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Quantum chemical calculations were performed to calculate the molecular properties of the 1 A g state of disulfur dinitride, S 2 N 2 , and the 1 A 1 state of the SN dimer, (SN) 2 , using density functional theory (DFT) and ab initio methods. The molecular structure of (SN) 2 is a trapezoid instead of a rectangle. Because the multireference character of (SN) 2 is considerable, most hybrid DFTs poorly describe its molecular properties. In contrast, old generalized gradient approximations give qualitatively correct descriptions of the molecular properties of (SN) 2 . Multi-state second-order multiconfigurational perturbation theory gives results that are close to those from multireference configuration interaction with the Davidson correction. The multireference character should be considered when calculating the molecular properties of poly sulfur nitride systems. [ABSTRACT FROM AUTHOR]

Highlights: [•] Li3N+CO2, a reaction recently suggested to capture carbon dioxide, is studied in the gas phase using ab initio methods. [•] With Li3N leaving its planar form while CO2 bends, a new species is formed with the sum formula Li3NCO2. [•] The practical value of the title reaction is tentatively assessed. [Copyright &y& Elsevier]

Abstract: Projective and expectation value energies, variance and full CI overlap calculations for single- and multi-reference coupled cluster and configuration interaction wavefunctions are carried out. The variance proves to give an accurate measure for the quality of the wavefunction and the error of the correlation energy with respect to full CI. Special attention to the consistency case (two non-interacting H2 and four non-interacting H2 systems) is drawn. Due to the lack of valence extensivity [M. Hanrath, Theor. Chem. Acc. 121 (2008) 187] the multi-reference exponential wavefunction approach [M. Hanrath, J. Chem. Phys. 123 (2005) 84102] lacks exact separability for delocalized orbitals. The single reference formalism based multi-reference coupled cluster approach [O. Oliphant, L. Adamowicz, J. Chem. Phys. 94 (1991) 1229; P. Piecuch, O. Oliphant, L. Adamowicz, J. Chem. Phys. 99 (1993) 1875] proves to be size consistent for localized and delocalized orbitals but does not treat all references on the same footing (lack of Fermi vacuum invariance) causing symmetry issues and discontinuities when calculating potential energy surfaces. Externally restricted exponential approaches are shown to remove extensivity and consistency properties of the coupled cluster approaches. [Copyright &y& Elsevier]

Abstract: The state-specific multi-reference coupled-cluster (SS-MRCC) theory of Mukherjee et al., in its singles and doubles truncation scheme (SS-MRCCSD), misses important couplings between the virtual functions reached by single and double excitations from different model functions. Since the SS-MRCC theory is not invariant with respect to the transformations among the active orbitals, the results are dependent on the active orbitals chosen. We demonstrate in this paper with results for potential energy curves for several example molecules involving single and multiple bond dissociation that the performance of SS-MRCCSD is significantly improved if localized active orbitals are used. The improvement is remarkable both in terms of the non-parallelity error and the magnitude of correlation energy recovered vis-a-vis the full configuration interaction results with the same basis set. The results bolster our claim that SS-MRCCSD with localized orbitals is an accurate general theory for potential energy surfaces. [Copyright &y& Elsevier]

Abstract: Multi-reference coupled-cluster calculations for the potential energy surface of the HF molecule including excited states are carried out. Using the multi-reference exponential wavefunction [M. Hanrath, J. Chem. Phys. 123 (2005) 84102] and the single-reference formalism based multi-reference coupled-cluster ansatz [J. Chem. Phys. 94 (1991) 1229, J. Chem. Phys. 99 (1993) 1875] the results are analyzed and discussed with respect to full CI results. The present work completes previous studies which considered calculations with partially restricted residual equations only. A certain attention is paid to spatial degeneracy issues. [Copyright &y& Elsevier]

