vachoppy 3.1.1

Python package for analyzing vacancy hopping mechanism

VacHopPy is a Python package for analyzing vacancy-mediated diffusion from molecular dynamics (MD) simulations. The package facilitates a reliable multiscale modeling workflow by calculating key effectuve hopping parameters that can be directly incorporated into continuum-scale models.

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Documentation

You can find the complete VacHopPy documentation, including comprehensive tutorials and API references, on Read The Docs.

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Key Features

• Effective Parameter Extraction

  Derives a single set of effective hopping parameters from the MD simulations, providing ready-to-use inputs for continuum-scale models.

• Ensemble Analysis

  Simultaneously processes an ensemble of multiple MD trajectories. This approach efficiently samples rare hopping events in high-barrier systems, avoiding the need for a single, prohibitively long simulation.

• Memory-Efficient Processing

  Reads and interprets large-scale trajectories via a streaming approach, enabling the analysis of massive datasets within just a few gigabytes of RAM.

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Effective Hopping Parameters

In ab initio calculations, such as the nudged elastic band (NEB) method, hopping parameters are typically determined for a specific, predefined migration path. Consequently, in a system with multiple distinct migration pathways, one obtains a collection of different hopping parameter sets, each corresponding to a unique path.

However, to compare these computational results with experimental data or to use them in continuum-scale models, a single, representative set of parameters that encapsulates the overall diffusion behavior is required. We term this representative set the effective hopping parameters, which serve as ready-to-use inputs for continuum-level analysis.

The effective hopping parameters self-consistently satisfy the following fundamental diffusion equations:

$$ D = \frac{1}{6} f z a^2 \nu \cdot \exp(-E_a / k_B T) $$

$$ \Gamma = \frac{1}{\tau} = z \nu \cdot \exp(-E_a / k_B T) $$

For a more detailed theoretical background, please refer to this paper. The key effective hoppping parameters extractable with VacHopPy are summarized below.

Symbol	Parameter	Unit
$D$	Vacancy diffusivity	m²/s
$\tau$	Vacancy residence time	ps
$f$	Correlation factor	-
$E_a$	Hopping barrier	eV
$a$	Hopping distance	Å
$z$	Coordination number	-
$\nu$	Attempt frequency	THz
$\nu^*$	Atomic vibration frequency	THz

All hopping parameters extracted by VacHopPy, such as those summarized above, are effective values. This means they represent the overall, averaged behavior of vacancy diffusion across all possible migration paths within the system. For instance, the calculated effective hopping barrier ($E_a$) is not tied to a single NEB path but reflects the system-wide average barrier influencing diffusion.

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Getting Started

You can install VacHopPy using pip:

pip install vachoppy

Alternatively, you can install the latest version in development from VacHopPy GitHub:

git clone [e-mail]:TY-Jeong/VacHopPy.git
cd VacHopPy
pip install -e .

Command-Line Interface (CLI)

In addition to its Python library, VacHopPy provides a powerful Command-Line Interface (CLI) to perform key tasks directly from your terminal. This allows you to run analyses quickly without writing custom scripts.

The general command structure is:

vachoppy <command> [options]

To see a list of all available commands and their brief descriptions, use the -h or --help option:

vachoppy -h

Here is a summary of the main commands available:

Command	Description
trajectory	Identify and visualize vacancy trajectories from a single trajectory file.
analyze	Extract hopping parameters from an ensemble of trajectories.
vibration	Extract atomic vibration frequency from a single trajectory.
distance	Trace change in cosine distance from a reference structure over time.
fingerprint	Calculate and plot the fingerprint for a single static structure.
msd	Calculate diffusivity from mean squared displacement (Einstein relation).
convert	Convert various MD trajectory formats to the standard HDF5 format.
concat	Concatenate two successive HDF5 trajectory files into a new one.
cut	Cut a portion of a HDF5 file and saves it as a new file.
show	Display a metadata summary of a HDF5 trajectory file.

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References

If you used VacHopPy package, please cite this paper

• T. Jeong et al., Comput. Phys. Commun. 320, 110010 (2026)