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Density-matrix refinement for molecular crystals.

The fitting of an idempotent density matrix to x-ray structure factors is discussed. If the density matrix is identified with a single Slater determinant of orbitals, then a model wave function can be recovered, enabling the computation of two-electron properties as well as the usual one-electron properties derived from the electron density. Hence this approach is potentially superior to the usual multipole expansion used in charge-density analysis, although the current formalisms are only useful for molecular crystals of very small molecules. A procedure is described, and applications to one theoretical example (methylamine) and one experimental example (formamide) are presented.