Singlet-triplet splitting, computation

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. 

Table 5 The singlet-triplet splitting of TMM computed with MR methods (in kcal/mol)...

Fig. 6 Singlet-triplet splitting for the M-acenes computed at MCSCF and MR-AQCC levels for three different cases ti-6-31G, ti-6-31G, and total-6-31G ...

The computed singlet-triplet splitting of 11.1 kcal mol in CI2 is higher by 2.5 kcal moP than what had been calculated before [23-25]. This is partly due to the more thorough treatment of basis set enlargement and CV cor-... 

The BS formalism was developed by Noodleman and Norman [5h,i] to compute singlet-triplet splitting using single determinants with either HF or DFT calculations. Let us consider the simplest case of two electrons in two non-degenerate molecular orbitals, (t>i and ( )2, having different symmetries. 

Finally, for the calculation of the singlet-triplet splitting equation (4) and (14) are equivalent. The last equation is a more convenient way of computing the overlap integral between magnetic orbitals and can be easily extended to cases in which two spin states, not necessarily singlet and triplet, arise from the exchange interaction. 

As evidenced by the very good agreement between the computed and measured values of AfsT in TMM and COT, ab initio calculations are currently capable of computing singlet-triplet splittings with high accuracy. The very recent development by Squires and co-workers of techniques for generating a wide variety of radical anions in the gas phase, will undoubtedly lead to accurate measurement by photoelectron of... 

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