DIP-EOM-CCSD method

Singlet-triplet separations of di-radicals treated by the DEA/DIP-EOM-CCSD methods... 

The relative IPs are also found to agree very well between the DIP-STEOM-CCSD md DIP-EOM-CCSD methods. The largest deviation is found for the two high-lying states of E symmetry. The lower of the two E states shows appreciable dependence on the basis set and differs by 0.4 eV between the DIP-SITEOM and DIP-EOM results. It is likely that the close proximity of two states of the same symmetry causes this higher sensitivity. Nevertheless, for nearly all of the states, the agreement is excellent. 

The NO3 vertical ionization spectrum was calculated by the DIP-STEOM-CCSD and DIP-EOM-CCSD methods. These methods avoid artifactual symmetry breaking of the reference wavefunction by starting from the symmetry-correct nitrate anion orbitals and provide a balanced treatment of dynamical and non-dynamical correlation effects. In general, the DIP-STEOM and DIP-EOM results agree well with the experiment of Wang (5) and with previous theoretical assignments (5,7). However, in our calculations, the vertical transition to the E" state nearly coincides with that of the E" state, in contrast to previous studies. Our calculations therefore do not support the assignment of the broad experimental feature near 14.05 eV to the E" state. 

The symmetry-correct anion orbitals can also be utilized in calculations of states of the N03 cation. The primary purpose of this work is to examine the NO3 ionization spectrum and the ground and low excited states of the N03 cation system by the DIP-STEOM-CCSD method (40) (double ionization potential similarity transformed equation-of-motion coupled-cluster singles and doubles). The DIP-STEOM-CCSD method is built upon the IP-EOM-CCSD method (32) (ionization potential equation-of-motion coupled-cluster singles and doubles), which in turn, has been shown to be equivalent (41,42) to singly ionized FSMRCC, such as the example of Kaldor above. The DIP-STEOM-CCSD method generates ground and excited states of the cation by deletion of... 

In contrast to the E" 1L2S minimum in DIP-STEOM-CCSD, a recent DFT study (14) reported a significantly distorted 1S2L structure for the E" state (Ri=1.152 A R2=R3=1.315 A Ai2=Ai3=132°). It is unclear if the stationary point was verified to be a true minimum by vibrational frequency calculations. As an additional confirmation of the DIP-STEOM-CCSD results, we also performed DIP-EOM-CCSD geometry optimizations and vibrational frequency calculations on the E" state. The DIP-EOM method finds C2v minimum and transition state structures nearly identical to the DIP-STEOM results in Table VII. 

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