Fig. 1. Optimized crystal structure of C₂H₂ at (a) 0 GPa and (b) 25 GPa. (c) Relative changes in unit cell parameters. (d) Selected d_C⋯C and (e) angles in the crystal as functions of pressure. d1 is d_C⋯C on the a–c plane, and d2 is d_C⋯C almost on the a–b plane. ∠1 or ∠2 is the angle between the c axis and the line between two carbon/hydrogen atoms, and ∠3 is the H–C–C bond angle.

Fig. 2. Electronic properties of the intermolecular C⋯C bond vs bond length. The bonding proceeds in three stages. At about 2.3 Å, the Mayer bond order (black) and the ELF value at the midpoint of the bond (red) sharply increase, suggesting the onset of bonding.

Fig. 3. Phonon spectra of the acetylene crystal and some of the vibrational modes at the Γ-point. (a) Spectrum of the optimized structure at 29 GPa. (b) Four phonon branches with imaginary frequencies from the Γ-point to the T-point (0.5, 0, −0.5) at 29 GPa. (c) Spectrum at 0 GPa (experimental 5.7 GPa). In the high-frequency part, the phonon dispersion curves are nearly horizontal. (d) Illustrations of two typical lattice vibrations leading to a decrease in d_C⋯C, originating from molecular translations and rotations.

Fig. 4. Vibrational displacement of carbon atoms from the equilibrated position with T-point symmetry applied. The instantaneous d_C⋯C at RT becomes 2.3 Å and reaches the threshold for intermolecular bonding.

Fig. 5. Reaction curve from acetylene to polyacetylene in the crystal. The MEP is shown as the black line, and the pathway through the transient “threshold model” is shown as the red line. The left part of each curve describes the enthalpy change of the supercell as the instantaneous d_C⋯C varies until intermolecular bonding occurs. The right part is the subsequent reaction displayed in steps. The bonding stage on the new path is rescaled for clarity. Three representative structures along the constructed path are illustrated. The enthalpies of the “threshold model” and the “TS” are almost the same.