In 1965, Gordon Moore predicted that the number of transistors in an integrated circuit would double about every two years (the so-called Moore's law). Rapidly, this "law" was interpreted as an expected two-fold increase in performance every 18 months. is became an industrial goal. e development of today's most popular electronic structure codes was initiated in the 1990's (or even before). At that time, the increase of computational power from one supercomputer a) Electronic mail: loos@irsamc.ups-tlse.fr b) Electronic mail: emmanuel.giner@lct.jussieu.fr c) Electronic mail: scemama@irsamc.ups-tlse.fr generation to the next was mostly driven by an increase of processors' frequency. Indeed, the amount of random access memory was small, the time to access data from disk was slow, and the energy consumption of the most powerful computer was 236 kW, hence far from being an economical concern. At the very beginning of the 21st century, having increased continuously, both the number of processors and their frequency raised the supercomputer power consumption by two orders of magnitude, in ating accordingly the electricity bill. e only way to slow down this frenetic growth of power consumption while keeping alive Moore's dream was to freeze the processor's frequency (between 1 and 4 GHz), and increase the number of CPU cores. e consequence of such a choice was that "free lunch" was over: the programmers now had to parallelize their programs to make them run faster. At the same time, computer scientists realized that the increase of performance in memory access was slower than the increase in computational power, and that the oating-point operation (or op) count would soon stop being the bo leneck. From now on, data movement would be the main concern.