# Calculate Si cohesive energy using VASP

• elsafo
In summary, to calculate the cohesive energy of a system in VASP, you must perform two separate calculations - one for the system and one for a single atom. By subtracting the energy of the single atom from the system, and dividing by the number of atoms, you can determine the cohesive energy of each atom. However, this method may not always be accurate.

#### elsafo

Hello, actually I am a newbie in solid state physics computational and I am starting using VASP.

I am practicing to do calculation with bulk Si. I want to calculate the cohesive energy of Si. As I know that cohesive energy equals to the difference between bulk energy/atom and single atom energy. I have calculated the bulk energy of fcc Si (2 atom/unit cell). Now, my problem is calculating the single atom energy. How to calculate the single atom energy? I tried to manipulate the POSCAR input with large size of lattice parameter and change the number of atom to 1, then change the KPOINTS to 1*1*1, however I didn't get appropriate value same with reference. Am I missing some procedure?

Please kindly to help me ^^

In order to calculate the cohesive energy of the system you must do two separate calculations in VASP. The first is the system that you are trying to find the cohesive energy of the atoms, the second is a system with just the lone atom. With both those energies (should be in eV) you can take the energy of the first system and subtract the energies of all the atoms in the system, and if all of the atoms are the same element you can divide this answer by the number of atoms in the first system to determine the cohesive energy of each atom. In your case E(2Si) - 2*E(Si) and that is the overall cohesive energy, then divide it by two to get individual energy. I realize it is probably a little bit late to help your case elsafo but hopefully this can help someone else.

TaraB