Recent advances in real-time equation-of-motion coupled cluster Green's function approach: triples and off-diagonal components

The real-time equation-of-motion coupled cluster (RT-EOM-CC) approach [1] has proven to be highly accurate for core and valence ionization energies. We have demonstrated that, for small molecules, the singles and doubles approximation (RT-EOM-CCSD) yields ionization energies that agree well (±0.2 eV) with both experiment and CI-based methods. Despite this agreement for quasiparticle (QP) properties, the RT-EOM-CCSD still overestimates the satellite peak positions in the spectral function by as much as 2 eV. In this talk, I discuss recent developments to improve the satellite description and expand the applicability of the method. For this purpose, we have implemented the full triples approximation (RT-EOM-CCSDT) and added the off-diagonal elements of the RT-EOM-CCSD Green's function. For core-excited H₂O, the inclusion of the triples reduces the overestimate of the QP-satellite gap by 50% while improving the overall shape of the satellite features. Comparison to full CI results obtained in a reduced active space show nearly full convergence for the position of the peaks. I also show that differences in the intensity of the QP in RT-EOM-CCSDT vs full CI arise from the use an uncorrelated CC reference. Finally, I present initial results for the full Green's function matrix.



[1] "Real-Time Equation-of-Motion CCSD Cumulant Green's Function", FD Vila et al., J. Chem. Theory Comput. (doi:10.1021/acs.jctc.1c01179).