This thesis investigates the existence of host-guest structure under high pressure in elemental metals such as Mg, Li, Ca, Sc and As. The new stable structure of magnesium (Mg) is predicted by ab initio random structure searching (AIRSS) technique under high pressure. The I4/mmm and Pnma structures have been predicted to give the normal metallic state at the pressure above 160 GPa. Moreover, for Li, the AIRSS technique is used to identify the host-guest (HG) of lithium (Li). The distribution of electrons between the host-host atoms has also been investigated by electron localization function. The strongly localized electron of π bond has led to the stability of the HG structure under high pressure. The calculation result from Ca in the metallic phase has led to the discovery of high-Tc superconductivity, with a Tc of 25.2 K at 160 GPa. The Tc of this monoclinic Cc structure is in good agreement with experiment. In addition, the explanation of Sc-II revealed that the 3cH and the 4cG structures coexist which form the HG structure with CH/CG ratio of 4/3 at 70 GPa. The AIRSS technique predicts the high pressure structure of Sc-III to be P41212 and also reveals Tc of 8.36 K which is in good agreement with the experimental result. Furthermore, the body-centered tetragonal (bct) structure in arsenic (As) has been predicted using AIRSS technique at 150 GPa. The calculation suggests transition sequence from the simple cubic structure transforms to the host-guest structure at 41 GPa and then to the bct structure at 81 GPa. The spectral function of bct structure is higher than those of the bcc structure. This result is also reported to be Tc 4.2 K at 150 GPa.

The Group IIA metal, also known as alkaline-earth, undergoes the structural phase transitions under high pressure due to s to d orbital electron transition. It has been reported to possess several novel crystal structures, however, the full structural details has not been identified. In this thesis, structural studies of these metals have been carried out using computational method. Density Functional Theory (DFT) and Molecular Dynamics (MD) are the computation approaches which have been exploited to calculate the transition behavior. For calcium, the ambient crystal structure is the face-centered cubic (fcc) with space group. Under higher pressure, it transforms to the body-centered cubic (bcc) with space group at 5.4 GPa and then transforms to the Ca-III structure which is the β-tin structure with space group at 33.2 GPa. Finally, it transforms to the structure at 91.8 GPa. For strontium, at medium pressure range (20 GPa-40 GPa), structures were investigated by ab initio calculation and special interest has been given to the variation of exchange-correlation functional, i.e., Perdew Burke Ernzerhof (PBE) and screened exhange-Local Density Aproximation (sX-LDA). By using PBE potential functional in the calculation, the fcc structure shows the transformation to the bcc structure at 1.4 GPa and then to the hcp structure at 23.8 GPa. On the contrary, when using sX-LDA as a potential functional, the enthalpy calculation of β-tin structure is significantly lower than those of the hcp structure. Therefore, sX-LDA is the most suitable potential functional for this type of phase transitions as sX-LDA can provide a significantly improved result when compared with the experimental data.