The PBC calculation is on a polymer chain in which non-covalent interactions is important. I'm attempting to calculate the crystal geometry, lattice parameters, and overall crystal packing. I'm wondering if somebody has a work around to this problem. I'm using Gaussian09. I've tried using both b97d, wb97xd, and b3lyp. The calculation runs with b3lyp without dispersion corrections, but I need dispersion corrections in order to attain meaningful and accurate results.