Wave Function Based Ab Initio Research Articles

Potential energy surfaces and dynamic properties via ab initio composite and density functional approaches.

Vibrational spectroscopy enables critical insight into the structural and dynamic properties of molecules. Presently, the majority of theoretical approaches to spectroscopy employ wavefunction-based ab initio or density functional methods that rely on the harmonic approximation. This approximation breaks down for large molecules with strongly anharmonic bonds or for molecules with large internuclear separations. An alternative to these methods involves generating molecular anharmonic potential energy surfaces (potentials) and using them to extrapolate the vibrational frequencies. This study examines the efficacy of density functional theory (DFT) and the correlation consistent Composite Approach (ccCA) in generating anharmonic frequencies from potentials of small main group molecules. Vibrational self-consistent field Theory (VSCF) and post-VSCF methods were used to calculate the fundamental frequencies of these molecules from their potentials. Functional choice, basis set selection, and mode-coupling are also examined as factors in influencing accuracy. The absolute deviations for the calculated frequencies using potentials at the ccCA level of theory were lower than the potentials at the DFT level. With DFT resulting in bending modes that are better described than those of ccCA, a multilevel DFT:ccCA approach where DFT potentials are used for single vibrational mode potentials and ccCA is used for vibrational mode-mode couplings can be utilized for larger polyatomic systems. The frequencies obtained with this multilevel approach using VCIPSI-PT2 were closer to experimental frequencies than the scaled harmonic frequencies, indicating the success of utilizing post-VSCF methods to generate more accurate representations of computed infrared spectra.

Revisiting the Electronic Structure of FeS Monomers Using ab Initio Ligand Field Theory and the Angular Overlap Model.

Iron-sulfur (FeS) proteins are universally found in nature with actives sites ranging in complexity from simple monomers to multinuclear sites from two up to eight iron atoms. These sites include mononuclear (rubredoxins), dinuclear (ferredoxins and Rieske proteins), trinuclear (e.g., hydrogenases), and tetranuclear (various ferredoxins and high-potential iron-sulfur proteins). The electronic structure of the higher-nuclearity clusters is inherently extremely complex. Hence, it is reasonable to take a bottom-up approach in which clusters of increasing nuclearity are analyzed in terms of the properties of their lower nuclearity constituents. In the present study, the first step is taken by an in-depth analysis of mononuclear FeS systems. Two different FeS molecules with phenylthiolate and methylthiolate as ligands are studied in their oxidized and reduced forms using modern wave function-based ab initio methods. The ab initio electronic spectra and wave function are presented and analyzed in detail. The very intricate electronic structure-geometry relationship in these systems is analyzed using ab initio ligand field theory (AILFT) in conjunction with the angular overlap model (AOM) parametrization scheme. The simple AOM model is used to explain the effect of geometric variations on the electronic structure. Through a comparison of the ab initio computed UV-vis absorption spectra and the available experimental spectra, the low-energy part of the many-particle spectrum is carefully analyzed. We show ab initio calculated magnetic circular dichroism spectra and present a comparison with the experimental spectrum. Finally, AILFT parameters and the ab initio spectra are compared with those obtained experimentally to understand the effect of the increased covalency of the thiolate ligands on the electronic structure of FeS monomers.

Assigning the Cerium Oxidation State for CH2CeF2 and OCeF2 Based on Multireference Wave Function Analysis.

The geometric and electronic structure of the recently experimentally studied molecules ZCeF2 (Z = CH2, O) was investigated by density functional theory (DFT) and wave function-based ab initio methods. Special attention was paid to the Ce-Z metal-ligand bonding, especially to the nature of the interaction between the Ce 4f and the Z 2p orbitals and the possible multiconfigurational character arising from it, as well as to the assignment of an oxidation state of Ce reflecting the electronic structure. Complete active space self-consistent field (CASSCF) calculations were performed, followed by orbital rotations in the active orbital space. The methylene compound CH2CeF2 has an open-shell singlet ground state, which is characterized by a two-configurational wave function in the basis of the strongly mixed natural CASSCF orbitals. The system can also be described in a very compact way by the dominant Ce 4f(1) C 2p(1) configuration, if nearly pure Ce 4f and C 2p orbitals are used. In the basis of these localized orbitals, the molecule is almost monoconfigurational and should be best described as a Ce(III) system. The singlet ground state of the oxygen OCeF2 complex is of closed-shell character when a monoconfigurational wave function with very strongly mixed Ce 4f and O 2p CASSCF natural orbitals is used for the description. The transformation to orbitals localized on the cerium and oxygen atoms leads to a multiconfigurational wave function and reveals characteristics of a mixed valent Ce(IV)/Ce(III) compound. Additionally, the interactions of the localized active orbitals were analyzed by evaluating the expectation values of the charge fluctuation operator and the local spin operator. The Ce 4f and C 2p orbital interaction of the CH2CeF2 compound is weakly covalent and resembles the interaction of the H 1s orbitals in a stretched hydrogen dimer. In contrast, the interaction of the localized active orbitals for OCeF2 shows ionic character. Calculated vibrational Ce-C and Ce-O stretching frequencies at the DFT, CASSCF, second-order Rayleigh-Schrödinger perturbation theory (RS2C), multireference configuration interaction (MRCI), as well as single, doubles, and perturbative triples coupled cluster (CCSD(T)) level are reported and compared to experimental infrared absorption data in a Ne and Ar matrix.

