Department of Chemistry - PhD Student Seminar - Time for a Paradigm Shift in Crystal Structure Refinement: Farewell to the Independent Atom Model & Welcome to Quantum Crystallography

Student: Ms. Cengan WANG

Department: Department of Chemistry, HKUST

Supervisor(s): Prof. Ian Duncan WILLIAMS, Prof. Zhihong GUO (Co)

Abstract

In single-crystal X-ray diffraction, conventional IAM methods employ spherical scattering factors. Although it is effective in solving the crystal structure and atomic positions, it fails to fully reflect bonding electrons. More accurate aspherical models—such as MM and TAAM— have been developed but faced challenges including database reliance, high computational time costs, and complex non-automatic operations, limiting their widespread adoption.

Today, quantum chemical calculations enable the calculation of the entire electron density of the molecules in a crystal structure, allowing atomic X-ray scattering to be adjusted according to the actual electron density distribution.

To achieve this, it requires the faster computation and a program that can output quantum calculated data to structure refinement. Additionally, an X-ray crystallographic refinement software package is needed that can accept quantum mechanical solutions and apply them in least-squares refinement of crystal structures.

Both requirements are now met, so the new approach NoSpherA2, as an interface module within Olex2, will gradually be accepted. Within a few years, it is expected to become standard.