VASP-Python-Stress-Tensor-Relaxation

This repository hosts a custom tool designed to enhance the functionality of VASP (Vienna Ab initio Simulation Package) by enabling structure relaxation towards a predefined stress tensor, a feature not originally provided by standard VASP. The tool is capable of automatically modifying the POSCAR file and invoking VASP in a loop until the stress reaches the desired convergence criteria. It utilizes isotropic linear elastic theory for POSCAR adjustments. While suitable for many materials, performance may vary with highly anisotropic substances.

Benchmarks

The tool has been successfully benchmarked on materials such as Tungsten (W), Molybdenum (Mo), Chromium (Cr), and alpha-Iron (α-Fe). However, caution is advised when dealing with highly anisotropic materials as convergence issues may arise. Implementing a full stiffness tensor (Cijkl) is a recommended solution for such cases.

Usage Instructions

1. Insert the desired stress tensor values into the script.
2. Enter the estimated elastic modulus into the script.
3. In your VASP INCAR file, ensure that ISIF=2.
4. Duplicate your initial POSCAR file and rename the copy to poscar.0.
5. Edit the script to configure the mpiexec commands as per your VASP installation and job management system.
6. Navigate (cd) to your job directory.
7. Submit your job using the command qsub fixpressure.py (assuming the utilization of a PBS job system).

Tips for Accurate Force Calculation

• Opt for a higher plane-wave cutoff energy (ECUT) and a dense k-point mesh for better convergence of forces and elastic constants, which tend to converge slowly with respect to k-points in VASP.
• Set LREAL=.FALSE. to avoid unrealistic forces that sometimes result from real space calculations.
• For large systems where reciprocal space calculations are computationally demanding, consider using LREAL=Auto coupled with PREC=Accurate and ADDGRID=.TRUE. to balance efficiency and accuracy.

Recommendations for Computational Efficiency

• Employ LWAVE=.TRUE. along with the default settings for ISTART and ICHARG to enable VASP to reuse the WAVECAR file from the previous iteration, reducing the need to restart calculations from scratch.

General Algorithm

1. Execute VASP with ISIF=2 to determine the current stress tensor.
2. Apply generalized Hooke's law using the provided elastic modulus to calculate the necessary modifications to POSCAR for achieving the target pressure.
3. Repeat steps 1 and 2 until the discrepancy between the current and target pressure falls within the predefined convergence threshold.

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