Towards expansion of the MATTS data bank with heavier elements: the influence of the wavefunction basis set on the multipole model derived from the wavefunction.

The MATTS (multipolar atom types from theory and statistical clustering) data bank is an advanced tool for crystal structure refinement and properties analysis. It applies a multipole model (MM), which describes the asphericity of the atomic electron density and helps to interpret X-ray or electron diffraction data better than approaches based on the spherical atoms approximation. The generation of MATTS data involves density functional theory calculations, and until recently we used the B3LYP/6-31G** level of theory for this stage. However, it was not so clear how the wavefunction level of theory, especially the basis set used, influenced the resulting MM. This study investigates the influence of the wavefunction basis set on the resulting MM from a charge density point of view. For this purpose, we used charge density related properties, such as correlation of electrostatic potentials, atomic electron populations and average electrostatic potential values. The complex analysis reveals that, within the framework of MATTS data generation, the size of the basis set used has the most significant impact on the MM's charge density quality, and switching from double- to triple-zeta basis sets helps notably improve the charge density related properties. This research sets the foundation for the creation of a new version of the MATTS data bank, which will be expanded to include atom types for elements heavier than Kr and selected metal complexes important for biological systems.