add_efield

This keyword is used to add force due to electric field on atoms in a selected group at each step during a run.

For qNEP models, the force equals to the dot product of the electric field and the Born effective charge (BEC). This is only meaningful when the qNEP model was trained with target BEC.

For other models, the force equals to the product of the electric field and the charge of the atom as specified in model.xyz via charge:R:1.

Syntax

This keyword is used in one of the following ways:

add_efield <group_method> <group_id> <Ex> <Ey> <Ez> # usage 1
add_efield <group_method> <group_id> <add_efield_file> # usage 2
add_efield <group_method> <group_id> <Ex> <Ey> <Ez> <mode> # usage 3
add_efield <group_method> <group_id> <add_efield_file> <mode> # usage 4

  • Electric field is applied to atoms in group group_id of group method group_method.

  • In usage 1, the constant electric field with components Ex, Ey, and Ez is applied to each selected atom.

  • In usages 2 and 4, a series of electric fields specified in the file add_efield_file will be periodically applied to each selected atom. The file format is:

    N
Ex(0) Ey(0) Ez(0)
Ex(1) Ey(1) Ez(1)
...
Ex(N-1) Ey(N-1) Ez(N-1)

    The first line contains the number of electric-field vectors \(N\), which must be at least 2. It is followed by \(N\) lines, each containing the three Cartesian components of one electric-field vector. At the initial force evaluation (\(t=0\)), vector 0 is used. At \(t=n\Delta t\), vector \(n \bmod N\) is used, so the sequence is repeated periodically.

  • In usages 1 and 2, if the potential model is qNEP, the added electric force equals to the dot product of the electric field and the BEC; otherwise it equals to the product of the electric field and the charge of the atom as specified in model.xyz via charge:R:1.

  • In usages 3 and 4, mode can be charge or bec. * When mode is charge, the electric force will be calculated via

    • the qNEP predicted charges for qNEP potential models

    • the user-specified charges for other potential models

    • When mode is bec, the potential model must be qNEP, and the BEC will be used to calculate the electric force.

  • Electric field is in units of V/Å.

Example 1

Add a constant electric field of 0.1 V/Å in the x direction on atoms in group 1 of group method 2:

add_efield 2 1 0.1 0 0

Example 2

Add electric field on atoms in group 2 of group method 0 using a sequence of electric-field vectors from add_efield.txt:

add_efield 0 2 add_efield.txt

For example, the following file applies three electric-field vectors periodically:

3
0.10 0.00 0.00
0.00 0.10 0.00
0.00 0.00 0.10

Note

This keyword can be used multiple times during a run.