Quantum crystallography

Quantum Crystallography (QCr) concerns the combining of crystallographic da ta with quantum-mechanical techniques in such a way that it should be possi ble to obtain information of enhanced value. The enhancement could be incre ased accuracy or information more readily obtained, or both. Our approach t o quantum crystallography makes use of molecular orbitals and a single dens ity matrix (projector matrix) formalism to form a quantum-mechanical model that includes information from a crystal structure analysis. Some experienc es involving data for crystalline maleic anhydride and also various substit uted cubanes are described. A method is also discussed for performing ab in itio quantum mechanics on large structures for which the calculation time i s essentially a linear function of the complexity. This method is based on a property of overlap integrals that permits the ab initio calculations to be performed on fragments of the structure. The fragment calculations are c ombined to form an accurate density matrix for the entire structure. (C) 19 98 Elsevier Science B.V. All rights reserved.