Output files

i-PI uses a very flexible mechanism to specify how and how often atomic configurations and physical properties should be output. Within the Output file tags tag of the xml input file the user can specify multiple tags, each one of which will correspond to a particular output file. Each file is managed separately by the code, so what is output to a particular file and how often can be adjusted for different files independently.

For example, some of the possible output properties require more than one force evaluation per time step to calculate, and so can considerably increase the computational cost of a simulation unless they are computed once every several time steps. On the other hand, for properties such as the conserved energy quantity it is easy, and often useful, to output them every time step as they are simple to compute and do not take long to output to file.

There are three types of output file that can be specified; property files for system level properties, trajectory files for atom/bead level properties, and checkpoint files which save the state of the system and so can be used to restart the simulation from a particular point. For a brief overview of the format of each of these types of files, and some of their more common uses, see Part 1 - NVT Equilibration run. To give a more in depth explanation of each of these files, they will now be considered in turn.


This is the output file for all the system and simulation level properties, such as the total energy and the time elapsed. It is designed to track a small number of important properties throughout a simulation run, and as such has been formatted to be used as input for plotting programs such as gnuplot.

The file starts with a header, which describes the properties being written in the different columns and their output units. This is followed by the actual data. Each line corresponds to one instant of the simulation. The file is fixed formatted, with two blank characters at the start of each row, then the data in the same order as the header row. By default, each column is 16 characters wide and every float is written in exponential format with 8 digits after the decimal point.

For example, if we had asked for the current time step, the total simulation time in picoseconds, and the potential energy in electronvolt, then the properties output file would look something like:

# column 1 –> step : The current simulation time step. # column 2 –>
timepicosecond : The elapsed simulation time. # column 3 –>
potential{electronvolt} : The physical system potential energy.
0.00000000e+00 0.00000000e+00 -1.32860475e+04 1.00000000e+00
1.00000000e-03 -1.32865789e+04 ...

The properties that are output are determined by the properties tag in the xml input file. The format of this tag is:

<properties stride= filename= flush= shape=> [ prop1name{units}(arg1;
... ), prop2name{units}(...), ... ] </properties>


The attributes have the following meanings:


The number of steps between each output to file


The name of the output file


The number of output lines between buffer flushes


The number of properties in the list.

The tag data is an array of strings, each of which contains three different parts:

  • The property name, which describes which type of property is to be output. This is a mandatory part of the string.

  • The units that the property will be output in. These are specified between curly brackets. If this is not specified, then the property will be output in atomic units. Note that some properties can only be output in atomic units.

  • The arguments to be passed to the function. These are specified between standard brackets, with each argument separated by a semi-colon. These may or may not be mandatory depending on the property, as some arguments have well defined default values. The arguments can be specified by either of two different syntaxes, (name1=arg1; …) or (arg1; …).

    The first syntax uses keyword arguments. The above example would set the variable with the name “name1” the value “arg1”. The second syntax uses positional arguments. This syntax relies on the arguments being specified in the correct order, as defined in the relevant function in the property.py module, since the user has not specified which variable to assign the value to.

    The two syntaxes may be mixed, but positional arguments must be specified first otherwise undefined behaviour will result. If no arguments are specified, then the defaults as defined in the properties.py module will be used.

See also the documentation of the properties tag, and the full List of available properties.

Trajectory files

These are the output files for atomic or bead level properties, such as the bead positions. In contrast to properties files, they output data for all atomic degrees of freedom, in a format that can be read by visualization packages such as VMD.

Multiple trajectory files can be specified, each described by a separate trajectory tag within the Output file tags section of the input file. The allowable file formats for the trajectory output files are the same as for the configuration input files, given in Configuration files.

These tags have the format:

<trajectory stride=`' filename=`' format=`' cell_units=`' flush=`' bead=`'>

This is very similar to the properties tag, except that it has the additional tags “format” and “cell_units”, and only one traj_name quantity can be specified per file. ‘format’ specifies the format of the output file, and ‘cell_units’ specifies the units in which the cell dimensions are output. Depending on the quantity being output, the trajectory may consist of just one file per time step (e.g. the position of the centroid) or of several files, one for each bead, whose name will be automatically determined by appending the bead index to the specified “filename” attribute (e.g. the beads position). In the latter case it is also possible to output the quantity computed for a single bead by specifying its (zero-based) index in the “bead” attribute.

See also the trajectory tag, and the full List of available trajectory files.

Checkpoint files

As well as the above output files, the state of the system at a particular time step can also be saved to file. These checkpoint files can later be used as input files, with all the information required to restore the state of the system to the point at which the file was created.

This is specified by the checkpoint tag which has the syntax:

<checkpoint stride= filename= overwrite=> step </checkpoint>

Again, this is similar to the properties and trajectory tags, but instead of having a value which specifies what to output, the value simply gives a number to identify the current checkpoint file. There is also one additional attribute, “overwrite”, which specifies whether each new checkpoint file overwrites the old one, or whether all checkpoint files are kept. If they are kept, they will be written not to the file “filename”, but instead an index based on the value of “step” will be appended to it to distinguish between different files.

If the ‘step’ parameter is not specified, the following syntax can also be used:

<checkpoint stride= filename= overwrite=/>

Soft exit and RESTART

As well as outputting checkpoint files during a simulation run, i-PI also creates a checkpoint automatically at the end of the simulation, with file name “RESTART”. In the same way as the checkpoint files discussed above, it contains the full state of the simulation. It can be used to seamlessly restart the simulation if the user decides that a longer run is needed to gather sufficient statistics, or if i-PI is terminated before the desired number of steps have been completed.

i-PI will try to generate a RESTART file when it terminates, either because total_time has elapsed, or because it received a (soft) kill signal by the operating system. A soft exit can also be forced by creating an empty file named “EXIT” in the directory in which i-PI is running.

An important point to note is that since each time step is split into several parts, it is only at the end of each step that all the variables are consistent with each other in such a way that the simulation can be restarted from them without changing the dynamics. Thus if a soft exit call is made during a step, then the restart file that is created must correspond to the state of the system before that step began. To this end, the state of the system is saved at the start of every step.

In order to restart i-PI from a file named “RESTART” one simply has to run

> python i-pi RESTART

Reading output files

It can be useful to parse the output files of i-PI into a format that can be readily manipulated in a custom Python script. To this end, i-PI provides a few utilities. ipi.read_output that can be used to parse a property output file, that returns data blocks as a dictionary of numpy array, and additional information from the header (such as units, and the description of each property) as a separate dictionary

from ipi import read_output
data, info = read_output("simulation.out")

Trajectory files can be read with ipi.read_trajectory. This reads the trajectory output into a list of ase.Atoms objects (hence this functionality has a dependency on ase), converting positions and cell to angstrom, and moving other properties to arrays and converting them to ASE units (e.g. forces are in eV/Å). extras output files (that contain either numerical data, or raw strings, returned by the driver code in addition to energy and forces) can be processed by using format=’extras’ and an option: the call will then return a dictionary with an entry having the name of the type of extra (if present) and either a list of the raw strings, or a numpy array with the data. A second dictionary entry contains the list of step numbers.

from ipi import read_trajectory
data = read_trajectory("simulation.dipoles", format="extras")