Internal units

All the units used internally by i-PI are atomic units, as given below. By default, both input and output data are given in atomic units, but in most cases the default units can be overridden if one wishes so. For details on how to do this, see and 3.2.1.



S.I. Value


Bohr radius

5.2917721e-11 m



2.4188843e-17 s


Electron mass

9.1093819e-31 kg



315774.66 K



4.3597438e-18 J



2.9421912e13 Pa

Regarding the specification of these units in the i-PI input files, the user is able to specify units both in the i-PI input file or in the structure file. If the structure file is of the .xyz format, the units specifications should be present in the comment line. Examples of such inputs can be found in examples/pes-regtest/io-units/ . The code then behaves in the following way, depending on the user’s choice:

  • If no units are specified in the input, i-PI tries to guess from the comment line of the structure file and it nothing is present, assumes atomic units, except for the cases discussed below.

  • If units are specified in both input and structure file and they match, conversion happens just once. If they do not match, an error is raised and i-PI stops.

There are a few exceptions:

  • The total simulation time, specified by the flag <total_time>, has to be provided in seconds.

  • The PDB standard defines units to be in angstrom, and so default units for PDB are angstrom.

  • When using ASE format for I/O, the units are dictated by the ASE defaults. So for example, the ASE extended xyz format expects positions in angstroms. (See for example examples/ase-io/)