Control in the Rate-Determining Step Provides a Promising Strategy To Develop New Catalysts for CO2 Hydrogenation: A Local Pair Natural Orbital Coupled Cluster Theory Study.

The development of efficient catalysts with base metals for CO2 hydrogenation has always been a major thrust of interest. A series of experimental and theoretical work has revealed that the catalytic cycle typically involves two key steps, namely, base-promoted heterolytic H2 splitting and hydride transfer to CO2, either of which can be the rate-determining step (RDS) of the entire reaction. To explore the determining factor for the nature of RDS, we present herein a comparative mechanistic investigation on CO2 hydrogenation mediated by [M(H)(η(2)-H2)(PP3(Ph))](n+) (M = Fe(II), Ru(II), and Co(III); PP3(Ph) = tris(2-(diphenylphosphino)phenyl)phosphine) type complexes. In order to construct reliable free energy profiles, we used highly correlated wave function based ab initio methods of the coupled cluster type alongside the standard density functional theory. Our calculations demonstrate that the hydricity of the metal-hydride intermediate generated by H2 splitting dictates the nature of the RDS for the Fe(II) and Co(III) systems, while the RDS for the Ru(II) catalyst appears to be ambiguous. CO2 hydrogenation catalyzed by the Fe(II) complex that possesses moderate hydricity traverses an H2-splitting RDS, whereas the RDS for the high-hydricity Co(III) species is found to be the hydride transfer. Thus, our findings suggest that hydricity can be used as a practical guide in future catalyst design. Enhancing the electron-accepting ability of low-hydricity catalysts is likely to improve their catalytic performance, while increasing the electron-donating ability of high-hydricity complexes may speed up CO2 conversion. Moreover, we also established the active roles of base NEt3 in directing the heterolytic H2 splitting and assisting product release through the formation of an acid-base complex.

Role of size and shape selectivity in interaction between gold nanoclusters and imidazole: a theoretical study.

We present a theoretical study on the structure, stability, spectra and electronic properties of imidazole (Im) adsorbed on gold nanoclusters (Aun, n = 2, 4, 6, 8, 10, and 20). These computations were performed using various density functional theories with and without inclusion of Grimme's (D3) dispersion correction. For small clusters, we also carried out wavefunction-based ab initio (MP2 and SCS-MP2) computations for comparison. Vibrational, atoms in molecules (AIM) and natural bond orbital (NBO) analyses clearly reveal the occurrence of charge transfer (CT) through covalent (N1-Au) and noncovalent interactions that play important roles in the stability of the Im@Aun complexes with anchor assisted H-bonds (Cα-H · Au). Therefore, gold clusters can act as H-bond acceptors with biomolecules for development of new materials and applications. Our study establishes also the ability and reliability of PBE0 and M05-2X functionals compared to B3LYP and PBE for an accurate description of covalent and noncovalent interactions between Im and gold clusters since they lead to close agreement with MP2. Finally, we show that the Au8 cluster may be viewed as large enough to mimic the 3D gold surface.

4f and 5d Levels of Ce3+ in D2 8-fold oxygen coordination

The effects of the first coordination shell geometry of the trivalent Cerium ion (Ce3+) on its 4f and 5d levels in Ce-doped oxides with a D2 8-fold site, like garnets, are studied with embedded cluster, wave function based ab initio methods. The only deformations of a D2 CeO8 moiety that are found to shift the lowest 4f→5d transition to the red (longer wavelengths) are the symmetric Ce–O bond compression and the tetragonal symmetric bond bending. In a first approximation, the lowest 5d level of Ce3+ in garnets can be understood as resulting from the cubic Eg level with a strong Eg×eg Jahn–Teller coupling. These results are analyzed in terms of 5d−4f centroid energy differences and ligand field stabilizations. The splittings of the upper 5d levels and of the 4f levels are also discussed.

