With the objective of analyzing which kind of reference data is appropriate for benchmarking quantum chemical approaches for transition metal compounds, we present the following, (a) a collection of 60 transition metal diatomic molecules for which experimentally derived dissociation energies, equilibrium distances, and harmonic vibrational frequencies are known and (b) a composite computational approach based on coupled-cluster theory with basis set extrapolation, inclusion of core-valence correlation, and corrections for relativistic and multireference effects. The latter correction was obtained from internally contracted multireference coupled-cluster (icMRCC) theory. This composite approach has been used to obtain the dissociation energies and spectroscopic constants for the 60 molecules in our data set. In accordance with previous studies on a subset of molecules, we find that multireference corrections are rather small in many cases and CCSD(T) can provide accurate reference values, if the complete basis set limit is explored. In addition, the multireference correction improves the results in cases where CCSD(T) is not a good approximation. For a few cases, however, strong deviations from experiment persist, which cannot be explained by the remaining error in the computational approach. We suggest that these experimentally derived values require careful revision. This also shows that reliable reference values for benchmarking approximate computational methods are not always easily accessible via experiment and accurate computations may provide an alternative way to access them. In order to assess how the choice of reference data affects benchmark studies, we tested 10 DFT functionals for the molecules in the present data set against experimental and calculated reference values. Despite the differences between these two sets of reference values, we found that the ranking of the relative performance of the DFT functionals is nearly independent of the chosen reference.

The internally contracted multireference coupled cluster (icMRCC) theory is reexamined in a multistate framework. The new derivation starts from the Bloch equations employing a wave operator similar to the one originally employed by Jeziorski and Monkhorst [Phys. Rev. A 24, 1668 (1981)], but allows for a multideterminantal model-space basis. The resulting working equations are a generalization of state-universal multireference coupled-cluster (SU-MRCC) theory. In the case of truncated cluster operators, energies and wave functions depend on the choice of the model-space basis. It is shown that the state-specific solutions of the original icMRCC theory exactly diagonalize the effective Hamiltonian of the multistate theory and thus decouple the multistate equations. While this in principle means that icMRCC is a multistate theory, we find that truncated icMRCC theory still shows minor artifacts close to avoided crossings, which can be removed by considering a true multistate ansatz. The results also shed new light on the orbital variance of the original SU-MRCC theory.

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This work reports a state-of-the-art theoretical characterization of four new sulfur-bromine species and five transition states on the [H, S(2), Br] potential energy surface. Our highest level theoretical approach employed the method coupled cluster singles and doubles with perturbative contributions of connected triples, CCSD(T), along with the series of correlation-consistent basis sets and with extrapolation to the complete basis set (CBS) limit in the optimization of the geometrical parameters and to quantify the energetic quantities. The structural and vibrational frequencies here reported are unique and represent the most accurate investigation to date of these species. The global minimum corresponds to a skewed structure HSSBr with a disulfide bond; this is followed by a pyramidal-like structure, SSHBr, 18.85 kcal/mol above the minimum. Much higher in energy, we found another skewed structure, HSBrS (50.29 kcal/mol), with one S-Br dative-type bond, and another pyramidal-like one, HBrSS (109.80 kcal/mol), with two S-Br dative-type bonds. The interconversion of HSSBr into SSHBr can occur via a transfer of either the hydrogen or the bromine atom but involves a very high barrier of about 43 kcal/mol. These molecules are potentially a new route of coupling the sulfur and bromine chemistry in the atmosphere, and conditions of high concentration of H(2)S like in volcanic eruptions might contribute to their formation. We note that HSSBr can act as a reservoir molecule for the reaction between the radicals HSS and Br. Also, an assessment of the methods DFT/B3LYP/CBS and MP2/CBS relative to CCSD(T)/CBS provides insights on the expected performance of these methods on the characterization of polysulfides and also of more complex systems containing disulfide bridges.

The internally contracted multireference coupled-cluster (icMRCC) method is analyzed through third order in perturbation theory. Up to second order, the icMRCC perturbation expansion is equivalent to that of the standard Rayleigh− Schrodinger perturbation theory, which is based on a linear ansatz for the wave function, and the resulting theory is, depending on the employed zeroth-order Hamiltonian, equivalent to either second-order complete active space perturbation theory (CASPT2), N-electron valence perturbation theory (NEVPT2), or Fink's retention of the excitation degree perturbation theory (REPT2). At third order, the icMRCC perturbation expansion features additional terms in comparison to the Rayleigh− Schrodinger perturbation theory, but these are shown to be nearly negligibly small by both analytic arguments and numerical examples. Considering these systematic cancellations, however, may be important in future work on approximations to icMRCC theory. In addition, we provide an extensive set of tests of the second and third-order perturbation theories based on three different zeroth-order Hamiltonians, namely, the projected effective Fock operator as used for CASPT, the Dyall Hamiltonian as used for NEVPT, and the Fink Hamiltonian used for REPT. While the third-order variant of REPT often gives absolute energies that are rather close to values from higher level calculations, the results for relative energies and spectroscopic constants such as harmonic frequencies, give a less clear picture and a general conclusion about any best zeroth-order Hamiltonian does not emerge from our data. For small active spaces, REPT is rather prone to intruder state problems.