Ab initio calculation of the high-pressure behaviour of hydrogen cyanide

The high-pressure behaviour of hydrogen cyanide, HCN, has been investigated by parameter-free ab initio calculations based on density functional theory, a generalized gradient approximation, pseudo-potentials and a constant-pressure relaxation based on a BFGS algorithm. At ambient pressure all calculated structural parameters coincide within 2% with experimental data. Phonon frequencies calculated with a frozen-phonon approach match experimental observations within 3%. The present calculations predict a second-order ferro-elastic pressure-induced structural phase transition at 51(2) GPa from the low-temperature, ambient-pressure orthorhombic structure to a tetragonal structure similar to that observed at ambient conditions. In HCN no symmetrical hydrogen bonds are formed at pressures up to 100 GPa.