Intermolecular Potential in Solid Methane. I. Cohesive Energy and Crystal Structure

The cohesive energy of crystalline methane is calculated for various possible structures, using a potential function consisting of repulsive and attractive interactions between nonbonded atoms. The necessary parameters were taken without adjustment from independent studies of other systems. It was found that the tetragonal structures of symmetry D3d have the best packing. The cohesive energy was increased when the unit cell was distorted from the cubic dimensions, the volume remaining constant. The maximum contribution of this distortion (about 1% of the total energy) occurs when the c axis of the unit cell is elongated 12%. The calculated stability of the crystal is not affected by slight displacements of the molecules along the c axis. On the other hand, the calculated stability of the crystal decreases upon rotation or upon distortions from Td symmetry of the individual molecules in the lattice.