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IT Program: Crystallography

The independent atom model (IAM) assumes spherical symmetry of the atomic density, where the effects of influence of the chemical environment on atomic density, such as chemical bonds, charge transfer, lone pairs, etc. are missed, leading to systematic errors. More-realistic models of electron density have been put in the literature with refinable parameterization of the electron density where X-ray diffraction data of subatomic resolution is required. Even though high-resolution data are not available, the multipole model can still be used in the so-called transferable aspherical atom mode. In this case, the parameters are constrained to values typical for the corresponding atom type. In this way, the bottlenecks of IAM are fixed and superior results are obtained. In a similar way to TAAM, the idea of transferability was also applied to constructing a databank of extremely localized molecular orbitals. Application in crystallographic refinement is one of the intended uses of the databank, yet there was no practical application for this purpose . There are some procedures for this problem, such as Hirshfeld atom refinement, Hansen-Coppens model (for macromolecular refinement), and some other alternatives. Since it seems that TAAM is a better substitute for the IAM model in routine refinement for molecular crystals, the availability of software implementations becomes a prominent issue. This is the point where the novelty of the invention arises. The novelty includes a new toolbox of software libraries aimed at facilitating integration of the aspherical atom model with a wide range of refinement programs commonly used in X-ray crystallography.



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