Intermolecular Forces and Rotational Phase Transition in the C2H2 Crystal

The potential energy of the acetylene crystal was calculated as a function of the rotational molecular orientations. The potential energy of this crystal has two kinds of minima, not a double minima; one arises from the quadrupole-quadrupole interaction, to which the high-temperature is due, while the other arises from the overlap repulsion interaction with which the low temperature structure is coincident. It was suggested that the mechanism of the phase transition in this crystal is a rotational transition between the two molecular orientations.