Ab initio study of anthracene under high pressure

The pressure effect on the internal molecular orientation, the electronic and optical properties of crystalline anthracene is calculated up to 10.2 GPa by performing density-functional calculations. As the only input for our ab initio calculations we use the lattice parameters experimentally determined by x-ray powder diffraction under pressure and optimize the internal geometry with respect to the three angles $\theta ,$ $\chi ,$ and $\delta ,$ which define the orientation of the molecules inside the unit cell. For the optimized structures the isothermal bulk moduli, the electronic band structures, and dielectric tensors as a function of the unit-cell volume are calculated. The structure optimizations using the local-density approximation and the generalized gradient approximation exchange-correlation potentials give very similar results and agree well with the internal geometry determined from experiment. This gives rise to the conclusion that the application of such approximations for the description of organic molecular crystals within density-functional theory is valid. Moreover, the electron distribution clearly shows a finite density between the molecules in the unit cell, which increases with pressure due to the enhancement of the intermolecular interactions. These findings support the interpretation that the bonding mechanism in anthracene is not solely van der Waals interaction.