Quantum ESPRESSO

Quantum ESPRESSO (Quantum opEn-Source Package for Research in Electronic Structure, Simulation, and Optimisation) is the prime open-source (set of) code(s) for quantum materials modelling using the plane-wave pseudopotential method; it has been the development platform for such important methodological innovations as Car-Parrinello molecular dynamics and Density-Functional Perturbation Theory. It is released under the GNU GPL. It implements density-functional theory (DFT) in the plane-wave pseudopotential approach, but it also includes more advanced levels of theory: DFT+U, hybrid functionals, various functionals for van der Waals forces, many-body perturbation theory, adiabatic-connection fluctuation-dissipation theory.

Among the various packages composing Quantum ESPRESSO, we mention

  • PWscf: structural optimisation and molecular dynamics on the electronic ground state, with self-consistent solution of DFT equations;
  • CP: Car-Parrinello molecular dynamics;
  • PHonon: vibrational and dielectric properties from Density-Functional Perturbation Theory;
  • TD-DFPT: spectra from Time-dependent Density-Functional Perturbation Theory;
  • EPW: calculation of electron-phonon coefficients in metals;
  • PWneb: reaction pathways and transition states with the Nudged Elastic Band method;
  • GWL: many-body perturbation theory in the GW approach using ultra-localised Wannier functions andLanczos chains.

During the first MaX project, and in collaboration with the ECAM CoE, the following libraries have been isolated and partially encapsulated in view of their release for usage in other codes as well:

  • UtilXlib performing basic MPI handling, error handling, timing handling.
  • FFTXlib parallel (MPI and OpenMP) distributed three-dimensional FFTs, performing also load-balanceddistribution of data (plane waves, G-vectors and real-space grids) across processors.
  • LAXlib parallel distributed dense-matrix diagonalization, using ELPA, SCALapack, or a custom algorithm.
  • KS Solver parallel iterative diagonalization for the Kohn-Sham Hamiltonian (represented as an operator),using block Davidson and band-by-band Conjugate-Gradient algorithms.
  • LRlib performs a variety of tasks connected with (time-dependent) DFPT, to be used also in connectionwith Many-Body Perturbation Theory.

Diffusion
The Quantum ESPRESSO manifesto published in 2009 has received more than 7200 citations in 8.5 years in the scientific literature. The users’ mailing list has currently 1650 active subscribers (i.e. receiving e- mails) and ∼ 2800 messages per year. Since 2001, more than 30 schools and tutorials based on Quantum ESPRESSO have been done in various parts of the world, including, thanks to the collaboration with ICTP, developing countries such as India, Iran, South Africa, Kenya. Quantum ESPRESSO is part of the PRACE Unified European Applications Benchmark Suite and is being used for validation and co-design by major hardware hip vendors: Intel, NVIDIA, and ARM.

Scaling
The Quantum ESPRESSO software was designed from the beginning having in mind complex condensed- matter systems described by large unit cells. Several levels of OpenMP and MPI parallelisation are present,  depending upon the kind of calculation and upon the package (PWscf and especially CP having been the main target for extensive parallelisation). A GPU-enabled version written in CUDA Fortran, replacing a pre- vious CUDA porting of selected routines, has been recently released. Important advances in parallelisation have resulted from the collaboration with PRACE1and from the MaX CoE2.

Scalability of a DFT calculation by the Quantum ESPRESSO suite, using the pw.x code. Blue dots represent the time to solution, red dots connected by the dash-dotted line represent the parallel efficiency. Calculations were run on Marconi at CINECA, A2 partition.

1 see paper http://www.prace-ri.eu/IMG/pdf/enabling_of_quantum_espresso_to_petascale_scientific_challenges.pdf

2 see benchmarks at https://github.com/electronic-structure/benchmarks