Extending the real-time equation-of-motion coupled cluster Green's function approach: CCSDT-n approximations for core and inner-valence ionization

The real-time equation-of-motion coupled cluster (RT-EOM-CC) approach [1] has demonstrated high accuracy for core and valence ionization energies. We previously showed that inclusion of full triples (RT-EOM-CCSDT) [2] improves the description of satellite features compared to the RT-EOM-CCSD level, particularly for inner-valence ionized states of H₂O. However, the steep computational cost of the full triples expansion limits its applicability. To address this, we explore reduced-scaling variants of the triples treatment, namely the RT-EOM-CCSDT-1 and RT-EOM-CCSDT-3 approximations. Preliminary results for the water monomer indicate that both RT-EOM-CCSDT-n methods (CCSDT-1 and CCSDT-3) produce nearly identical spectra to one another, with quasiparticle energies close to the converged values and satellite positions that fall between the RT-EOM-CCSD and full RT-EOM-CCSDT results. These findings suggest that RT-EOM-CCSDT-n methods can offer a balanced compromise between accuracy and computational cost. Additional results for core and inner-valence ionizations will also be presented.

 

[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).

[2] “Efficient Variable-Time Implementation of the RT-EOM-CCSDT Approach for Core and Valence Ionization Spectral Functions”, FD Vila et al., J. Chem. Theory Comput. (doi:10.1021/acs.jctc.5c00430)