post-Hartree Fock Methods Research Articles

How the Amount of Hartree-Fock Exchange Affects the Observation of the Pseudo Jahn-Teller Effect

Heavy group 14 analogs of planar carbon molecules are well known to prefer bent or pyramidalized geometries. The pseudo Jahn-Teller effect (pJTE) is a general approach to understanding the origin of such molecular geometries. Computationally intensive multi-reference post Hartree-Fock methods are generally required to obtain values relevant to the pJTE. However, an alternative approach, using more computationally efficient density functional theory (DFT) and fitting a model vibronic Hamiltonian, can account for the pJTE parameters. Previous studies have detailed the effect of both integration grid size and exact exchange on various properties including the nature of stationary points (i.e., transition structure v. minimum) on potential energy surfaces but none have addressed the root cause of such discrepancies. In this work we examine two contentious stationary points belonging to planar disilene and 2Si TCNQ from the perspective of the pseudo Jahn-Teller effect (pJTE) using DFT methods. First the planar stationary points are characterized using a variety of model chemistries and integration grids. The effect of the amount of Hartree-Fock exchange is then studied and the usage of DFT for assessing pJT parameters is explored. It is shown that the DFT approach is a viable alternative for determining values relevant to the pJTE. A complementary cusp catastrophe theory approach to the problem is also introduced. Difficulties associated with the DFT-based approach to the study of the pJTE are discussed and changes in the electronic structure are explored using the Quantum Theory of Atoms in Molecules.

Amino acids as bio-organocatalysts in ring-opening copolymerization for eco-friendly synthesis of biobased oligomers from vegetable oils.

Herein, we present an innovative synthetic approach for producing a diverse set of biobased oligomers. This method begins with olive oil and employs a wide variety of commercially available amino acids (AAs) as bio-organocatalysts, in addition to tetrabutylammonium iodide (TBAI) as a cocatalyst, to synthesize various biobased oligomers. These biobased oligomers were strategically prepared starting from epoxidized olive oil (EOO) and a variety of cyclic anhydrides (phthalic, PA; maleic, MA; succinic, SA; and glutaric, GA). Among the amino acids tested as bio-organocatalysts, L-glutamic acid (L-Glu) showed the best performance for the synthesis of both poly(EOO-co-PA) and poly(EOO-co-MA), exhibiting 100% conversion at 80 °C in 2 hours, whereas the formation of poly(EOO-co-SA) and poly(EOO-co-GA) required more extreme reaction conditions (72 hours under toluene reflux conditions). Likewise, we have succeeded in obtaining the trans isomer exclusively for the MA based-oligomer within the same synthetic framework. The obtained oligomers were extensively characterized using techniques including NMR, FT-IR, GPC and TGA. A series of computational simulations based on density functional theory (DFT) and post-Hartree Fock (post-HF) methods were performed to corroborate our experimental findings and to obtain an understanding of the reaction mechanisms.

In silico-based identification of new anti-pfdhfr drug candidates via 1,3,5-triazine derivatives

Quantitative structure-activity relationship study was used to investigate the relationship between anti-pfdhfr activity and structure of twenty-eight 1,3,5-triazine derivatives. We performed benchmark studies on the molecular geometry, electron properties of 1,3,5-triazine using semi-empirical(PM3), density functional theory and post Hartree-Fock methods. Followed by a QSAR study using multiple linear regression (MLR) and artificial neural networks (ANN). The QSAR models developed allow identify/describe the relationship between the biological activity of the molecules and their molecular descriptors (topological, physicochemical, electronic...). A further external set of compounds was used for validation where a high correlation between experimental and predicted anti-pfdhfr activity values is noticed. This QSAR study provides useful information for developing novel pfdhfr inhibitors. The set’s ADME properties and drug similarities, as well as newly produced compounds and reference ligand, were investigated. These findings would be extremely useful in guiding optimization for the development of new anti-pfdhfr drug candidates.

Vibronic excitations in resonant inelastic x-ray scattering spectra of K2RuCl6

We present the fingerprints of dynamic Jahn-Teller effect in resonant inelastic x-ray scattering (RIXS) spectra of K$_2$RuCl$_6$. We determined the dynamic Jahn-Teller model Hamiltonian of an embedded Ru$^{4+}$ ion using post Hartree-Fock methods, and derived the vibronic states by numerically diagonalizing the Hamiltonian. With the obtained vibronic states, we reproduced the RIXS spectra. The shape and the temperature dependence of the RIXS spectrum agree well with the experimental data. We found that some peaks emerge due to the dynamic Jahn-Teller effect rather than the crystal field splitting. Our study indicates the significance of the Jahn-Teller coupling to adequately interpret RIXS spectra.

Chalcogen Bond as a Factor Stabilizing Ligand Conformation in the Binding Pocket of Carbonic Anhydrase IX Receptor Mimic.

