Exploring the properties of silica under high pressure has also gathered interest within the scientific community, as such studies aid our understanding of processes occurring in Earth's mantle. For example, MD simulations of silica melts revealed that the self diffusion coefficient (SDC) of Si and O is pressure-dependent and that fivefold and sixfold coordinated Si sites become widespread under high pressure. A study combining MD and high-pressure in situ neutron diffraction indicated that the fivefold coordinated Si sites play a crucial role as intermediates in the transformation of SiO 2 glass from tetrahedral to octahedral coordination under pressure. Zhang et al. investigated silica in the TPa regime by applying MD, thereby explaining the nature of the bond dissociation process in SiO 2 in early Earth and making a contribution to the phase diagram of SiO 2 (Figure ). Another system that has been scrutinized under pressure is hydrogen cyanide, which crystallizes in long chains. DFT calculations have revealed that HCN undergoes phase transitions at elevated pressure, and that its high-pressure phases can exhibit insulating, semiconducting, metallic, or ferroelastic behavior. Under extreme pressure, HCN polymerizes (cf. Section 3.2.2 for more examples of pressure-induced polymerization processes). Due to the simplicity of the molecular structure and the chain-like geometry of crystalline HCN, this system is suitable for comparing simulations of the full crystal in a PBC setup to model systems with a finite number of molecules. Using X-HCFF and GOST-SHYP it was recently found that simulating a chain with approx. 15-20 HCN molecules reproduces most of the high-pressure structural parameters of a HCN crystal occurring along the crystallographic c axis, i.e., the direction along which the HCN molecules are aligned in the crystal. A similar comparison has been made using XP-PCM in the case of the realgar crystal. Melicherová and Martoňák studied the polymerization of nitrogen at high pressure and temperature using NPT MD simulations. The authors investigated liquid-liquid and liquid-solid transitions and found that the polymerization of nitrogen at 3000 K takes place between 110 and 115 GPa. With this, the authors achieved better agreement with experimental observations compared to previous studies. This was traced back to the use of a larger simulation cell that allowed chain formation, which might not be possible for smaller cells. The simplest molecular system, hydrogen, belongs to the most heavily researched elements, and computational studies focusing on its high-pressure liquid and crystalline phases are abundant.