  Recent developments in path integral methodology have significantly reduced
the computational expense of including quantum mechanical effects in the
nuclear motion in ab initio molecular dynamics simulations. However, the
implementation of these developments requires a considerable programming
effort, which has hindered their adoption. Here we describe i-PI, an interface
written in Python that has been designed to minimise the effort required to
bring state-of-the-art path integral techniques to an electronic structure
program. While it is best suited to first principles calculations and path
integral molecular dynamics, i-PI can also be used to perform classical
molecular dynamics simulations, and can just as easily be interfaced with an
empirical forcefield code. To give just one example of the many potential
applications of the interface, we use it in conjunction with the CP2K
electronic structure package to showcase the importance of nuclear quantum
effects in high pressure water.