Abstract: According to the recently presented generalized extensivity test [M. Nooijen, K.R. Shamasundar, D. Mukherjee, Mol. Phys. 103 (2005) 2277] the concepts of and relations between connectivity, separability, and consistency are explained and illustrated by partitioned diagrams. The concept of minimal rigorous partitionings is introduced and serves to limit the number of necessary partitionings. In order to check the exponential multi-reference wavefunction ansatz (MRexpT) [M. Hanrath, J. Chem. Phys. 123 (2005) 84102] for core extensivity the H–H – model system is introduced and numerical test calculations are carried out. MRexpT passes the test of core extensivity (= linear scaling with the number of electrons in inactive orbitals) completing the algebraic analysis [M. Hanrath, Theor. Chem. Acc. 121 (2008) 187]. The validity and approval of core extensivity is an important result as it allows MRexpT to give accurate results when applied to large molecular systems. [Copyright &y& Elsevier]

Abstract: In 1985 Kutzelnigg showed that a large percentage of the electron correlation energy for helium can be recovered using a single explicitly correlated basis function, chosen to fit the cusp at the correlation hole. In particular the simple wave function returned more than 80% of the correlation energy. In this paper we return to Kutzelnigg’s simple ansatz and remove the conventional double excitations in explicitly correlated CC2 theory (denoted as CCS(F12)), applying all established developments in modern F12 theory, such as replacing linear r 12 with f(r 12)=exp(−γr 12) and the use of auxiliary basis sets for the standard RI approximation in R12 theory. Analysing different approximations we show that in general the CCS(F12) approach yields 80–95% of the CC2 correlation energy, which is astonishingly large considering the small number and restricted form of the geminal basis functions. [Copyright &y& Elsevier]

Abstract: Within this work, we introduce local-correlation approximations into a hybrid approach which couples short-range density functional-based with long-range wave-function-based methods. The local treatment of long-range ab initio correlation is expected to greatly enhance the efficiency of the mixed approach for large systems. We start from our previous implementation, where a short-range gradient-corrected density functional of the Perdew–Burke–Ernzerhof (PBE) type is coupled with long-range Møller–Plesset second order perturbation theory and coupled-cluster methods, and extend the scheme to include the corresponding local long-range ab initio methods. The accuracy of the resulting approach is tested using the benchmark database for non-bonded (or, more precisely, weakly bonded) interactions of Zhao and Truhlar. The effect of the local-correlation approximation as well as the influence of counterpoise corrections is discussed for the different types of methods. It is shown that the mixed short-range/long-range approach is less sensitive to the ab initio correlation level and the one-particle basis-set used than pure ab initio methods, while the overall performance is significantly improved with respect to pure PBE. [Copyright &y& Elsevier]

Abstract: Bosons in optical lattices and rings are attractive and active fields of research in cold-atom physics. Here, we apply our recently developed coupled-cluster approach for bosons in external traps to these systems, and extend it to the lowest-in-energy excited states with total quasi- or angular-momentum k. In the coupled-cluster approach the exact many-boson ground state is obtained by applying an exponential operator exp{T}, to the ground configuration, which is (usually) the state where the bosons occupy a single orbital. For excited states, a second exponential operator exp{T (k)}, is employed to accommodate the remaining excitations from the unperturbed excited configuration. Due to the conservation of momentum, T 1 and can vanish. Working equations for coupled-cluster (singles) doubles (CCD) are provided and their implications are briefly discussed. [Copyright &y& Elsevier]

Abstract: Fock space (FS) multi-reference (MR) coupled-cluster (CC) has been established to be state-of-the-art method for energies of open-shell radicals, ionized, electron attached states and excited states. Due to the multi-root nature of the method, the linear response of MRCC, however, is non-trivial. Constrained variation approach (CVA) makes it plausible to calculate response of the MRCC method efficiently. However, the approach is used for only one root of the problem at a time. In the present article, we make first implementation of the approach and present test results of dipole moments of small radicals. [Copyright &y& Elsevier]

