PHASESHIFTS PACKAGE

This package is a Python-based implementation of the Barbieri/Van Hove phase shift (a.k.a. phshift) calculation package needed to produce elastic electron atom scattering (EEAS) phase shifts for modelling within various LEED packages (including CLEED), as well as for certain XPD packages. To quote the original authors' site:

"The phase shift calculation is performed in several steps:

1. Calculation of the radial charge density for a free atom.
2. Calculation of the radial muffin-tin potential for atoms embedded in a surface defined by the user (the surface is represented by a slab that is periodically repeated in 3 dimensions, within vacuum between the repeated slabs); various approximations to the exchange potential are available; relativistic effects are taken into account.
3. Calculation of phase shifts from the muffin-tin potential.
4. Elimination of pi-jumps in the energy dependence of the phase shifts."

[!NOTE] You can get the original Fortran source (& learn more about the phshift programs) from Michel Van Hove's LEED Calculation Home Page:

https://www.icts.hkbu.edu.hk/VanHove_files/leed/leedpack.html

A local copy of the source files can be found under phaseshifts/lib/.phsh.orig/phsh[0-2].f.

Running

The phsh.py script (available after installing the package) aims to simplify these steps with a single command. For more information please read the documentation at http://pythonhosted.org/phaseshifts/ (latest PyPI release) or GitHub Pages (latest master)

The simplest and most reliable cross-platform way to run phsh.py is through docker:

# obtain the image
docker pull ghcr.io/Liam-Deacon/phaseshifts:latest  # should only need to do this once

# run phsh.py via the docker image
docker run ghcr.io/Liam-Deacon/phaseshifts:latest  # will display usage

# or more generally (adjust as needed)
docker run ghcr.io/Liam-Deacon//phaseshifts:latest -v /path/to/host/input/data:/data [<phsh-args> ...]

[!TIP] Development docker images can be built locally, e.g. DOCKER_TAG=dev make docker

[!WARNING] There is a known possible bug where the compiled libphsh.f is not thread-safe (as ascertained by the fortran compiler), as such if you anticipate using this library in concurrent environments then it is advised to run phsh.py via docker run ghcr.io/Liam-Deacon/phaseshifts:latest as this works around this limitation due to the emphereal nature of container instances created using docker run.

Install

TDLR;

For python 3.11 or older:

#  install latest release
pip install phaseshifts

# development install
pip install wheel numpy setuptools  # needed for older python/pip versions
pip install -e .
phsh --help

Details

The phaseshifts package requires CPython 2.7 or later and also uses the numpy, scipy and periodictable packages. Currently, it has only been tested most extensively with Python 2.7 on Windows, so there are no guarantees with other platforms. To perform a setup follow the steps below.

1. Install the numpy, scipy and periodictable packages.

   On systems compatible with PyPI this can be done using the command:

   pip install numpy scipy periodictable
   

   Or if you have the easy_install package:

   easy_install install numpy scipy periodictable
   

   Older versions of numpy & scipy did not allow simultaneous installation -if you experience problems then try first installing numpy before attempting to install scipy. The periodictable package allows lookup of the most common crystal structure for a given element and is instrumental in many of the convenience functions contained in the model module.

   Alternatively download and install these packages manually following the instructions provided for the respective packages.

2. To install the phaseshifts package:

   python setup.py install
   

   With any luck the package has been installed successfully. A set of test scripts are provided, however a simple check may suffice using an interactive session of the python interpreter:

   >>> import phaseshifts
   >>> from phaseshifts.lib import libphsh # compiled FORTRAN .pyd or .so using f2py
   

   If these execute without errors then it is likely that all is well, but in case of problems or bugs please use the contact provided below and I will do my best to address the problem quickly.

[!TIP] On Windows systems it may be easier to install a scientific python distibution rather than install the dependencies from source - Python(x,y) or Anaconda with mingw (gcc & gfortran) installed is highly recommended. Mac OS X users can simply do brew install gfortran and Debian/Ubuntu users can do sudo apt-get install -y gfortran.

[!NOTE] On unix systems, setup the virtualenv on Python 3.10 or lower, activate it and run make.

[!WARNING] Python 3.12 compatibility is a work in progress due to the removal of numpy.distuils build backend for f2py preventing simple installation via pip install, this github issue tracks progress on fixing this known issue.

About the code

The example source codes provided in this package are intended to be instructional in calculating phase shifts. While it is not recommended to use the example code in production, the code should be sufficient to explain the general use of the library.

