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Table 1 A comparison of the CC energies obtained for the LiF model in the cc-pVTZ basis set (see text for more details) with C1 and C2 energies obtained in the active space defined by the 13 lowest-lying RHF orbitals

From: Coupled Cluster Downfolding Theory: towards universal many-body algorithms for dimensionality reduction of composite quantum systems in chemistry and materials science

Method 1.0R e 2.0R e 5.0R e
RHF -106.980121 -106.850430 -106.728681
CCSD -107.283398 -107.153375 -107.022451
CCSDT -107.291248 -107.161817 -107.028098
CCSDT(2) Q -107.291453 -107.162103 -107.028288
CCSDTQ -106.980480 -106.850899 -106.728(8)
in act. space    
C1 -107.276752 -107.147287 -107.019105
C2 -107.281461 -107.142401 -107.032819