Relativistic Fock-space multireference coupled cluster theory with full iterative triples for the one-, two-, and three-hole sectors

The relativistic Fock-space coupled cluster method with full iterative inclusion of connected triple excitations (FS-CCSDT) for the 1h0p, 2h0p, and 3h0p Fock space sectors was presented and implemented. The newly developed methods were benchmarked in a series of calculations of both atomic (Ar1/2/3+, I0/1/2+) and molecular (Ar2+, HI+, HI2+) systems for which high-quality experimental data are available. Typical uncertainties in ionization potential and adiabatic excitation energy calculations for FS-CCSD and FS-CCSDT in the low sectors (1h0p and 2h0p) are ∼0.05–0.10 and 0.005–0.02 eV, respectively. The accuracy of the FS-CCSD model is quite similar to that of the relativistic third-order algebraic diagrammatic construction and the closely related equation-of-motion IP-EOM-CCSD and DIP-EOM-CCSD methods. The newly developed relativistic FS-CCSDT model is ∼3 to 5 times more accurate in these sectors. Models formulated for the 3h0p sector provide an acceptable accuracy in calculations of excitation energies of the Ar3+ and I2+ ions with average errors not exceeding 0.13 and 0.06 eV for FS-CCSD and FS-CCSDT, respectively.