It is postulated that the overexpression of Carbonic Anhydrase isozyme IX in some cancers contributes to the acidification of the extracellular matrix. It was proved that this promotes the growth and metastasis of the tumor. These observations have made Carbonic Anhydrase IX an attractive drug target. In the light of the findings and importance of the glycoprotein in the cancer treatment, we have employed quantum-chemical approaches to study non-covalent interactions in the binding pocket. As a ligand, the acetazolamide (AZM) molecule was chosen, being known as a potential inhibitor exhibiting anticancer properties. First-Principles Molecular Dynamics was performed to study the chalcogen and other non-covalent interactions in the AZM ligand and its complexes with amino acids forming the binding site. Based on Density Functional Theory (DFT) and post-Hartree-Fock methods, the metric and electronic structure parameters were described. The Non-Covalent Interaction (NCI) index and Atoms in Molecules (AIM) methods were applied for qualitative/quantitative analyses of the non-covalent interactions. Finally, the AZM-binding pocket interaction energy decomposition was carried out. Chalcogen bonding in the AZM molecule is an important factor stabilizing the preferred conformation. Free energy mapping via metadynamics and Path Integral molecular dynamics confirmed the significance of the chalcogen bond in structuring the conformational flexibility of the systems. The developed models are useful in the design of new inhibitors with desired pharmacological properties.

Is There a Quadruple Fe-C Bond in FeC(CO)3?

A recent computational paper (Kalita et al., Phys. Chem. Chem. Phys. 2020, 22, 24178–24180) reports the existence of a quadruple bond between a carbon and an iron atom in the FeC(CO)3 molecule. In this communication, we perform several computations on the same system, using both density functional theory and post-Hartree–Fock methods and find that the results, and in particular the Fe-C bond length and stretching frequency depend strongly on the method used. We ascribe this behavior to a strong multireference character of the FeC(CO)3 ground state, which explains the non-conclusive results obtained with single-reference methods. We therefore conclude that, while the existence of a Fe-C quadruple bond is not disproved, further investigation is required before a conclusion can be drawn.

On deformability of atoms—comparative study between atoms and atomic nuclei

Atomic nuclei can be spontaneously deformed into non-spherical shapes as many-nucleon systems. We discuss to what extent a similar deformation takes place in many-electron systems. To this end, we employ several many-body methods, such as the unrestricted Hartree–Fock method, post-Hartree–Fock methods, and the density functional theory, to compute the electron density distribution in atoms. We show that the electron density distribution of open-shell atoms is deformed due solely to the single-particle valence orbitals, while the core part remains spherical. This is in contrast to atomic nuclei, which can be deformed collectively. We qualitatively discuss the origin for this apparent difference between atoms and nuclei by estimating the energy change due to deformation. We find that nature of the interaction plays an essential role for the collective deformation.

Dual XH-π Interaction of Hexafluoroisopropanol with Arenes.

The dual XH (OH and CH) hydrogen-bond-donating property of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and the strong dual XH–π interaction with arenes were firstly disclosed by theoretical studies. Here, the high accuracy post-Hartree–Fock methods, CCSD(T)/CBS, reveal the interaction energy of HFIP/benzene complex (−7.22 kcal/mol) and the contribution of the electronic correlation energy in the total interaction energy. Strong orbital interaction between HFIP and benzene was found by using the DFT method in this work to disclose the dual XH–π intermolecular orbital interaction of HFIP with benzene-forming bonding and antibonding orbitals resulting from the orbital symmetry of HFIP. The density of states and charge decomposition analyses were used to investigate the orbital interactions. Isopropanol (IP), an analogue of HFIP, and chloroform (CHCl3) were studied to compare them with the classical OH–π, and non-classical CH–π interactions. In addition, the influence of the aggregating effect of HFIP, and the numbers of substituted methyl groups in benzene rings were also studied. The interaction energies of HFIP with the selected 24 common organic compounds were calculated to understand the role of HFIP as solvent or additive in organic transformation in a more detailed manner. A single-crystal X-ray diffraction study of hexafluoroisopropyl benzoate further disclosed and confirmed that the CH of HFIP shows the non-classical hydrogen-bond-donating behavior.

Post-Hartree–Fock method in quantum chemistry for quantum computer

Quantum computational chemistry is a potential application of quantum computers that is expected to effectively solve several quantum-chemistry problems, particularly the electronic structure problem. Quantum computational chemistry can be compared to the conventional computational devices. This review comprehensively investigates the applications and overview of quantum computational chemistry, including a review of the Hartree-Fock method for quantum information scientists. Quantum algorithms, quantum phase estimation, and variational quantum eigensolver, have been applied to the post-Hartree-Fock method.

