Periodic boundary conditions simple metals

Another interesting possibility is die use of plane waves as basis sets in periodic infinite systems (e.g., metals, crystalline solids, or liquids represented using periodic boundary conditions). "While it takes an enormous number of plane waves to properly represent the decidedly aperiodic densities that are possible within the unit cells of interesting chemical systems, the necessary integrals are particularly simple to solve, and dius diis approach sees considerable use in dynamics and solid-state physics (Dovesi et al. 2000). 

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