If you aren't familiar with the phase shift calculation process, you can read further information in doc/ folder:

• phshift2007 - a brief user guide/documentation concerning the input files (& details of the original fortran phshift package).
• phaseshifts API - a more detailed overview of the library functions and how to calculate phase shifts using the convenience functions in this package. This is not yet finished and so the reader is referred to the above document for the time being.

For those wanting a crash course of the Van Hove / Tong programs, I advise reading the phsh2007 document. See the examples/ directory to get an idea of the structure of the input files (for a random selection of models & elements). In particular see the cluster_Ni.i file for helpful comments regarding each line of input.

Those of you who are eager to generate phase shifts - first look at the example cluster files for a bulk and slab calculation, noting that the atoms in the model are in fractional units of the a basis vector for the unit cell (SPA units). Next, after creating a bulk and slab model in the cluster.i format, simply use the following python code:

>>> from phaseshifts.phsh import Wrapper as phsh
>>> phsh.autogen_from_inputs(bulk_file, slab_file)

This will hopefully produce the desired phase shift output files (at least for simple models) and works by assessing the two models to determine what output to produce. For more detailed documentation and function use refer to the pdf manual.

[!TIP] A standalone command line utility phsh.py is provided as a way of automating the generation of phase shifts as part of a script. For more information use:

phsh.py --help

[!NOTE] The phaseshifts.leed module provides a conversion class for CLEED .inp and .bul files. This is included as part of the phsh.py module, however the file extension is important (needs .inp, .pmin, .bul, or .bmin) and error checking is limited. There are also plans to include a validator to check the files for malformatted input at some point in the future.

Alternatives

A number of alternatives are available, notably the following:

1. AQuaLEED (with a useful poster overview of phaseshifts calculations). This is an officially mentioned piece of software on Michel Van Hove's LEED Calculation Homepage. Although the poster mentions that the software is written in python, this software is not (currently) distributed on https://PyPI.org (or via alternative means such as a docker image on DockerHub) and therefore harder to integrate with other python LEED-related projects such as CLEED and cleedpy.
2. A fortran program is described in "McGreevy, E., & Stewart, A.L. (-Apr 1978). A program for calculating elastic scattering phase shifts for an electron colliding with a one-electron target using perturbation theory. Computer Physics Communications, 14(1-2), 99-107.", however this code is not publicly available online (pay-walled by journal).

[!NOTE] Should you know of alternatives, please either open an issue or (better yet) create a PR with changes to this documentation to keep this list up to date.

Acknowledgements

As with all scientific progress, we stand on the shoulders of giants. If this package is of use to you in publishing papers then please acknowledge the following people who have made this package a reality:

• A. Barbieri and M.A. Van Hove - who developed most of the original fortran code. Use A. Barbieri and M.A. Van Hove, private communication. (see doc/phsh2007.txt for further details).
• E.L. Shirley - who developed part of the fortran code during work towards his PhD thesis (refer to the thesis: E.L. Shirley, "Quasiparticle calculations in atoms and many-body core-valence partitioning", University of Illinois, Urbana, 1991).
• Christoph Gohlke - who developed the elements.py module used extensively throughout for the modelling convenience functions (see 'elements.py' for license details).

I would also be grateful if you acknowledge this python package (phaseshifts) as: L.M. Deacon, private communication.

Thanks

I wish to personally add a heart-felt thanks to both Eric Shirley and Michel Van Hove who have kindly allowed the use of their code in the libphsh.f file needed for the underlying low-level functions in this package.

Contact

This package is developed/maintained in my spare time so any bug reports, patches, or other feedback are very welcome.

The project is (still) in the early developmental stages and so anyone who wishes to get involved are most welcome. Please either create an issue or (better yet) submit a pull request.

[!TIP] Please star it on GitHub as this will help to easily indicate that others find the package useful.

To Do

1. Documentation - the manual has been started, but is not complete and thus is a high priority. The current aim is to use sphinx to generate html and latex documents for semi-automated generation of both the tutorial and supporting website. If you have the phaseshifts source and the sphinx and the numpydoc PyPi packages then you can try making html or latex manuals using make html or make latexpdf commands from the doc/ directory.
2. Test suit to verify the package is working as expected.
3. GUI frontend (Qt ui files are provided in the gui/ directory for anyone wishing to undertake this challenge). Other front ends are welcome (I use Qt due to familiarity/experience). For those wishing a sneak preview, try executing main.pyw

See either todo issues or TODO.rst for more information.

Contacts

• Liam Deacon - current maintainer
• Michel Van Hove - Contact for original LEEDPACK phsh[0-3].f programs