A wavefunction and energy of the azide ion in potassium azide obtained by a quantum-mechanically constrained fit to X-ray diffraction data

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A wavefunction for the azide ion in solid potassium azide has been obtained by fitting the elements of a density matrix to high-resolution X-ray structure factors, subject to an idempotency constraint. The method used includes both molecular symmetry and idempotency constraints on the density matrix, and allows use of large contracted Gaussian basis sets. Using a 6-31G∗ basis set, the energies calculated from the fitted wavefunction and the theoretical wavefunction are −162.29 and −163.29 hartree, respectively. The electron deformation density distribution constructed from the fitted wavefunction shows some features that are also present in multipole model densities. However, it also shows some features found in theoretical densities that are not generally present in multipole model densities.