #<Mass of atom type 1 is 39.48 [mass units grams/mole]> 

pair_style eam

pair_coeff * * Al_jnp.eam

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

group Al type 1 #<Group all the aluminium types (aluminium type is of type 1. All atoms of type 1 are in group Al)

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 596.3K

velocity all create 596.3 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_solid.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

thermo 1000

run 10000

unfix 1

####End

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

Visit LAMMPS Manual for a detailed explanation of various commands used.

Result

Explanation

After running the simulation, the RDF values will be stored in "rdf_solid.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 high. So we get a curve like this. The plot will be similar for any solid.

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