Approaching the bulk limit with finite cluster calculations using local increments: The case of LiH

Finite-cluster calculations employing high-level wavefunction-based ab initio methods and extended atomic-orbital basis sets are used to determine local energy increments for bulk LiH. It is shown that these increments can be converged with respect to cluster size and point-charge embedding so as to yield bulk cohesive energies with an accuracy of better than 1 mE(h), both at the Hartree-Fock and at correlated levels. Instrumental for the efficiency of the scheme is the introduction of non-orthogonal orbitals, at an intermediate stage.

Correlated wavefunction methods in bioinorganic chemistry

In this commentary the challenges faced in the application of wavefunction-based ab initio methods to (open-shell) transition metal complexes of (bio)inorganic interest are briefly touched on. Both single-reference and multireference methods are covered. It is stressed that the generation and nature of the reference wavefunction is a subject of major importance. How erroneous results can be easily obtained even with coupled-cluster theory is illustrated through the example of the septet-quintet separation in iron(IV)-oxo complexes. Second, the interplay between relativistic and correlation effects is important. This is demonstrated with coupled-cluster calculations on models for dinuclear copper active sites, where relativity has a major influence on the relative stabilities of the bis(μ-oxo) and side-on peroxo species.

Coupling of Short-range Density-functional with Long-range Post-Hartree-Fock Methods

Abstract A review is given on recent work with range-separated hybrid density-functional approaches, using accurate wavefunction-based ab initio methods for the treatment of the long-range inter-electronic interaction. It is shown that in many cases results can be improved with respect to the borderline methods, since the hybrid approach is less basis-set dependent than in the pure ab initio case, and deficiencies of pure DFT for weak interactions are efficiently removed. However, there is still need for improvement of currently available short-range density functionals.

A Multiconfigurational ab Initio Study of the Zero-Field Splitting in the Di- and Trivalent Hexaquo−Chromium Complexes

A detailed analysis of the value of zero-field splitting for the di- and trivalent chromium hexaquo complexes is presented. The effect of the Jahn-Teller distortion was studied, for the case of the divalent complex, through the use of state-averaged CASSCF calculations, for the mapping of the potential energy surface along the e(g) normal modes. At the minima of the surface, multiconfigurational ab initio calculations (spectroscopy oriented configuration interaction, SORCI, and difference dedicated configuration interaction, DDCI) were used for the calculation of the D tensor and the analysis of the individual contributions to it. The final value calculated with the SORCI method (D = -2.45 cm(-1)) for the divalent complex is in excellent agreement with the experimental estimate (D = -2.3 cm(-1)). The importance of inclusion of the direct spin-spin coupling contribution to D is pointed out ( approximately 16%). At the same time, contributions of the higher than the lowest (3)T(1g) triplets were found to be non-negligible as well ( approximately 11%). The accuracy of second-order perturbation theory for the calculation of SOC was investigated and found to be satisfactory. For comparison, DFT calculations were performed with hybrid (B3LYP) and nonhybrid (BP86) functionals and were found to be inferior to the wave function based ab initio methods.

Adsorption of single Cu atoms at differently stabilized polar ZnO surfaces: An ab initio study

In this study, the adsorption of a single Cu atom on the polar ZnO(0001) and ZnO(0001) surfaces was investigated by means of highly accurate wave function based ab initio methods. Different modifications of the polar surfaces were investigated: unstabilized surfaces, surfaces with hydrogen and OH overlayers, and defectious surfaces. Cu is strongly bound at the unstabilized surfaces by a charge transfer mechanism which induces a stabilization of the surface. At regular adsorption sites of stabilized surfaces the binding energies are much smaller. At oxygen defects, the Cu electron is transferred to the defect and a Cu(+) ion sits on top of the defect. At the Zn terminated surface the Cu moves into the Zn defects.

A comparative electron correlation treatment in H 2S–benzene dimer with DFT and wavefunction-based ab initio methods

Three major conformations of H 2S–benzene dimer have been located with a variety of density functional theories (DFT) and second order Møller–Plesset perturbation (MP2). In line with an experiment, MP2 results indicate that the tilted C s symmetry structure is a stable dimer, yet a C 2v symmetry structure is only a second-order saddle point. Although all of the examined DFT methods also predict the binding between H 2S and benzene as MP2 and the coupled cluster method with single and double excitations and perturbative triples (CCSD(T)) do, they considerably underestimate binding energies as compared with CCSD(T) results. However, PW91LYP and MPWB1K reproduce the binding sequence obtained with MP2 for the dimers and provide the best binding energies among the tested DFT methods. The method of increments with the orbitals of H 2S and π orbitals of the benzene recovers 99% of the total binding from the full CCSD(T).

