# 9.4. Run path-integral MD with i-PI

The i-PI works in a client-server model. The i-PI provides the server for integrating the replica positions of atoms, while the DeePMD-kit provides a client named dp_ipi (or dp_ipi_low for low precision) that computes the interactions (including energy, force and virial). The server and client communicates via the Unix domain socket or the Internet socket. Installation instructions of i-PI can be found here. The client can be started by

i-pi input.xml &
dp_ipi water.json


It is noted that multiple instances of the client is allow for computing, in parallel, the interactions of multiple replica of the path-integral MD.

water.json is the parameter file for the client dp_ipi, and an example is provided:

{
"verbose":		false,
"use_unix":		true,
"port":		31415,
"host":		"localhost",
"graph_file":	"graph.pb",
"coord_file":	"conf.xyz",
"atom_type" : {
"OW":		0,
"HW1":		1,
"HW2":		1
}
}


The option use_unix is set to true to activate the Unix domain socket, otherwise, the Internet socket is used.

The option port should be the same as that in input.xml:

<port>31415</port>


The option graph_file provides the file name of the frozen model.

The dp_ipi gets the atom names from an XYZ file provided by coord_file (meanwhile ignores all coordinates in it), and translates the names to atom types by rules provided by atom_type.