The Cheng group develops novel electronic-structure methods and applies them in chemical and spectroscopic studies. The ultimate goal in the area of electronic-structure theory is perhaps to have a set of quantum-chemical methods and computer programs that are generally applicable to the entire periodic table and to both ground and excited states. We share this “dream”. Our method development work consists of

(1) relativistic quantum chemistry to relates the non-relativistic world (Schroedinger equation) with the relativistic world (Dirac equation).

(2) new methods to enhance our capabilities for treating electronically excited states.

We develop new quantum-mechanical methods, new algorithms as well as efficient computer implementations.

We use the new methods to solve problems that cannot be solved before. We are actively working on applications to two topics:

(1) chemistry of early actinides.

(2) X-ray spectroscopy and dynamics.

We work closely with experimental colleagues on these topics. Below  is the recent work in collaboration with the atomic,  molecular, and optics science group of Argonne national laboratory on x-ray absorption spectrum of XeF2. It is the first study to elucidate the pre-edge features of hard x-ray absorption spectra.

Model fits to the Xe K-edge of XeF2 using X-ray fluorescence yield. Red curve: 1s ionization continuum. Orange curve: 1s to 6pz Rydberg state. Blue curve: 1s to 6px and 6py Rydberg states. Green curve: 1s-> LUMO. Black curve: total fit. Open circles: measured yields.