Relaxation of open crystal structures with a density functional tight-binding scheme: Illustration in the case of the acetonitrile crystal

In view of its interest for high pressure simulations of molecular crystals, the self-consistent field density functional tight-binding model is coupled to a symmetry-constrained optimization routine and applied to a stringent test case: the open structure of the acetonitrile crystal. The symmetry-constrained relaxation is compared with two symmetry-unconstrained relaxations, one starting from the observed X-ray structure and the other one from a structure obtained after a optimization of the atoms positions within the observed unit cell. According to the procedure employed for the crystal relaxation, three different energy minima with similar energies are found.