Getting Started

Starting i-PI is very easy!

Prerequisites

Mandatory:

  • A decent version of python >=2.5 (< 3)
  • A decent version of numpy

Optional:

  • A F90 compiler (gfortran preferred)
  • A program to plot time series and to visualize molecular structures (e.g. gnuplot and VMD)
  • Other codes which are compatible with i-PI (Quantum Espresso, CP2K, LAMMPS, etc.).

How to install i-PI


If you want only to test i-PI you can download our VirtualBox image which already contains i-PI and several clients properly configured.


Being written completely in Python, i-PI do not need a real installation. To keep a low barrier for the new users, we decided to provide a simple script that sets all the necessary environment variable in a BASH console. Once you downloaded, you can move the i-PI root directory where you prefer and then simply source the env.sh file:

$ cd <path_to_ipi>
$ source env.sh

Now i-PI and all his tools are available in the actual console. If you want to avoid this operation in the future, you can just run the following command:

$ echo 'source <path_to_ipi>/env.sh' >> ~/.bashrc

Obviously the <path_to_ipi> must be replaced with the absolute path to the i-PI folder.

At this point i-PI should work. To test if i-PI is working just run the following command:

$ i-pi

and check that the output resemble the following:

Usage: i-pi [options] <input file>

i-pi: error: No input file name provided.

Installing an example driver

To do anything, i-PI needs to communicate with a client code able to provide forces for a atomic configuration. We provide a simple suite of potential energy surfaces coded into the “driver” that allows the users to readily test i-PI.

To compile the driver a F90 compiler is needed. We recommend gfortran.

To prepare the driver executable, just enter the driver directory and write

$ cd <path_to_ipi>/driver
$ make

If you have a non standard linux installation or you want to use a different compiler than gfortran, you can edit the Makefile contained in this directory.

First run with i-PI

To run i-PI one must know that i-PI works as a server that only moves the atoms accordingly to the user input. To provide some valuable results, we needs a client that connect to i-PI and computes forces, energy and stresses on the atomic configurations provided by i-PI. Thus, you will need two terminals: both must have been configured to work with the i-PI framework (source <path_to_ipi>/env.sh).

As a first example move both terminals to the examples/driver/zundel-aimd directory and run the following:

Terminal 1

$ i-pi input.xml

After a few lines of output describing the i-PI set-up, you should get the following text:

@ForceField: Starting the polling thread main loop

At this point i-PI is running and is waiting for a client. In Terminal 2 write:

$ i-pi-driver -m zundel -u -h zundel-md & 

Now, the Terminal 1 should report all the i-PI operations.

Congratulation! Your first simulation with i-PI is now running!

Please note that you can stop the client (Terminal 2) when you want (also pressing Ctrl+c into the terminal directly). Since you are running a single client, i-PI will stop too but will not crash. If you decide to restart the client, running the last command in the Terminal 2, the simulation will restart exactly from where you stopped it! Wonderful!


To go deeper in understanding how i-PI works you can check the documentation and our tutorials.