Benchmarks

The standard Castep benchmarks have not changed for several years, and many are now too small to be useful for parallel scaling tests. The smallest benchmark, al1x1, is a small slab of aluminium oxide and runs in less that 6 minutes on 8 PEs of HECToR. The larger titanium nitride benchmark, TiN (which also contains a single hydrogen atom), only takes an hour on 16 PEs because the DM algorithm converges slowly - its time per SCF cycle is little more than twice the al1x1 benchmark. For this reason we settled on the al3x3 test system as the main benchmark for the parallel scaling tests, since this was large enough to take a reasonable amount of time per SCF cycle, yet small enough to run over a wide range of nodes.

The al3x3 benchmark is essentially a 3x3 surface cell of the al1x1 system, and has:

• 270 atoms (108 Al, 162 O)
• 88,184 G-vectors
• 778 bands (1296 electrons, non-spin-polarised, plus 130 conduction bands)
• 2 k-points (symmetrised 2$\times$2$\times$1 MP grid)

However the parameter files for this calculation do not specify Castep's optimisation level. In general it is advisable to tell Castep how to bias it's optimisation, e.g. opt_strategy_bias : 3 to optimise for speed (at the expense of using more RAM). Since the default optimisation level is not appropriate for HPC machines such as HECToR, most of our calculations were performed with the addition of opt_strategy_bias : 3 to the Castep parameter file al3x3.param.