NWChem (website) is a widely used open-source computational chemistry software package, written primarily in Fortran. It supports scalable parallel implementations of atomic orbital and plane-wave density-function theory, many-body methods from Moller-Plesset perturbation theory to coupled-cluster with quadruple excitations, and a number of methods for computing multiscale methods and molecular and macroscropic properties. It is used on computers from Apple M1 laptops to the largest supercomputers, supporting multicore CPUs and GPUs with OpenMP and other programming models. NWChem uses MPI for parallelism, usually hidden by the Global Arrays programming model, which uses one-sided communication to support a data-centric abstraction of multidimensional arrays across shared and distributed memory protocols. NWChem was created by Pacific Northwest National Laboratory in the 1990s, and has been under continuous development by a team based on national laboratories, universities, and industry for 25 years. NWChem has been cited thousands of times and was a finalist for the Gordon Bell Prize in 2009.

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The ICON (ICOsahedral Nonhydrostatic) earth system model is a unified next-generation global numerical weather prediction and climate modelling system. It consists of an atmosphere and an ocean component. The system of equations is solved in a grid point space on a geodesic icosahedral grid, which allows a quasi-isotropic horizontal resolution on the sphere as well as the restriction to regional domains. The primary cells of the grid are triangles resulting from a Delaunay triangulation which in turn allows C-grid type discretization and straight forward local refinement in selected areas. ICON contains parameterization packages for scales from ~100km for long term coupled climate simulations to ~1km for cloud (atm.) or eddy (ocean) resolving regional simulations. ICON has proven to have high scalability while running on the largest German and European HPC machines.

The ICON model has been introduced into DWD's (German Weather Service) operational forecast system in January 2015 and is used in several national and international climate research projects targeting high resolution simulations.

ESIWACE (EU project)
CLICCS (German project)
Monsoon 2.0 (German-Chinese Collaboration)
HD(CP)2 (German project)

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Xcompact3d is a Fortran-based framework of high-order finite-difference flow solvers dedicated to the study of turbulent flows. Dedicated to Direct and Large Eddy Simulations (DNS/LES) for which the largest turbulent scales are simulated, it can combine the versatility of industrial codes with the accuracy of spectral codes. Its user-friendliness, simplicity, versatility, accuracy, scalability, portability and efficiency makes it an attractive tool for the Computational Fluid Dynamics community.

XCompact3d is currently able to solve the incompressible and low-Mach number variable density Navier-Stokes equations using sixth-order compact finite-difference schemes with a spectral-like accuracy on a monobloc Cartesian mesh. It was initially designed in France in the mid-90's for serial processors and later converted to HPC systems. It can now be used efficiently on hundreds of thousands CPU cores to investigate turbulence and heat transfer problems thanks to the open-source library 2DECOMP&FFT (a Fortran-based 2D pencil decomposition framework to support building large-scale parallel applications on distributed memory systems using MPI; the library has a Fast Fourier Transform module).

When dealing with incompressible flows, the fractional step method used to advance the simulation in time requires to solve a Poisson equation. This equation is fully solved in spectral space via the use of relevant 3D Fast Fourier transforms (FFTs), allowing the use of any kind of boundary conditions for the velocity field. Using the concept of the modified wavenumber (to allow for operations in the spectral space to have the same accuracy as if they were performed in the physical space), the divergence free condition is ensured up to machine accuracy. The pressure field is staggered from the velocity field by half a mesh to avoid spurious oscillations created by the implicit finite-difference schemes. The modelling of a fixed or moving solid body inside the computational domain is performed with a customised Immersed Boundary Method. It is based on a direct forcing term in the Navier-Stokes equations to ensure a no-slip boundary condition at the wall of the solid body while imposing non-zero velocities inside the solid body to avoid discontinuities on the velocity field. This customised IBM, fully compatible with the 2D domain decomposition and with a possible mesh refinement at the wall, is based on a 1D expansion of the velocity field from fluid regions into solid regions using Lagrange polynomials or spline reconstructions. In order to reach high velocities in a context of LES, it is possible to customise the coefficients of the second derivative schemes (used for the viscous term) to add extra numerical dissipation in the simulation as a substitute of the missing dissipation from the small turbulent scales that are not resolved.

Xcompact3d is currently being used by many research groups worldwide to study gravity currents, wall-bounded turbulence, wake and jet flows, wind farms and active flow control solutions to mitigate turbulence.

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Coding Challenge

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