Ab Initio Wave Function Theory Research Articles

Theoretical insights into the coordination structures, stabilities and electronic spectra of Cm3+ species at the gibbsite-water interface: A computational study combing ab initio molecular dynamics and wave function theory

The information of structure and stability of actinide species is key to understand the sorption mechanism of actinides at mineral–water interface. Such information is approximately derived from experimental spectroscopic measurements and needs to be accurately obtained by a direct atomic-scale modelling. Herein, systematic first-principles calculations and ab initio molecular dynamics (AIMD) simulations are carried out to study the coordination structures and absorption energies of Cm(III) surface complexes at gibbsite-water interface. Eleven representative complexing sites are investigated. The most stable Cm3+ sorption species are predicted to be a tridentate surface complex in weakly acidic/neutral solution condition and a bidentate one in the alkaline solution condition. Moreover, luminescence spectra of the Cm3+ aqua ion and the two surface complexes are predicted based on the high-accuracy ab initio wave function theory (WFT). The results give a gradually decreasing emission energy in good agreement with experimental observation of a red shift of peak maximum with pH increasing from 5 to 11. This work is a comprehensive computational study involving AIMD and ab initio WFT methods to gain the coordination structures, stabilities, and electronic spectra of actinide sorption species at the mineral–water interface, thus providing important theoretical support for geological disposal of actinide waste.

Ab Initio Methods in First‐Row Transition Metal Chemistry

AbstractMolecules containing 3d transition metals (TMs) are usually associated with versatile reactivity, partly due to their complicated electronic structures involving multiple close‐lying spin states. An accurate description of different electronic states is notoriously difficult and still a challenge for computational methodologies. Density functional theory, although repeatedly shown to give less reliable results for near‐degeneracy problems, remains the workhorse to explore the reactivity of TM complexes. Ab initio wavefunction theory has been lagging behind due to its high computational cost. However, tremendous efforts have been put to improve their applicability over the past few years, particularly local coupled cluster and low‐scaling multireference methods. In this mini‐review, we highlight major advancements of these ab initio techniques and discuss their recent applications in the calculations of spin state energetics of first‐row TM complexes, ranging from spin crossover compounds and metalloproteins to biomimetic complexes capable of activating C−H bonds.

Magnetic Properties and Electronic Structure of the S = 2 Complex [MnIII{(OPPh2)2N}3] Showing Field-Induced Slow Magnetization Relaxation

The high-spin S = 2 Mn(III) complex [Mn{(OPPh2)2N}3] (1Mn) exhibits field-induced slow relaxation of magnetization (Inorg. Chem. 2013, 52, 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zero-field-splitting (zfs) parameter D. In order to explore the magnetic and electronic properties of 1Mn in detail, a combination of experimental and computational studies is presented herein. Alternating-current magnetometry on magnetically diluted samples (1Mn/1Ga) of 1Mn in the diamagnetic gallium analogue, [Ga{(OPPh2)2N}3], indicates that the slow relaxation behavior of 1Mn is due to the intrinsic properties of the individual molecules of 1Mn. Investigation of the single-crystal magnetization of both 1Mn and 1Mn/1Ga by a micro-SQUID device reveals hysteresis loops below 1 K. Closed hysteresis loops at a zero direct-current magnetic field are observed and attributed to fast quantum tunneling of magnetization. High-frequency and -field EPR (HFEPR) spectroscopic studies reveal that, apart from the second-order zfs terms (D and E), fourth-order terms (B4m) are required in order to appropriately describe the magnetic properties of 1Mn. These studies provide accurate spin-Hamiltonian (sH) parameters of 1Mn, i.e., zfs parameters |D| = 3.917(5) cm-1, |E| = 0.018(4) cm-1, B04 = B42 = 0, and B44 = (3.6 ± 1.7) × 10-3 cm-1 and g = [1.994(5), 1.996(4), 1.985(4)], and confirm the negative sign of D. Parallel-mode X-band EPR studies on 1Mn/1Ga and CH2Cl2 solutions of 1Mn probe the electronic-nuclear hyperfine interactions in the solid state and solution. The electronic structure of 1Mn is investigated by quantum-chemical calculations by employing recently developed computational protocols that are grounded on ab initio wave function theory. From computational analysis, the contributions of spin-spin and spin-orbit coupling to the magnitude of D are obtained. The calculations provide also computed values of the fourth-order zfs terms B4m, as well as those of the g and hyperfine interaction tensor components. In all cases, a very good agreement between the computed and experimentally determined sH parameters is observed. The magnetization relaxation properties of 1Mn are rationalized on the basis of the composition of the ground-state wave functions in the absence or presence of an external magnetic field.

