## Porting VASP from MPI to MPI+OpenMP [SIMD]

• We describe for the VASP application (a widely used electronic structure code written in FORTRAN) the transition from an MPI-only to a hybrid code base leveraging the three relevant levels of parallelism to be addressed when optimizing for an effective execution on modern computer platforms: multiprocessing, multithreading and SIMD vectorization. To achieve code portability, we draw on MPI parallelization together with OpenMP threading and SIMD constructs. Combining the latter can be challenging in complex code bases. Optimization targets are combining multithreading and vectorization in different calling contexts as well as whole function vectorization. In addition to outlining design decisions made throughout the code transformation process, we will demonstrate the effectiveness of the code adaptations using different compilers (GNU, Intel) and target platforms (CPU, Intel Xeon Phi (KNL)).

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