Understanding Electron Correlation in Excited States

Electronic structure methods are fundamentally categorized based on their ability to account for dynamic and nondynamic electron correlation effects [1]. Within the first category, we find single-reference (SR) methods, for which a single Slater determinant often serves as a satisfactory zero-order approximation of the system, while methods designed to allow strong coupling among more than one Slater determinant are broadly recognized as multireference (MR) methods. The accurate depiction of molecular systems is contingent upon the judicious selection of an electronic structure method. Specifically, systems that exhibit considerable nondynamic correlation may be grossly misrepresented if analyzed using SR methods. Conversely, many MR methods may fall short in incorporating dynamic correlation, making the discernment of the predominant type of electron correlation in a molecule an invaluable asset. The latter is particularly relevant for the study of excited states, where MR methods such as CASSCF which suffer from the lack of dynamic correlation, hampering the characterization of excited states and impacting in the accurate simulation of photophysical and photochemical processes.

For this internship, we are looking for a candidate with background in chemistry or physics (preferably with knowledge on quantum chemistry). The candidate will learn about the concepts of electron correlation and how to quantify it [1-4]. Together with the candidate we will study the problem of electron correlation in excited states, trying to put the focus in quantifying the amount of dynamic electron correlation. To this end, will use similar strategies to those already employed in our group [1-4]. These tools will be applied to develop new accurate methods for excited states.

[1] Ramos-Cordoba E., Salvador P., Matito E.; Separation of Dynamic and Nondynamic Correlation. Phys. Chem. Chem. Phys. 18, 24015 (2016) doi:10.1039/C6CP03072F
[2] Ramos-Cordoba E., Matito E.; Local Descriptors of Dynamic and Nondynamic Correlation. J. Chem. Theory Comput. 13, 2705 (2017) doi:10.1021/acs.jctc.7b00293
[3] Xu X., Soriano-Agueda L., Lopez X., Ramos-Cordoba E., Matito E.; An All-Purpose Measure of Electron Correlation for Multireference Diagnostics. J. Chem. Theory Comput. 20, 721 (2024) doi:10.1021/acs.jctc.3c01073
[4] Xu X., Soriano-Agueda L., López X., Ramos-Cordoba E., Matito E.; How Many Distinct and Reliable Multireference Diagnostics Are There? J. Chem. Phys. (accepted)