Smooth potential-energy surfaces in fragmentation-based local correlation methods for periodic systems

ABSTRACT Local approximations facilitate the application of post-Hartree–Fock methods in the condensed phase, but simultaneously introduce errors leading to discontinuous potential-energy surfaces. In this work, we explore how these discontinuities arise in periodic systems, their implications, and possible ways of controlling them. In addition, we present a fully periodic Divide-Expand-Consolidate second-order Møller–Plesset approach using an attenuated resolution-of-the-identity approximation for the electron repulsion integrals and a convenient class to handle translation-symmetric tensors in block-Toeplitz format.

Reduction of the molecular hamiltonian matrix using quantum community detection

Quantum chemistry is interested in calculating ground and excited states of molecular systems by solving the electronic Schrödinger equation. The exact numerical solution of this equation, frequently represented as an eigenvalue problem, remains unfeasible for most molecules and requires approximate methods. In this paper we introduce the use of Quantum Community Detection performed using the D-Wave quantum annealer to reduce the molecular Hamiltonian matrix in Slater determinant basis without chemical knowledge. Given a molecule represented by a matrix of Slater determinants, the connectivity between Slater determinants (as off-diagonal elements) is viewed as a graph adjacency matrix for determining multiple communities based on modularity maximization. A gauge metric based on perturbation theory is used to determine the lowest energy cluster. This cluster or sub-matrix of Slater determinants is used to calculate approximate ground state and excited state energies within chemical accuracy. The details of this method are described along with demonstrating its performance across multiple molecules of interest and bond dissociation cases. These examples provide proof-of-principle results for approximate solution of the electronic structure problem using quantum computing. This approach is general and shows potential to reduce the computational complexity of post-Hartree–Fock methods as future advances in quantum hardware become available.

Revisiting the OH + H2 → H2O + H reaction at the molecular level: the plausible catalytic role of ice in its own reconstruction

Context. In spite of the permanent damage suffered from the radiation field (cosmic rays, X-rays, and intense UV-visible radiations), interstellar grains are still covered by ices mantles whose role in interstellar chemistry is well beyond any doubt. This clearly means that the destruction of the ice cover has to be counterbalanced by efficient reconstruction mechanisms. Aims. Our goal is to determine whether the ice, which is still present after irradiation, has a catalytic role in the OH + H2 → H2O + H reaction for its own reconstruction. We focus on the three plausible reaction paths depending on the way reactants OH or H2 are adsorbed at the ice surface. Methods. Calculations were performed in both cluster and solid state approaches, using ab-initio post Hartree-Fock methods for small systems, standard density functional theory (DFT) for larger clusters, and periodic solid state DFT with specific formalisms accounting for weak interactions in systems of infinite dimensions. Results. Although the end product is the same, that is namely the reconstruction of one H2O on the subjacent ice, three different reaction paths are found depending on whether H2 reacts with adsorbed OH(ads), wether OH reacts with adsorbed H2(ads) or wether both OH(ads) and H2(ads) are adsorbed on the ice before reacting. In the first case, there is an activation barrier of ~6 kcal mol−1, requiring the tunneling effect for the reaction to proceed, which is in agreement with preceding studies. In the second case, the reaction is a barrierless process leading to the direct reconstruction of the ice. In the third case, the double adsorption increases the activation barrier due to the lowering of the starting energy. This is found regardless of the dimension of the supporting ice aggregates. Conclusions. Icy grain surfaces play a critical role for their own reconstruction in cold, dense interstellar clouds. The prevalence of tunneling over the direct mechanism should strongly depend on the temperature and local environment.

A quantum chemical study of LIX63 hydroxyoxime syn/anti isomerisation

ABSTRACT Quantum mechanical studies of the Z/E (syn/anti) acetoin oxime interconversion have been conducted using both density functional theory (DFT) and ab initio post-Hartree–Fock methods in order to understand the thermodynamic properties of the syn/anti isomerisation of the aliphatic α-hydroxyoxime 5,8-diethyl-7-hydroxydodecan-6-oxime. The anti isomer of this molecule is the active component in LIX63, a commercial reagent used in hydrometallurgical solvent extraction (SX). The effect of different sized alkyl (R) groups on both kinetic and thermodynamic properties of the equilibrium between syn and anti isomers of aliphatic α-hydroxyoxime has been studied. Theoretically, it has been shown that using R = C3H8 on both ends of the oxime is the minimum substituent size required to obtain good agreement for the thermodynamics in comparison to experimental data for LIX63. Further increases in the size of the R group do not have any significant effect on the final distribution between syn and anti isomers of LIX63 hydroxyoxime. Inversion was determined to be the most energetically favourable hydroxyoxime isomerisation mechanism, with the energy barrier lowered when isomerisation is considered between syn and anti isomers with larger R groups.

