AB-INITIO CALCULATION OF THE HIGH-PRESSURE BEHAVIOR OF HYDROGEN-CYANIDE

The high-pressure behaviour of hydrogen cyanide, HCN, has been investi gated by parameter-free ab initio calculations based on density functi onal theory, a generalized gradient approximation, pseudo-potentials a nd a constant-pressure relaxation based on a BFGS algorithm. At ambien t pressure all calculated structural parameters coincide within 2% wit h experimental data. Phonon frequencies calculated with a frozen-phono n approach match experimental observations within 3%. The present calc ulations predict a second-order ferro-elastic pressure-induced structu ral phase transition at 51(2) GPa from the low-temperature, ambient-pr essure orthorhombic structure to a tetragonal structure similar to tha t observed at ambient conditions. In HCN no symmetrical hydrogen bonds are formed at pressures up to 100 GPa.