  We report on scaling and timing tests of the SIESTA electronic structure code
for ab initio molecular dynamics simulations using density-functional theory.
The tests are performed on six large-scale supercomputers belonging to the
PRACE Tier-0 network with four different architectures: Cray XE6, IBM
BlueGene/Q, BullX, and IBM iDataPlex. We employ a systematic strategy for
simultaneously testing weak and strong scaling, and propose a measure which is
independent of the range of number of cores on which the tests are performed to
quantify strong scaling efficiency as a function of simulation size. We find an
increase in efficiency with simulation size for all machines, with a
qualitatively different curve depending on the supercomputer topology, and
discuss the connection of this functional form with weak scaling behaviour. We
also analyze the absolute timings obtained in our tests, showing the range of
system sizes and cores favourable for different machines. Our results can be
employed as a guide both for running SIESTA on parallel architectures, and for
executing similar scaling tests of other electronic structure codes.