Am(C5Me4H)3]: An Organometallic Americium Complex

AbstractWe report the small‐scale synthesis, isolated yield, single‐crystal X‐ray structure, 1H NMR solution spectroscopy /solid‐state UV/Vis‐nIR spectroscopy, and density functional theory (DFT)/ab initio wave function theory calculations on an Am3+ organometallic complex, [Am(C5Me4H)3] (1). This constitutes the first quantitative data on Am−C bonding in a molecular species.

Am(C5 Me4 H)3 ]: An Organometallic Americium Complex.

We report the small-scale synthesis, isolated yield, single-crystal X-ray structure, 1 H NMR solution spectroscopy /solid-state UV/Vis-nIR spectroscopy, and density functional theory (DFT)/ab initio wave function theory calculations on an Am3+ organometallic complex, [Am(C5 Me4 H)3 ] (1). This constitutes the first quantitative data on Am-C bonding in a molecular species.

What is semiempirical molecular orbital theory approximating?

We elucidate the approaches used to incorporate electron correlation in existing semiempirical molecular orbital theory (SEMO) methods and compare them with the techniques used in other quantum chemical methods. After analyzing expressions for electron correlation in ab initio wavefunction theory, density functional theory, and density functional-based tight-binding (TB) methods, we suggest a framework for developing hybrid TB-SEMO methods. We provide a numerical proof-of-concept for such a method based on the OM2 method.

Coupled Cluster and Density Functional Studies of Atomic Fluorine Chemisorption on Coronene as Model Systems for Graphene Fluorination

Potential energy curves (PECs) of atomic fluorine adsorption on coronene as a model for graphene or nanocarbon surfaces have been computed. The PECs were obtained by scanning the fluorine atom distance to one of the center carbon atoms of the coronene molecule as model system in a “top” position from 4.0 to1.0 A in intervals of 0.1 A using a variety of quantum chemical methods. Various density functional theory (DFT) functionals, such as B3LYP, PBE, PBE0, CAM-B3LYP, and LC-ωPBE; approximate DFT methods such as several levels of the density-functional tight-binding (DFTB) method, as well as ab initio wave function theory methods such as MP2, CCSD, CCSD(T), and G2MS extrapolations, were used to evaluate energies for B3LYP/cc-pVDZ PEC geometries. G2MS is an approximation to the highly accurate CCSD(T)/cc-pVTZ level of theory in our work. We found that fluorine is chemically adsorbed on coronene with a binding energy of 22.9 and 23.3 kcal/mol at the B3LYP/cc-pVDZ and G2MS levels of theory, respectively, and 1...

Projected hybrid orbitals: a general QM/MM method.

A projected hybrid orbital (PHO) method was described to model the covalent boundary in a hybrid quantum mechanical and molecular mechanical (QM/MM) system. The PHO approach can be used in ab initio wave function theory and in density functional theory with any basis set without introducing system-dependent parameters. In this method, a secondary basis set on the boundary atom is introduced to formulate a set of hybrid atomic orbtials. The primary basis set on the boundary atom used for the QM subsystem is projected onto the secondary basis to yield a representation that provides a good approximation to the electron-withdrawing power of the primary basis set to balance electronic interactions between QM and MM subsystems. The PHO method has been tested on a range of molecules and properties. Comparison with results obtained from QM calculations on the entire system shows that the present PHO method is a robust and balanced QM/MM scheme that preserves the structural and electronic properties of the QM region.

Actinide–Silicon Multiradical Bonding: Infrared Spectra and Electronic Structures of the Si(μ-X)AnF3(An = Th, U; X = H, F) Molecules

