A combined quantum chemistry and classical molecular interaction energy method for the determination of crystal geometries and energies

Abstract

Theoretical molecular solid crystal structure optimization presents many computational challenges. Calculation of both intramolecular interactions and intermolecular interactions are major obstacles. In this study, the intramolecular interactions are treated quantum mechanically and the intermolecular interactions are approximated by a force field, which is in part determined by a partial charge analysis of the quantum treatment. This combined approach, called the quantum coupled unit cell description (QCUCD) method, treats short-range and long-range intermolecular interactions with convergence-accelerated lattice sum techniques. QCUCD finds the internal molecular geometry; then crystal parameters are optimized until a fully consistent solution between the unit cell description and the quantum chemistry of the molecular electronic structure is achieved. The single molecule experiences the periodically repeating potential at one molecular site of the crystal. The solution of the electronic structure of the molecule under the QCUCD Hamiltonian determines the atomic partial charges that are positioned in a periodic array to establish the electrostatic contribution to the potential. The parameters for the Lennard-Jones interactions are taken from the force field of the widely used molecular simulation program AMBER as a convenient source for a large number of atom-based interactions. All geometric parameters for definition of the molecular crystal are optimized: the lattice constants, the orientational and positional degrees of freedom of the molecules at their crystal lattice sites, and the internal molecular geometry. QCUCD correctly determines the crystalline structures of the simple molecular solids N2, CO, and CO2 and also obtains the correct crystal packing orders of two different phases of N2 crystal in terms of crystal packing energies.