Ab initio study on geometrical structures of the TTTA molecular crystal

The crystal made of organic radical, l,3,5-trithia-2,4,6-triazapentalenyl (TTTA), exhibits a large first-order magnetic and structural phase transition between a paramagnetic high-temperature (HT) phase and a diamagnetic low-temperature (LT) phase, with a surprisingly wide thermal hysteresis loop over room temperature. We investigate theoretically the crystal structures for the two phases by means of the ab initio structural optimization method based on the local density approximation. We present necessary and sufficient atomic coordinates in this paper. The resulting structures are in good agreement with the recent X-ray diffraction experiment. In the HT phase, uniform one-dimensional stacking of the radical molecule appears, while, in the LT phase, strong dimerization along the stacking direction appears.