Double electron-attachment equation-of-motion coupled-cluster methods with up to 4-particle-2-hole excitations: improved implementation and application to singlet-triplet gaps in ortho-, meta-, and para-benzyne isomers

Efficient new codes enabling double electron-attachment equation-of-motion coupled-cluster (DEA-EOMCC) calculations with up to 4-particle-2-hole (4p-2h) excitations, treated with active orbitals, have been developed. They can be applied to medium size diradicals using larger basis sets and modest computational resources. The new codes have been used to determine the singlet-triplet (S-T) gaps characterising the ortho-, meta-, and para-benzyne isomers using the correlation consistent cc-pVxZ basis sets with x=D, T, and Q and complete-basis-set (CBS) extrapolations. Our best estimates of the S-T gaps in the ortho-, meta-, and para-benzynes obtained in the DEA-EOMCC calculations with an active-space treatment of 4p-2h terms, extrapolated to the CBS limit and corrected for vibrations, are 37.4, 20.7, and 4.5 kcal/mol, respectively, in excellent agreement with experiment.