Understanding DFT to Calculate Potential Energy of Atoms

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Discussion Overview

The discussion revolves around the use of Density Functional Theory (DFT) to calculate the potential energy of atoms within a structure. Participants explore methods for obtaining potential energy values, particularly in comparison to classical molecular dynamics (MD) approaches, and address challenges related to non-crystalline materials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks guidance on how to calculate the potential energy of each atom using DFT, referencing their experience with classical MD methods in LAMMPS.
  • Another participant mentions that forces can be derived from VASP relaxation calculations and questions whether potential energy can similarly be obtained from such calculations.
  • Concerns are raised about the definition of energy per atom in non-crystalline materials, with one participant suggesting that this concept is ill-defined in such contexts.
  • A later reply clarifies that for non-crystalline materials, obtaining relative potential energy may involve complex considerations, such as removing an atom from a calculation and addressing basis set superposition error.
  • It is suggested that for solids, using a large supercell may be necessary to accurately assess potential energy.

Areas of Agreement / Disagreement

Participants express differing views on the definition and calculation of potential energy in non-crystalline materials, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

Participants highlight challenges related to the definition of energy per atom in non-crystalline materials and the complexities involved in calculating relative potential energy, including issues like basis set superposition error and the need for large supercells.

wkxez
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I am a freshman in DFT calculation. I don't understand the DFT method clearly,but I want to use DFT to do some calculations.
My question is that how how can I get the potential energy of each atom in a structure using DFT package ,which I have done by using classical MD method in Lammps(the command: compute 1 all pe/atom ,Lammps manual :The per-atom energy is calculated by the various pair, bond, etc potentials defined for the simulation. If no extra keywords are listed, then the potential energy is the sum of pair, bond, angle, dihedral,improper, and kspace energy.).
I want to know how can I do such a calculation.Please give some suggestions.
 
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I now know that i can get the force F of each atom from the results of vasp relaxation calculation .In the classical theory,F= dU/dr(U is the potential energy),,so if it's possible to get the potential energy from the VAsp or other package calculation?
 


The notion of energy per atom is ill defined for a non-crystalline material. IF you are modelling a crystal , you simply divide the total energy by the number of unit formulas of the crysta.
 


Useful nucleus said:
The notion of energy per atom is ill defined for a non-crystalline material. IF you are modelling a crystal , you simply divide the total energy by the number of unit formulas of the crysta.


Thank you for your reply,Why is it ill defined for non-crystalline material? And you may misunderstand my question. I want to get the relative potential energy of each atom in a structure,not energy per atom.
 


wkxez said:
Thank you for your reply,Why is it ill defined for non-crystalline material? And you may misunderstand my question. I want to get the relative potential energy of each atom in a structure,not energy per atom.

In an infinite solid this is difficult, in a molecule you can in principle remove that atom from the calculation and compare the energy of the molecule with and without the atom. In practice there is a problem called basis set superposition error, and you may have to replace the atom by a ghost atom with the same basis set but zero nuclear charge.
For a solid you would have to consider probably some large super cell.
 

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