Single-reference methods assessment

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. 

Both within a single laboratory or in an interlaboratory study, the approach to assessing bias and recovery is similar. The method is assessed using either reference materials with known quantity values, or the analysis of a test material is compared with the results of analysis of the same material... 

All of the previously mentioned exposure methods can be used to estimate either chronic exposure (over a period of years) or acute exposure (single day) for the United States population and population subgroups. Both chronic and acute assessments are usually based on a no observed adverse effect level (NOAEL) in an animal species. Acute exposure is defined relative to an acute (single dose) toxicological endpoint (usually a NOAEL) and may be expressed as a margin of exposure (MOE) or as a percentage of an acute reference dose that is based on a NOAEL and an uncertainty factor (see below). 

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