Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using ab initio calculations. The calculations reveal that -FeSe makes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe are also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from -FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between FeSe bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from -FeSe to the other phases of FeSe. The experimentally observed decrease in T _c of superconducting -FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest an increase of the T _c of -FeSe if such phase transitions are frustrated by suitable methods.