PLUGINS/FORCE AND STRESS

PLUGINS/FORCE_AND_STRESS = .True. | .False.
Default: PLUGINS/FORCE_AND_STRESS = .False. 

Description: PLUGINS/FORCE_AND_STRESS calls the Python plugin for the force and stress interface for each ionic relaxation step

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When PLUGINS/FORCE_AND_STRESS=.TRUE., VASP calls the force_and_stress Python function at the end of each ionic relaxation step. The primary use-case of this tag is to modify forces and the stress tensor to be consistent with modifications to the potential performed with PLUGINS/LOCAL_POTENTIAL

Expected inputs

The force_and_stress Python function expects the following inputs,

def force_and_stress(constants, additions):

where constants and additions and Python dataclasses. The constants dataclass consists of the following inputs, listed here with their associated datatypes

@dataclass(frozen=True)
class ConstantsForceAndStress:
    ENCUT: float
    NELECT: float
    shape_grid: IntArray
    number_ions: int
    number_ion_types: int
    ion_types: IndexArray
    atomic_numbers: IntArray
    lattice_vectors: DoubleArray
    positions: DoubleArray
    ZVAL: DoubleArray
    POMASS: DoubleArray
    forces: DoubleArray
    stress: DoubleArray
    charge_density: Optional[DoubleArray] = None

Note that the INCAR tags are capitalized. shape_grid is a three dimensional integer array which stores the shape of the real space grid, NGXF, NGYF and NGZF, number_ions is the total number of ions listed in the POSCAR file, number_ion_types is the number of ion corresponding to each ion type in the convention of the POSCAR file, ion_types stores the total number of ion types, atomic_numbers contains the atomic number for each atom type, lattice_vectors and positions contain the lattice vectors and positions of the current SCF step forces and stress are the computed forces and stress tensor and charge_density contains the charge density on the real space grid.

The additions dataclass consists of the following modifiable outputs

@dataclass
class AdditionsForceAndStress:
    total_energy: float
    forces: DoubleArray
    stress: DoubleArray

Modifying quantities

Modify the quantities listed in additions by adding to them. For example, if you wanted to add one to the forces

import numpy as np
def force_and_stress(constants, additions)
    additions.forces += np.ones((constants.number_ions,3))

Mind: You may not make modifications to quantities in constants

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