How Many Atoms Can Be Computed with QuantumATK

In principle, there is no limit on the system size, since ATK uses dynamic memory allocation. In practice, the system size is naturally limited by the amount of RAM on your computer. The memory required for a specific calculation depends very strongly on parameters like method (DFT vs. extended Hückel), the number and type of atoms, the size of the unit cell, the mesh cut-off, the basis set, the number of k-points, FFT method vs. multigrid, and a few other parameters. It should be possible to include between 500-1000 atoms with DFT if you have 8 Gb of RAM and don’t use too high accuracy settings, and the FFT method. On the other hand, we have run an example with over 4,400 carbon atoms, using the Hückel method with multigrid; it allocated just below 14 Gb. An important thing to note is that each MPI node uses the same amount of RAM. So if one assigns more than one MPI processes to the same node, the memory requirement goes up quickly on that node.