Theoretical investigations of molecules composed only of fluorine, oxygen and nitrogen: determination of the equilibrium structures of FOOF, (NO)2 and FNNF and the transition state structure for FNNF cis-trans isomerization

Ab Initio Thermochemistry of the Hydrogenation of Hydrocarbon Radicals Using Silicon-, Germanium-, Tin-, and Lead-Substituted Methane and Isobutane

“…Whereas CCSD(T) is very reliable for well-behaved systems, it can suffer in the case of electronic near-degeneracies, as may arise in bond-breaking and bond-making situations. In an examination of reactions 1 and 2, we found that several of the transition states feature values of the T 1 diagnostic 61,62 close to the value of 0.02, above which multireference character often becomes important. Considering the alternative D 1 diagnostic [63][64][65] for these systems, we find several values for transition states that approach 0.05, whereas for open-shell systems such as those considered here, the quality of the coupled-cluster wavefunction is thought to degrade for values over 0.025.…”

Communication: Electric properties of the ThO(X Σ1+) molecule

“…They provided the equilibrium distance r = 1.845 Å, in close agreement with literature values ͑see below͒, and established the interval of internuclear distances safe for the single-reference treatment. The decision was based on the T 1 diagnostics, 12 which slowly increased with internuclear distance from r = 1.5 Å staying close to "critical" 0.02 mark up to 1.9 Å. Then more rapid growth brought it to 0.03 at 2.3 Å.…”

Theoretical study of the helio hydrogen cyanide dication HeCNH2+

“…Energies obtained by optimizing the geometries for the ccsd(t) model are labelled E ccsd(t) . The t1 diagnostic suggested by Lee et al [44,45] was used in the ccsd(t) calculations to determine whether a single reference wave function is adequate for each of the geometries examined. For the ccsd(t) calculations in which the geometry was optimized within that model, the t1 diagnostic took the values 0.0138 and 0.0352 for the L and N structures, repectively.…”