Double ionization potentials

Abstract The singlet-triplet splittings of the di-radicals methylene, trimethylene-methane, ortha-, meta- and para-benzynes, and cyclobutane-l,2,3,4-tetrone have become test systems for the applications of various multi-reference (MR) coupled-cluster methods. We report results close to the basis set limit computed with double ionization potential (DIP) and double electron attachment (DBA) equation-of-motion coupled-cluster methods. These diradicals share the characteristics of a 2-hole 2-particle MR problem and are commonly used to assess the performance of MR methods, and yet require more careful study unto themselves as benchmarks. Here, using our CCSD(T)/6-311G(2d,2p) optimized geometries, we report DIP/DEA-CC results and single-reference (SR) CCSD, CCSD(T), ACCSD(T) and CCSDT results for comparison. 

ADC = algebraic diagrammatic con.struction DIP = double ionization potential GF = Green s function p-p = particle-particle rhs = right-hand side RPA = random pha.se approximation. 

The structure of the p-p propagator is very similar to that of G. This popagator describes the simultaneous annihilation of two electrons from the system as well as the simultaneous attachment of two electrons to the system. In particular, the pole positions of the first and second part of FI represent double ionization potentials (DIP) and double attachment energies. DIPs, EAs, and IPs are, of course, important quantities of electronic systems. But not only the pole positions are related to experiment. The residue amplitudes and contain relevant information on the single and double ionization and attachment (or scattering) processes. For instance, the partial-channel photoionization cross section for production of molecular ions in the nth state reads in the sudden approximation ... 

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... 

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