# Simulation using lj unit

Hi

I am confused with lj unit system. I am good with other unit systems. I am using effective potential in tabular form and it doesn’t allow me to run the input file in metal units.

I tried it with lj unit system and it runs successfully. For example if I want to calculate it for aluminum at (943 K) melting then what is my reduced density and reduced temperature?

I found reduced density 1.088 corresponding to 2.7 gm/cm3. (Using sigma=2.62 Angstrom)

I came across one paper and found value of epsilon/kb=4551 K for aluminum. If I use this value and calculate the reduced temperature T*= 0.2 corresponding melting (T=943 K). Using this value I am not able to get liquid structure and even the dependent properties.

I know I have misunderstood something and need your help to figure it out.

Thanks,
Amit

Dear Mr. Amit

Thermodynamically, reduced properties are nothing but the ratio of any given property to its critical value. So in your case, for example, if you wanted to calculate reduced temperature at 943K, it would be:
Tr = 943/Tc (where Tc is critical temperature of Aluminium, which you will get from literature)
Now since lj units use reduced T, put the value of Tc (available in literature) in above equation and set the temperature or rather reduced temperature in your simulation (for lj unit system). Same follows for the reduced density. Get value of critical density of Al and find reduced density at desired condition via:
Density® = Density(at desired condition)/critical density of Al
Hence, you can enter the above calculated value if required in your simulation (for lj unit system).

Thanks
Deep Patel

Also, see the Allen & Tildesley canonical MD book for a disucssion of reduced units
in one of its appendices.

Steve