It should be clear that connecting two parallel computer codes such as FAST and SIESTA so that they will cooperate is not a trivial undertaking. Users of FAST who will be doing one-off or infrequent computations e.g. at a single geometry for different systems are best advised to keep to communication via SIESTA output files.
However, FAST can be coupled directly to SIESTA using the MPICPL (MPI Coupling) framework. MPICPL is dedicated to the coupling of parallel scientific codes, based on the well-known MPI standard. It is divided into several independent layers for coupling, data redistribution and steering. The codes to be coupled are launched and connections between them are set up by mpicpl, according to information derived from an xml file.
The direct coupling option might be attractive to perform a series of TDDFT calculations on the fly, for example during a SIESTA molecular dyanmics run. This is feasible once the range of frequencies and FAST parameters have been tuned to extract resonances in the desired frequency window, e.g. usually, experimental data will be available only for the first one or two excited states. Such repetitive calculations are handled on the SIESTA side of the coupling by a set of patches, in which a call to FAST, communicating via MPICPL is inserted in the 'move' loop of the SIESTA main program.
MPICPL is downloadable at https://gforge.inria.fr/projects/mpicpl/https://gforge.inria.fr/projects/mpicpl/.
The SIESTA patches are available at https://gforge.inria.fr/frs/?group_id=1179 https://gforge.inria.fr/frs/?group_id=1179 .