We report a series of Si(μ-X)AnF3 (An = Th, U; X = H, F) complexes with silicon-actinide(IV) single bonds and unexpected multiradical features that form rare triplet silylenes. These bridged molecules have been prepared in microscopic scale through reactions of laser-ablated uranium and thorium atoms with silicon fluorides and identified from infrared spectra in argon and neon matrixes and relativistic quantum chemical calculations. Similar neon matrix experiments for the reactions of uranium with CF4 and CHF3 were carried out for comparison. Our density functional theory calculations show that the Si-U single-bonded species Si(μ-X)UF3 (X = H, F) with U(IV) oxidation state and the quasi-agostic bridge ligand of H or F are most stable among all the isomers, whereas the naively anticipated triple-bonded species XSi≡UF3 with U(VI) oxidation state and the double-bonded species XSi(•)═(•)UF3 with U(V) oxidation state lie markedly higher in energy. Similar thorium products from reactions with XSiF3 are also found to prefer the Si(μ-X)ThF3 structures with Si-Th single bonds and bridged H or F ligands. High level ab initio wave function theory calculations with the CCSD(T) and CASPT2 methods confirm that the ground states are quintet for Si(μ-X)UF3 and triplet for Si(μ-X)ThF3 with two unpaired electrons on the silylene group. These silicon-bearing molecules as the lowest-energy isomer of XSiAnF3 represent the first silicon-actinide systems with unusual "triplet" silylenes and Si-An single bonds with multiradical character. They are in dramatic contrast to the uranium-carbon analogs, XC≡UF3, which form triple-bonded singlet ground states with C3v symmetry. The calculated vibrational frequencies of the Si(μ-X)AnF3 complexes agree well with experimental observations. These results accentuate the critical difference of chemical bonding of 3p- and 2p-row main-group elements with actinides. The Lewis electron-pair model and the octet rule break down for these silicon compounds.

An extensible interface for QM/MM molecular dynamics simulations with AMBER.

We present an extensible interface between the AMBER molecular dynamics (MD) software package and electronic structure software packages for quantum mechanical (QM) and mixed QM and classical molecular mechanical (MM) MD simulations within both mechanical and electronic embedding schemes. With this interface, ab initio wave function theory and density functional theory methods, as available in the supported electronic structure software packages, become available for QM/MM MD simulations with AMBER. The interface has been written in a modular fashion that allows straight forward extensions to support additional QM software packages and can easily be ported to other MD software. Data exchange between the MD and QM software is implemented by means of files and system calls or the message passing interface standard. Based on extensive tests, default settings for the supported QM packages are provided such that energy is conserved for typical QM/MM MD simulations in the microcanonical ensemble. Results for the free energy of binding of calcium ions to aspartate in aqueous solution comparing semiempirical and density functional Hamiltonians are shown to demonstrate features of this interface.

Theoretical Studies on the Photoelectron and Absorption Spectra of MnO4–and TcO4–

The tetraoxo pertechnetate anion (TcO4(-)) is of great interest for nuclear waste management and radiopharmceuticals. To elucidate its electronic structure and to compare with that of its lighter congener MnO4(-), the photoelectron and electronic absorption spectra of MnO4(-) and TcO4(-) are investigated with density functional theory (DFT) and ab initio wave function theory (WFT). The vertical electron detachment energies (VDEs) of MnO4(-) obtained with the CR-EOM-CCSD(T) method are in good agreement with the lowest two experimental VDEs; the differences are less than 0.1 eV, representing a significant improvement over the IP-EOM-CCSD(T) result in the literature. Combining our CCSD(T) and CR-EOM-CCSD(T) results, the first five VDEs of TcO4(-) are estimated between 5 and 10 eV with an estimated accuracy of about ±0.2 eV. The vertical excitation energies are determined by using TD-DFT, CR-EOM-CCSD(T), and RAS-PT2 methods. The excitation energies and the assignments of the spectra are analyzed and partly improved. They are compared with reported SAC-CI results and available experimental data. Both dynamic and nondynamic electron correlations are important in the ground and excited states of MnO4(-) and TcO4(-). Nondynamical correlations are particularly relevant in TcO4(-) for reliable prediction of excitation energies. In TcO4(-) one Rydberg state interlaces but does not mix with the valence excited states, and it disappears in the condensed phase.

Accurate Prediction of Hydrocarbon Interactions with Zeolites Utilizing Improved Exchange-Correlation Functionals and QM/MM Methods: Benchmark Calculations of Adsorption Enthalpies and Application to Ethene Methylation by Methanol

The adsorption enthalpy of light hydrocarbon molecules in both acidic and neutral zeolite MFI has been investigated with a range of computational methods. The role of cluster model size and density functional theory methodology is examined by comparison with high quality ab initio wave function theory MP2 results and experimentally determined heats of adsorption. The commonly applied B3LYP functional performs poorly in benchmark studies due to an inadequate description of intermolecular interactions. The functionals ωB97X-D and M06-2X predict adsorption enthalpies consistent with experimental values over three classes of adsorbates. A hybrid quantum mechanics/molecular mechanics (QM/MM) method is required to converge calculated thermochemical properties with respect to cluster model size in a manner that is computationally efficient. The accuracy of both QM and QM/MM methods is highly sensitive to choice of level of theory and cluster size, requiring the use of large basis sets, large cluster models, and ...