Transition metal spin state energetics and noninnocent systems: challenges for DFT in the bioinorganic arena

Although density functional theory (DFT) provides a generally good description of transition metal systems, we have identified several cases, involving Fe(III) porphyrins and related systems, where common functionals fail to correctly describe the energetics of the different low-lying spin states. The question of metal- versus ligand-centered oxidation in high-valent transition metal complexes is also a challenging one for DFT calculations, as I have tried to illustrate with examples from among porphyrin, corrole, biliverdine, and NO complexes. In a number of cases, I have compared results obtained with different exchange-correlation functionals; in addition, I have added a discussion on the relative performance of pure versus hybrid functionals. Finally, I have offered some thoughts on the role that traditional wavefunction-based ab initio methods, now essentially absent from the bioinorganic arena, might play in the future.

Density Functional Theory and Its Application to Nuclear Magnetic Resonance Shielding Constants

AbstractFor Abstract see ChemInform Abstract in Full Text.

From ab Initio Calculations to Model Hamiltonians: The Effective Hamiltonian Technique as an Efficient Tool to Describe Mixed-Valence Molecules

The excitation spectrum of the Creutz-Taube molecule in slightly modified form, that is, [HCN (NH 3 ) 4 Ru pz Ru (NH 3 ) 4 NCH] 5 + , where pz = pyrazine, is calculated with wave function-based ab initio methods. An effective matrix is built using Bloch's procedure in a model space generated by the d orbitals of the ruthenium atoms and the lowest π* orbital of the bridging ligand. A model Hamiltonian is deduced. The effect of the mixing of the orbitals is discussed. A large part of the spectrum (33 states) can be well described in terms of two parameters: t, the transfer integral between a d orbital of the ruthenium and the π* orbital, and U, the excitation energy from the d orbital to the π* orbital. These parameters have been estimated at different levels of correlation: the dynamical correlation mostly affects U, which is negative without correlation and becomes almost zero with correlation. This article shows that a two-band Hubbard Hamiltonian is an excellent model for describing bridged mixed-valence molecules and proposes a procedure that allows the determination of the parameters of this model Hamiltonian by wave function-based ab initio calculations.

Ab initio theoretical studies on U3+ and on the structure and spectroscopy of U3+ substitutional defects in Cs2NaYCl6, 5f 26d1 manifold

In this paper we present the results of spin–orbit relativistic ab initio model potential embedded cluster calculations of the 5f26d1 excited manifold of (UCl6)3− embedded in a reliable representation of the Cs2NaYCl6 elpasolite host. They are aimed at interpreting the 5f3→5f26d1 absorption bands reported by Karbowiak et al. [J. Chem. Phys. 108, 10181 (1998).] An excellent agreement is found between the calculated energies of the absorption transitions from the ground state 5f3 1 Γ8u(4I9/2) and the experimental data, which supports a detailed interpretation of the electronic nature of the absorption spectrum in the energy region 14 000–23 000 cm−1. In particular, the three unidentified electronic origins that had been experimentally detected are now assigned, and the observed bands are interpreted as having multiple electronic origins. From the structural point of view, the excited states of the 5f26d1 manifold are classified in two sets of main configuration 5f26d(t2g)1 and 5f26d(eg)1 with bond distances Re [5f26d(t2g)1]<Re[5f3]<Re[5f26d(eg)1]. The energies of the 5f26d1 manifold of free U3+ have also been calculated; experimental data on them are not available in the literature to the best of our knowledge. These results contribute to show that wave function based ab initio methods can provide useful structural and spectroscopic information, complementary to the experimental data, in studies on actinide ion impurities doping ionic hosts, where large manifolds of 5dn−16d1 excited states are involved.

Large Basis Set Calculations on Model Zerovalent Palladium Systems

Wave function based ab initio and nonlocal density functional calculations employing large basis sets have been performed on the model systems PdCO and PdPH3 in order to investigate the influence of the basis set and electron correlation on properties of the Pd-C and Pd-P metal-ligand bonds. Three relativistic effective core potentials (RECPs) on palladium were combined with various valence basis sets and ligand basis sets. Electron correlation was included using Moller-Plesset methods MPn (n = 2-4), configuration interaction including single and double excitations (CISD), quadratic configuration interaction (QCISD(T)), and coupled-cluster methods (CCSD and CCSD(T)). Full geometry optimizations were performed at all of these levels, and the results were compared to local and nonlocal density functional calculations using the same basis sets. Geometries of reasonable accuracy were obtained using a small-core pseudopotential with double ζ basis sets and polarization functions on the ligands with either the MP2 level of theory or nonlocal density functional theory. The reaction energy for the process Pd-PH3 + CO → PH3 + Pd-CO was also investigated and found to be less consistent than the geometries; levels of theory lower than CCSD(T) or QCISD(T) gave results which deviated significantly (>7 kJ/mol) from the values calculated at these levels. Convergence of the reaction energy with increasing basis set size required large basis sets with diffuse functions and polarization functions on the metal and ligands. © 1995 American Chemical Society.