An option for user to output relative energy in ABACUS

[QuantumMisaka]

Background

In VASP, the total electronic energy output is shift to the formation energy which relative to the energy of elements, but ABACUS can only output the absolute energy.

The absolute energy have no meaning in principle, but they could lead to some numerical problem, especially in the training process of machine learning potential. DeePMD-kit can deal this through a element bias tactic, but other MLPs like NEP have no forehead process like DP.

Also, this absolute energy can be comparable to VASP

Describe the solution you'd like

• Maintain a reference energy of each elements, which can be the PBE energy of single atom, which is only for energy shift
• Allow user to specify the relative energy output as E_TOT instead of absolute energy

Task list only for developers

• Notice possible changes of behavior
• Explain the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi

Notice Possible Changes of Behavior (Reminder only for developers)

No response

Notice any changes of core modules (Reminder only for developers)

No response

Notice Possible Changes of Core Modules (Reminder only for developers)

No response

Additional Context

No response

Task list for Issue attackers (only for developers)

• Review and understand the proposed feature and its importance.
• Research on the existing solutions and relevant research articles/resources.
• Discuss with the team to evaluate the feasibility of implementing the feature.
• Create a design document outlining the proposed solution and implementation details.
• Get feedback from the team on the design document.
• Develop the feature following the agreed design.
• Write unit tests and integration tests for the feature.
• Update the documentation to include the new feature.
• Perform code review and address any issues.
• Merge the feature into the main branch.
• Monitor for any issues or bugs reported by users after the feature is released.
• Address any issues or bugs reported by users and continuously improve the feature.

[kirk0830]

the main difference between the pseudopotential used by VASP and ABACUS/QE is the former has the copyright to publish pseudopotential, but for the latter, the development of software and pseudopotentials are always decoupled and are done by different groups, so it is not very possible to change the content of pseudopotential, but it may be of value to consider when ABACUS implements the "neutral potential" for mitigating the eggbox effect and reduce the computational cost on grid integration, in which there would be SCF for isolated atom(s) be performed before the practical calculation, and after which an atomic energy can be obtained.