Ab initio wavefunctions in bioinorganic chemistry: More than a succès d’estime?

Ab initio wavefunctions in bioinorganic chemistry: More than a succès d’estime?

Geometries, electronic structures, and excited states of UN2, NUO+, and UO 2 2+ : a combined CCSD(T), RAS/CASPT2 and TDDFT study

The ground- and excited-state geometries and electronic structures of the isoelectronic series of molecules UN2, NUO+, and UO22+ are investigated by using relativistic density functional theory (DFT) and ab initio wavefunction theory (WFT). Scalar relativistic and spin–orbit-coupled quantum chemical methods at the CASPT2, RASPT2, CCSD(T), DFT and TDDFT levels are applied. Relativistic effects as elucidated by Pekka Pyykko play an important role in these uranium compounds, in particular for the excited states. The three molecular species exhibit significantly different spectroscopic properties, concerning their excitation energies, bond lengths and vibrations. Density functional approaches yield qualitatively correct results for the ground states and the valence → U.7s,6d excited states. However, the performance of TDDFT for valence → U.5f type excitations (in particular of UN2 and NUO+) is less satisfactory, indicating the importance of the self-interaction correction for such excitations.

Ab initio DFT – the seamless connection between WFT and DFT

Orbital-dependent exchange-correlation functionals and potentials play an increasingly important role in Density Functional Theory (DFT). Methods which use explicit orbital-dependent functionals can be viewed as a natural extension to the standard Kohn–Sham (KS) procedure in DFT, that traditionally have used functionals with explicit density-dependence but only implicit orbital-dependence. Ab initio DFT, invented at the Quantum Theory Project, is the method which could define rigorous orbital-dependent exchange-correlation functionals and potentials in the context of KS DFT theory. The local and multiplicative exchange-correlation potentials are derived from a general theoretical framework based on the density condition in KS theory and from coupled-cluster theory and many-body perturbation theory. Ab initio DFT guarantees to converge to the right answer in the correlation and basis set limit, just as does ab initio Wave Function Theory (WFT) and solves in a rigorous way most of the shortcomings of standard density-dependent KS DFT. It is also the route toward understanding the relationships between traditional ab initio WFT and DFT. The Optimized Effective Potential ‘journey’ on the borderline of WFT and DFT was inspired and possible only because of the Quantum Theory Project where we stayed as postdocs in 1999–2001. It seems to us then we were in the right place and at the right time, and certainly with the right people. The QTP scientific melting pot and Sanibel's meetings gave us an excellent possibility to work together, learn and hopefully solve many important scientific problems.

Theoretical Investigations on the Formation and Dehydrogenation Reaction Pathways of H(NH2BH2)nH (n = 1−4) Oligomers: Importance of Dihydrogen Interactions

The H(NH(2)BH(2))(n)H oligomers are possible products from dehydrogenation of ammonia borane (NH(3)BH(3)) and ammonium borohydride (NH(4)BH(4)), which belong to a class of boron-nitrogen-hydrogen (BNH(x)) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNH(x)-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wave function theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH(2)BH(2))(n)H (n = 1-4) oligomers, polyaminoborane (PAB), from NH(3)BH(3) monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through computational transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH(3)BH(3) molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH(3)BH(3) molecules. We find that the formation of H(NH(2)BH(2))(n+1)H oligomers occurs first through reactions of the H(NH(2)BH(2))(n)H oligomers with BH(3) followed by reactions with NH(3) and the release of H(2), where the BH(3) and NH(3) intermediates are formed through dissociation of NH(3)BH(3). We also find that the dimerization of the NH(3)BH(3) molecules to form cyclic c-(NH(2)BH(2))(2) is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H(2) molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H(2)), with the largest exothermicity for n = 3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (H(N)) hydrogens interact with hydridic (H(B)) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the R-BH(2)(eta(2)-H(2)) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane.

