Pressure induced phonon softening and electronic topological transition in HgBa₂CuO₄

Results of total energy local density calculations for the effect of pressure on the lattice parameters, bond lengths, electronic structure and A_1g phonon frequency in H_gBa₂CuO₄ have been carried out. The effects of pressure on the frequency of the apical-oxygen (O_A) A_1g phonon in the Hg-O_A-Cu bonds are studied in order to understand the role of pressure in increasing T_c of mercury-based superconductors. Theoretically determined zero pressure lattice parameters and phonon frequencies are in good agreement with experiments. An electronic topological transition that occurs when the van Hove singularity is shifted close to the vicinity of E_F by pressure causes considerable phonon softening and anomalous behavior of c-axis length, the Hg-O_A bond and Ba z-coordinate.