The Socket Interface

Although the modular design of the code allows for direct, library-like calls to force evaluators, the main mode of functioning of i-PI allows to use a separate ab initio or empirical force evaluation code to compute interatomic forces. 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.

i-PI was originally designed to perform ab initio PIMD - where the cost of the force evaluation is overwhelming relative to the ionic dynamics. However, it has since grown to do much more than that. The implementation of i-PI is efficient enough that it can be used with empirical forcefields and machine-learned potentials without incurring a large overhead. If you experience very large overheads, please contact the developers! It is likely that some easy solutions are possible. See more about i-PI implementation in the publication that accompanies its release.