Assessment of the Accuracy of Theoretical Methods for Calculating 27Al Nuclear Magnetic Resonance Shielding Tensors of Aquated Aluminum Species

Assessment of the accuracy of methods including 29 DFT methods and 2 ab initio wave function theory (WFT) methods for predicting (27)Al nuclear magnetic resonance shielding tensors of aquated Al(III) species was carried out. Among all of the tested methods, HF and MP2 methods give the best performance for the calculations of chemical shifts. Among all of the DFT methods with GIAO calculations, O3LYP and MPWKCIS1K are the most accurate models for calculations of chemical shifts, followed in order by BHandHLYP, B98, B97-1, mPW1PW91, PBE1PBE, and MPW1KCIS. Among all of the DFT methods with CSGT calculations, VSXC is the best method for the prediction of chemical shifts, followed in order by TPSSh, B97-2, O3LYP, TPSS, TPSS1KCIS, MPWKCIS1K, BHandHLYP, B97-1, and B98. The popular B3LYP method overestimates largely the chemical shifts with both GIAO and CSGT methods. The calculated results indicate that the predictions of (27)Al chemical shifts on the base of the model that includes both explicit solvent effect and bulk solvent effect are most accurate for aquated Al(III) species.

Spin Symmetry Requirements in Density Functional Theory: The Proper Way to Predict Magnetic Coupling Constants in Molecules and Solids

In this paper it is argued that the use of density functional theory (DFT) to solve the exact, non-relativistic, many-electron problem, for magnetic systems requires impos- ing space and spin symmetry constraints exactly in the same way as it is currently done in ab initio wave function theory. This strong statement is supported on pertinent calculations for selected systems representative of organic diradicals, molecular magnets and antiferromagnetic solids. These calculations include several wave function methods of increasing accuracy and different forms of the exchange- correlation functional. The comparisons of numerical results carried out always within the same standard Gaussian Type Orbital atomic basis set show that imposing or not the spin and space constraints (restricted or unrestricted formalisms) leads to contradictory results. Therefore, it appears that, in the case of the Heisenberg magnetic constant, the present exchange-correlation functionals may provide reasonable numerical results although for the wrong physical reasons thus evidencing the failure of the current DFT methods to properly describe magnetic systems.

Accurate orbital-dependent correlation and exchange-correlation potentials from non-iterative ab initio dft calculations

A new approximate non-iterative procedure to obtain accurate correlation and exchange-correlation potentials of Kohn-Sham (KS) density functional theory (DFT) is presented. By carrying out only one step of the correlated optimized effective potential (OEP) iterations following the standard iterative exchange-only OEP, one can recover accurate correlation potentials corresponding to the orbital-dependent second-order many-body perturbation theory [MBPT(2)] energy functional that are hardly discernible from those obtained by the more expensive, fully iterative procedure. This new ‘one-step’ OEP-MBPT(2) algorithm reflects the non-iterative, perturbative algorithm of standard, canonical MBPT(2) of ab initio wave function theory, while it allows the correlation potentials to readjust and include the majority of the MBPT(2) correlation effect. It is also flexible in the treatment of exchange and the Hartree-Fock orbitals may be used in lieu of the exchange-only OEP orbitals, when the correlation or exchange-correlation potential is of interest.

Benchmark Database of Barrier Heights for Heavy Atom Transfer, Nucleophilic Substitution, Association, and Unimolecular Reactions and Its Use to Test Theoretical Methods

A benchmark database of forward and reverse barrier heights for 19 non-hydrogen-transfer reactions has been developed by using Weizmann 1 calculations, and 29 DFT methods and 6 ab initio wave-function theory (WFT) methods have been tested against the new database as well as against an older database for hydrogen atom transfer reactions. Among the tested hybrid DFT methods without kinetic energy density, MPW1K is the most accurate model for calculations of barrier heights. Among the tested hybrid meta DFT methods, BB1K and MPWB1K are the two most accurate models for the calculations of barrier heights. Overall, the results show that BB1K and MPWB1K are the two best DFT methods for calculating barrier heights, followed in order by MPW1K, MPWKCIS1K, B1B95, MPW1B95, BH and HLYP, B97-2, mPW1PW91, and B98. The popular B3LYP method has a mean unsigned error four times larger than that of BB1K. Of the methods tested, QCISD(T) is the best ab initio WFT method for barrier height calculations, and QCISD is second best, but QCISD is outperformed by the BB1K, MPWB1K, MPWKCIS1K, and MPW1K methods.