More Accurate Green’s Function Simulations

The Science

The electron dynamics at play in ultrafast spectroscopy are highly suited to theoretical study. Modeling the excited electron states and their dynamics generally involves Green’s function calculations at certain theoretical levels. Researchers tested several time-dependent wavefunction formulations, or ansatze, in an approach that combines time-dependent coupled cluster theory with Green’s function structure. They identified an ansatz that produced more accurate one-particle Green’s function calculations. This ansatz can be modified with an approximation approach that maintains most of the accuracy while decreasing the computational burden.

The Impact

Ultrafast spectroscopy can provide insight into the electron dynamics that control the movement of charge, conductivity, and ionization, among other possibilities. However, the experimental results often require insights from theory to develop a full picture of the chemical processes at play. The new ansatz developed in this work can help researchers accurately describe the dynamics of more complex electronic excited states in the ultraviolet and X-ray energy regimes that have strong electron correlation and multiple configurations. 

Summary

Ultrafast processes are an important study ground for time-dependent calculations, as the electronic dynamics are rapid enough to be uncoupled from the movement of nuclei. This allows researchers to exclusively focus on the time-dependent electronic dynamics representations that directly correspond to experimental set ups. Researchers used time-dependent coupled cluster theory to inform one-particle Green’s function calculations that represent excited electronic states. They tested different ansatze for the time-dependent coupled cluster calculations to find the formulation with the highest accuracy. They identified an ansatz that approaches the exact limit for the overall simulations by incorporating higher order electronic information into the calculations. The team also tested approaches to approximating the ansatz, an important process for its use in real calculations. They found a structure that maintained an acceptable amount of accuracy while decreasing the complexity of the calculation. Throughout the evaluation process, the team developed a method for analyzing the Green’s function components. This can aid other researchers in identifying observed features in experimental spectra.

Contact

Bo Peng, Pacific Northwest National Laboratory, peng398@pnnl.gov 

Funding

This material was based upon the work supported by the “Transferring exascale computational chemistry to cloud computing environment and emerging hardware technologies (TEC⁴)” project, which is funded by the Department of Energy (DOE), Office of Science, Basic Energy Sciences program, the Division of Chemical Sciences, Geosciences, and Biosciences (under Grant No. FWP 82037). B.P. also acknowledges the support from the Early Career Research Program by the DOE, Office of Science, under Grant No. FWP 83466. F.D.V. and J.J.R. acknowledge the support from the Center for Scalable Predictive methods for Excitations and Correlated phenomena (SPEC), which is funded by DOE Office of Science, Basic Energy Sciences program, Division of Chemical Sciences, Geosciences and Biosciences as part of the Computational Chemical Sciences program at Pacific Northwest National Laboratory under Grant No. FWP 70942. B.P. thanks Dr. Niri Govind for the fruitful discussion during the preparation of this manuscript.