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/ These are the examples of ABACUS program.
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# External Electric Field Examples

This directory contains examples for performing calculations with external electric field in ABACUS. The external electric field is implemented as a saw-like potential, which simulates the effect of a gate electrode separated by a dielectric.

## What is External Electric Field Calculation?

External electric field calculations are used to study how materials respond to an applied electric field. This is particularly important for:

- Field-effect devices
- Catalysis under electric field
- Ferroelectric materials
- Electronic structure changes under electric field
- Polarization and dielectric properties

ABACUS implements the external electric field as a saw-like potential, which is a simplified model of field-effect measurements where the system is separated from a gate electrode by a dielectric layer (e.g., silicon oxide).

## External Electric Field in ABACUS

To enable external electric field in ABACUS, set the following parameters in the INPUT file:

```
INPUT_PARAMETERS
efield_flag        1    # Enable external electric field
efield_dir         2    # Direction of the electric field (1, 2, or 3 for x, y, or z)
efield_pos_max     0.5  # Position of the maximum field (fraction of cell length)
efield_pos_dec     0.1  # Decay length of the field (fraction of cell length)
efield_amp         0.001  # Amplitude of the electric field (Ha/bohr)
```

### Key Parameters:

- `efield_flag`: Set to 1 to enable external electric field
- `efield_dir`: Direction of the electric field (1 for x, 2 for y, 3 for z)
- `efield_pos_max`: Position of the maximum field strength as a fraction of the cell length (0 to 1)
- `efield_pos_dec`: Decay length of the field as a fraction of the cell length
- `efield_amp`: Amplitude of the electric field in Hartree/bohr (1 Ha/bohr ≈ 51.42 V/Å)

Both plane wave (PW) and LCAO basis sets are supported for external electric field calculations.

## Examples Included

### 1. 01_Pt_slab
- **System**: Platinum slab under external electric field
- **Basis**: Plane wave (PW)
- **Purpose**: Demonstrates external electric field calculation for a metal slab
- **Input Files**:
  - `INPUT`: Contains the external electric field parameters
  - `STRU`: Describes the platinum slab structure
  - `run.sh`: Script to run the calculation
  - `running_scf.ref`: Reference output for comparison

## How to Run

1. Navigate to the example directory:
   ```bash
   cd /abacus/examples/28_efield/01_Pt_slab
   ```

2. Run the calculation using the provided script:
   ```bash
   bash run.sh
   ```

3. Check the output files in the `OUT.ABACUS` directory.

## Output Files

After running the calculation, you will find the following key output files in the `OUT.ABACUS` directory:

- Energy output file: Contains energy and convergence information, including the electric field energy contribution
- `running_scf.log`: Log file for the SCF calculation, includes details about the electric field setup
- `forces.dat`: Contains atomic forces, which include contributions from the electric field

### Energy Contribution

The energy contribution from the external electric field is given as `E_efield` in the output files. This term represents the interaction energy between the system and the applied electric field.

## Notes

- The electric field strength is specified in atomic units (Ha/bohr). To convert to V/Å, use the conversion factor: 1 Ha/bohr ≈ 51.42 V/Å
- The saw-like potential is a simplified model that approximates the field from a gate electrode
- For accurate results, ensure that the unit cell is sufficiently large in the direction of the electric field to avoid interactions between periodic images
- The choice of `efield_pos_max` and `efield_pos_dec` affects the shape of the potential; adjust these parameters based on your system geometry
- External electric field calculations can be combined with other features like spin-orbit coupling and vdW corrections
- The computational cost of external electric field calculations is similar to standard DFT calculations
- The examples provided are for demonstration purposes; for production calculations, you should carefully test the convergence of results with respect to basis set, k-point sampling, and electric field parameters

## Applications

External electric field calculations in ABACUS can be used to study:

- Field-induced phase transitions
- Electric field effects on catalytic activity
- Band bending at interfaces
- Field-emission properties
- Ferroelectric switching
- Dielectric response of materials