Abstract: Based upon the exact separation of the Dirac–Coulomb Hamiltonian into a spin-free and a spin-dependent part, we present high-level correlated calculations using the spin-free formulation. A newly-implemented large-scale configuration interaction program and the coupled-cluster method are applied to the gold dimer, the ground state of which is determined by electron correlation and scalar relativistic effects. We compare the results to results obtained with the approximate scalar relativistic Douglas–Kroll–Hess Hamiltonian obtained by truncated expansions in unitary transformations of the Dirac–Coulomb Hamiltonian. Furthermore, we determine the effect of spin–orbit coupling on the ground state properties in a coupled-cluster approach and briefly discuss the problem of separating scalar relativistic and spin–orbit effects in the present context. [Copyright &y& Elsevier]

The ground state potential energy surface for He–F2 has been generated using the coupled-cluster singles and doubles excitation approach with perturbative treatment of triple excitations [CCSD(T)] and multi-reference configuration interaction (MRCI) methodologies, with augmented correlation consistent quadruple zeta basis set and diffused functions. Both the CCSD(T) and MRCI surfaces are compared and the results analyzed. The CCSD(T) surface exhibits van der Waals minima at different distances for different orientations of He approaching F2 and is adequate to describe accurately only in the region around the equilibrium bond distance of F2. The MRCI surface, on the other hand, yields reliable results for a wider range of F–F bond distances leading to the correct asymptote. Davidson correction to the MRCI surface makes it purely repulsive over the regions investigated. [Copyright &y& Elsevier]

By employing density functional theory (DFT) and coupled cluster (CCSD(T)) calculations, the energy difference between the highest occupied molecular orbital (HOMO) energy of water molecule and the lowest unoccupied molecular orbital (LUMO) energy of metal cation is proven to serve as a good indicator for predicting the experimental acidity constant (pKa) of metal cations in aqueous solutions. Mono-, di- and tri-valent metal cations comprising main group and transition group elements are investigated. This frontier-molecular-orbital linear correlation is useful to rationalize deprotonation trends of aqueous metal ions without having to perform time-consuming calculations and is useful to clarify the interdependence of various aqueous reactions such as the hydration and deprotonation reactions of metal cations. [Copyright &y& Elsevier]

• The pseudo-Jahn-Teller distortion is an intrinsic property of triplet p -benzoquinone. • The lowest triplet state involves an inequivalent n -π* transition located at one C O. • The quasi-degeneracy of the two n -π* states is the origin of the distortion. Various experiments have suggested the symmetry distortion of para -benzoquinone (PBQ) at the lowest triplet (n -π*) state. The vibronic coupling of electronic states known as the pseudo-Jahn-Teller effect could be the origin of the symmetry distortion. However, previous quantum chemical calculations could not provide reliable evidence for the symmetry distortion. Here we show the pseudo-Jahn-Teller distortion is an intrinsic property of PBQ due to the vibronic coupling between two low-lying n -π* states. Consequently, PBQ takes a C 2v geometry having inequivalent two carbonyl groups, but the molecule remains planar. This state involves an inequivalent n -π* electronic transition and yields a mixed-valence state. [ABSTRACT FROM AUTHOR]

• Selected-CI is an efficient tool to construct a compact trial wavefunction in QMC. • The Molybdenum dimer is studied by means of the fixed-node QMC. • The fixed-node QMC results were compared with the CCSD(T) results. We have studied the molybdenum dimer ( Mo 2 ) system. The binding energy was calculated by means of the fixed-node DMC (FN-DMC) method. The Slater part of the trial wave function was constructed by the Selected-CI method by using the orbitals generated by the KS-DFT method with a hybrid meta-GGA exchange and correlation functional, TPSSh. We also carried out CCSD(T) calculations which were subsequently extrapolated to the complete basis set (CBS) limit. The results are presented. [ABSTRACT FROM AUTHOR]