Welcome to i-PI’s documentation!

i-PI is a interface for advanced molecular simulations written in Python, designed to be used together with an ab initio evaluation of the interactions between the atoms. The main goal is to decouple the problem of evolving the ionic positions to sample the appropriate thermodynamic ensemble and the problem of computing the inter-atomic forces. i-PI was initially developed for Path Integral Molecular Dynamics (PIMD) simulations, and contains probably the most comprehensive array of PIMD techniques. Since v2.0, however, it also contains several general-purpose methods for molecular simulations, ranging from phonon calculators to replica exchange molecular dynamics.


Schematic representation of the functioning of i-PI.

The implementation is based on a client-server paradigm, where i-PI acts as the server and deals with the propagation of the nuclear dynamics, whereas the calculation of the potential energy, forces and the potential energy part of the pressure virial is delegated to one or more instances of an external code, acting as clients. Since the main focus is on performing ab initio PIMD – where the cost of the force evaluation is overwhelming relative to the ionic dynamics – clarity has been privileged over speed. Still, the implementation of i-PI is efficient enough that it can be used with empirical forcefields to perform simple benchmarks and preparatory simulations.

The documentation including the python help strings of the keywords is best accessed through the latex-generated PDF file that can be compiled from the code repository for the most recent version or downloaded from here (static version updated periodically): manual-pdf

This documentation is structured as follows: