DRAFT: Implement the Calculation of Hubbard U Parameters via the Linear Response Method

Goal:

• Implement a lightweight routine to calculate Hubbard U parameters according to the linear response method outlined here (pg. 64, eqn. 3.26) and here.

Motivation:

• DFT+U is required for systems with highly correlated d/f electrons
• Rigorously speaking, U values are not only atom-specific but structure-specific as well
• VASP exposes an API for calculating U parameters, but it requires tedious modifications to POSCAR and POTCAR files
• Enter ASE: automated input file creation ++; inconsistent atomic indexing --

Methodology:

• Preserve ASE's input file creation workflows
• Setting the setups keyword argument of the Vasp calculator constructor yields consistent POSCAR and POTCAR files
• Expose a Vasp.get_hubbard_u method by which Users can specify an ase.Atoms index of an atom to calculate its U parameter
• Optional: strictly enforce sensical parameters (e.g., lmaxmix>=4)
• Optional: specify grid for differentiation

Public API Changes:

• Vasp.get_hubbard_u method
• vasp.get_ionic_charges function (This is a helper function that may, more appropriately, be relocated to ase.io.vasp_parsers.vasp_outcar_parsers.)

TODO:

• more testing
• optionally enforce parameters or provide adequate warning in documentation?
• vasp.get_ionic_charges -> ase.io.vasp_parsers.vasp_outcar_parsers