Ground-state wavefunctions energetics

Correction for Coulomb energy differences sets the Be and C ground states at an excitation such that either the 1.74 MeV or the 2.15 MeV level of B could be the analogous state. The positron decay of C is allowed for transitions to both the 0.72 and 1.74 MeV levels for Be decay is possible energetically only to the J = y ground state, and is second forbidden. This suggests that the 0.72 and 1.74 MeV states are probably the (/, T) = (1,0) and (0,1) components of the lowest multiplet, so that their wavefunction has the same spatial symmetry as that of C (and of Be ). The assignment of isotopic spin T = to the 1.74 MeV state is supported by the fact that this state is not observed in the dd ) reaction. 

At the wavefunction level methods based on coupled cluster (CC) theory are among the most reliable ones. For ground-state energetics the CCSD(T) approach is the gold standard of chemistry, whereas for excited states one can use the equation-of-motion (EOM) CC (EOM-CC) method or CC linear response theory (CC-LRT) [4] approaches. Note that the CC-LRT is size-extensive for both energies and properties such as intensities, however for EOM-CC this is true only for energies (unless one uses the closely related similarity transformed (ST)EOM-CC method [18]). As the computational cost of fully iterative (e.g., CCSD, CCSDT, etc.) methods can quickly become prohibitive, perturbative methods [4]... 

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