This a short tutorial on how to run I-PI with CP2K.


i-PI is a Python interface for ab initio path integral molecular dynamics simulations. 
i-PI is composed of a Python server (i-pi itself, that does not need to be compiled but only requires a relatively recent version of Python and Numpy) that propagates the (path integral) dynamics of the nuclei, and of an external code that acts as a client and computes the electronic energy and forces.

The i-PI manual can be found in {{:howto:i-pi-manual.pdf | i-PI Manual}}


Please cite [[https://www.sciencedirect.com/science/article/abs/pii/S0010465518303436|Computer Physics Communications 236 (2019) 214–223]] and [[https://aip.scitation.org/doi/10.1063/1.4941091|J. Chem. Phys. 144, 054111 (2016)]] if you use I-PI with CP2K.

Published work using I-PI with CP2K: 

[[https://pubs.acs.org/doi/10.1021/acs.jpclett.0c01025|J. Phys. Chem. Lett. 2020, 11, 9, 3724–3730]]

[[https://www.pnas.org/content/116/4/1110.short | PNAS 01, 22, 2019 116 (4) 1110-1115 ]]

[[https://www.nature.com/articles/s41467-021-20914-0 | Nature Communications 12, 766 (2021) ]]

===== Download i-PI from Github =====

<code>
git clone https://github.com/i-pi/i-pi.git
</code>

To run i-PI, one need to source the environment file from the i-PI directory

<code>
source ${PATH_TO_IPI}/env.sh
</code>

Then one can run i-PI by using 

<code>
i-pi input.xml > log &
</code>

There are many input examples in i-pi/examples/ folder, which contains different methods, e.g. NVE, NVT, NPT, PIMD, REMP, etc.


===== Run i-PI with INET socket =====
To use CP2K as the client code using an **internet** domain socket on the host address “host_address” and on the port number “port” the following lines must be added to its input file:

<code>
&MOTION 
...
   &DRIVER
      HOST host_address
      PORT port
   &END DRIVER
...
&END MOTION   
</code>
In the **input.xml**, one need to use the same host_address and port to CP2K input.
<code>
  <ffsocket mode='inet' name='driver'>
    <address>host_address</address>
    <port>port</port>
    <latency>0.01</latency>
    <timeout>5000</timeout>
  </ffsocket>

</code>


===== Run i-PI with UNIX socket =====

If instead a **unix** domain socket is required then the following modification is necessary:

<code>
&MOTION 
...
   &DRIVER
      HOST host_address
      PORT port
      UNIX
   &END DRIVER
   ...
&END MOTION
</code>

In the **input.xml**, one to specify the mode=unix to enable the communication via UNIX socket.
<code>
  <ffsocket mode='unix' name='driver'>
    <address>host_address</address>
    <port>port</port>
    <latency>0.01</latency>
    <timeout>5000</timeout>
  </ffsocket>

</code>

===== Run I-PI with CP2K on a supercomputer =====
To run I-PI with CP2K on a supercomputer is also straightforward, one needs to get the hostname where the i-PI is executed, and then replace this variable in the CP2K input files. Here is an example script one can use to run I-PI with CP2K on the Daint machine (CSCS). 

<code>
HOST=$(hostname)

source ~/i-pi/env.sh

if [ -e simulation.restart ]; then
   sed -i "s/address>[^<]*</address>$HOST</" simulation.restart
   i-pi simulation.restart  >> log.ipi 2>&1 &
else
   sed -i "s/address>[^<]*</address>$HOST</" input.xml
   i-pi input.xml &> log.ipi &
fi
sleep 5


sed -i "s/HOST.*/HOST $HOST/" cp2k.inp

srun cp2k.psmp -i cp2k.inp -o cp2k.out

wait


</code>