[Input: recipes/interfaces/ase/sockets/]

For calculations that heavily rely on file-IO, it is better to use communication between DFTB+ and ASE with the i-PI protocol. In these cases, the reduction of the wallclock time can be significant.

Note: At this time, only communication with ASE or i-PI has been tested. In general, however, other software which correctly implements the i-PI protocol should be supported.


To enable socket-communication in DFTB+ the WITH_SOCKETS flag in the configuration file config.cmake must be set to TRUE, before starting the compilation process!

Geometry Optimization by ASE

Providing the input for DFTB+

To establish a connection via socket communication, DFTB+ is called with the appropriate Socket{} driver option:

Driver = Socket {
  File = "dftbplus"
  Protocol = i-PI {}
  MaxSteps = 1000
  Verbosity = 0

Which instructs DFTB+ to read geometry driving commands via a named temporary communication file (stored in /tmp/). In this example, due to a bug in the current ASE implementation of the i-PI protocol (see note below), the code is asked to perform up to 1000 geometry steps under control of the external interface then stop. For external driving codes that support the EXIT command in the protocol, instead DFTB+ can continue running until told to stop by using:

MaxSteps = -1

Due to the nature of the i-PI protocol, a dummy geometry must be provided, from which the number and species of the atoms, as well as unit cell vectors will be read. In general, the specification of arbitrary positions and cell vectors is possible. However, to avoid errors due to a different ordering of the atoms in the ASE-geometry and the species in the dummy-geometry, this example uses the ability of ASE to read in and write out .gen files. The output geometry (geo.gen) is then used as input for DFTB+, avoiding the possibility of the above mentioned errors:

Geometry = GenFormat {
  <<< "geo.gen"

In this example, calculation of a water molecule is performed without charge self-consistency and the required Slater-Koster files are stored in the top-level directory (same folder as dftb_in.hsd):

Hamiltonian = DFTB {
  SCC = No

  MaxAngularMomentum = {
    O = "p"
    H = "s"

  SlaterKosterFiles = Type2FileNames {
    Prefix = "./
    Separator = "-"
    Suffix = ".skf"

For the full input file assembled from the code snippets shown here, please consult the archive, whose location is stated at the begin of this section.

Geometry and main script

The geometry used is a simple water molecule, whose file location is specified in the main script (GEO_PATH):

3  C
 O H
     1    1    0.00000000000E+00  -0.10000000000E+01   0.00000000000E+00
     2    2    0.00000000000E+00   0.00000000000E+00   0.78306400000E+00
     3    2    0.00000000000E+00   0.00000000000E+00  -0.78306400000E+00

Following the necessary imports, the main script defines the type of socket to be used (UNIX socket) as well as the path to the DFTB+ executable (DFTBP_PATH) and the geometry (GEO_PATH). The main method then first reads in the geometry and immediately writes it out for the reasons mentioned above. Subsequently, the ASE driver BFGS() for geometry optimization is specified and where to write the trajectory and the logfile.

Finally, the convergent forces and the corresponding energy of the system can be read out directly by ASE:

import sys
from subprocess import Popen
from import read, write
from ase.optimize import BFGS
from ase.calculators.socketio import SocketIOCalculator

UNIXSOCKET = 'dftbplus'
GEO_PATH = './H2O_cluster.gen'

def main():
    '''Main driver routine.'''

    system = read(GEO_PATH, format='gen')
    write('geo.gen', system, format='gen')

    opt = BFGS(system, trajectory='opt.traj', logfile='opt.log')

    with SocketIOCalculator(log=sys.stdout, unixsocket=UNIXSOCKET) as calc:

    forces = system.get_forces()
    energy = system.get_potential_energy()

if __name__ == "__main__":


To correctly close sockets on the ASE side, call calc.close() at the end or, more elegantly, enclose the class SocketIOCalculator using the with statement as done in the example shown here. Nevertheless, in the current state of ASE, the socket gets closed without warning missing the ‘EXIT’ string of the i-PI protocol, which always leads to an error message issued by DFTB+ at the end of a calculation driven by socket-communication.