A Python interface for ab initio path integral molecular dynamics simulations. i-PI is composed of a Python server (i-pi itself, that does not need to be compiled but only requires a relatively recent version of Python and Numpy) that apply an algorithm that updates the positions of the nuclei, and of an external code that acts as a client and computes the electronic energy and forces.

This is typically a patched version of an electronic structure code, but a simple self-contained Fortran driver that implements Lennard-Jones and Silvera-Goldman potentials is included for test purposes.

i-PI was originally developed to simulate the quantum mechanical nature of light nuclei by performing path integral molecular dynamics simulations, and it implements most of the state-of-the-art methods to accelerate this kind of calculations. It has since grown to also provide all sorts of simulation strategies, from replica exchange to geometry optimization.

To use i-PI with already existing drivers, install and update using Pip:

Last version::

python -m pip install git+https://github.com/i-pi/i-pi.git

Last Release::

pip install -U i-PI

Test with Pytest::

pip install pytest
pytest --pyargs ipi.tests

To develop i-PI or test it with the self-contained driver, follow these instructions. It is assumed that i-PI will be run from a Linux environment, with a recent version of Python, Numpy and gfortran, and that the terminal is initially in the i-pi package directory (the directory containing this file), which you can obtain by cloning the repository

git clone https://github.com/i-pi/i-pi.git

Source the environment settings file env.sh as source env.sh or . env.sh. It is useful to put this in your .bashrc or other settings file if you always want to have i-PI available.

cd drivers
make
cd ..

This will first start the wrapper in the background, redirecting the output to a log file, and then run a couple of instances of the driver code. The progress of the wrapper is followed by monitoring the log file with the tail Linux command.

Optionally, you can make a copy of the directory with the example somewhere else if you want to keep the i-PI directory clean.

cd examples/tutorial/tutorial-1/
i-pi tutorial-1.xml > log &
i-pi-driver -h localhost -p 31415 -m sg -o 15 &
i-pi-driver -h localhost -p 31415 -m sg -o 15 &
tail -f log

The monitoring can be interrupted with CTRL+C when the run has finished (5000 steps).

The automatic test suite can be run by calling the i-pi-test script. You need to have the pytest package installed

i-pi-test

See more details in the README file inside the ipi_tests folder.

If you have new features you want to implement into i-PI, your contributions are much welcome. See CONTRIBUTING.md for a brief set of style guidelines and best practices. Before embarking into a substantial project, it might be good to get in touch with the developers, e.g. by opening a wishlist issue.