Computation of NMR Shifts for Paramagnetic Solids Including Zero-Field-Splitting and Beyond-DFT Approaches. Application to LiMPO4 (M = Mn, Fe, Co, Ni) and MPO4 (M = Fe, Co)

A protocol for the computation of NMR shifts of paramagnetic spin-coupled solids is described and applied to the olivine-type lithium-ion battery cathode materials LiMPO4 (M = Mn, Fe, Co, Ni) and to the related binary phosphates MPO4 (M = Fe, Co). The computational approach includes Fermi-contact (FC), pseudocontact (PC), as well as orbital contributions to the shifts and combines periodic density functional computations with multireference post-Hartree–Fock methods. A shift formalism based on EPR spin-Hamiltonian parameters corrected for residual spin couplings within the Curie–Weiss regime is used. Orbital shieldings as well as hyperfine couplings (HFCs) are obtained at DFT levels for large simulation cells using the CP2K code, taking advantage of hybrid functionals for the HFCs. g-Tensors and zero-field-splitting tensors are computed within an incremental cluster model, using CASSCF and NEVPT2 approaches, providing thus the first post-Hartree–Fock treatments of these properties for extended solids. Com...

Mechanistic study of the photoexcitation, photoconversion, and photodissociation of CS2.

Carbon disulfide is a prototype molecular system for studies of photophysical processes in molecules at different time scales and is also relevant to astrophysics. Here, reliable molecular properties are computed for linear SCS, bent cyc-CS2, and linear CSS forms using highly correlated post Hartree-Fock methods in conjunction with large basis sets. Structures are identified using explicitly correlated and standard coupled cluster techniques. Evolution of the lowest-lying singlet and triplet electronic states of the three isomers along the SS and CS stretching coordinates and along the bending angle are mapped at the multireference configuration interaction (MRCI)/aug-cc-pV(5+d)Z level of theory. The computations suggest that the 1B2(1Σ+) electronic state of the SCS isomer plays an important role in the photoconversion of CS2 to cyc-CS2 and CSS. Photoconversion competes with photodissociation. Plausible mechanisms for the production of S2 and CS diatomics after the photoexcitation of SCS are proposed. To aid in the identification of CSS in the laboratory and in astrophysical media, a set of spectroscopic constants and rovibrational levels for CSS are reported.

Optimized auxiliary basis sets for density fitted post-Hartree–Fock calculations of lanthanide containing molecules

ABSTRACTOptimised auxiliary basis sets for lanthanide atoms (Ce to Lu) for four basis sets of the Karlsruhe error-balanced segmented contracted def2 - series (SVP, TZVP, TZVPP and QZVPP) are reported. These auxiliary basis sets enable the use of the resolution-of-the-identity (RI) approximation in post Hartree–Fock methods – as for example, second-order perturbation theory (MP2) and coupled cluster (CC) theory. The auxiliary basis sets are tested on an enlarged set of about a hundred molecules where the test criterion is the size of the RI error in MP2 calculations. Our tests also show that the same auxiliary basis sets can be used together with different effective core potentials. With these auxiliary basis set calculations of MP2 and CC quality can now be performed efficiently on medium-sized molecules containing lanthanides.

Imidazole derivatives as angiotensin II AT1 receptor blockers: Benchmarks, drug-like calculations and quantitative structure-activity relationships modeling

We performed benchmark studies on the molecular geometry, electron properties and vibrational analysis of imidazole using semi-empirical, density functional theory and post Hartree-Fock methods. These studies validated the use of AM1 for the treatment of larger systems. Then, we treated the structural, physical and chemical relationships for a series of imidazole derivatives acting as angiotensin II AT1 receptor blockers using AM1. QSAR studies were done for these imidazole derivatives using a combination of various physicochemical descriptors. A multiple linear regression procedure was used to design the relationships between molecular descriptor and the activity of imidazole derivatives. Results validate the derived QSAR model.

Insights on the interaction of Zn2 + cation with triazoles: Structures, bonding, electronic excitation and applications

At present, we investigate the structures, the stability, the bonding and the spectroscopy of the Zn2+-triazole complexes (Zn2+-Tz), which are subunits of triazolate based porous materials and Zn-enzymes. This theoretical work is performed using ab initio methods and density functional theory (DFT) where dispersion correction is included. Through these benchmarks, we establish the ability and reliability of M05-2X+D3 and PBE0+D3 functionals for the correct description of Zn2+-Tz bond since these DFTs lead to close agreement with post Hartree-Fock methods. Therefore, M05-2X+D3 and PBE0+D3 functionals are recommended for the characterization of larger organometallic complexes formed by Zn and N-rich linkers. For Zn2+-Tz, we found two stable σ-type complexes: (i) a planar structure where Zn2+ links to unprotonated nitrogen and (ii) an out-of-plane cluster where carbon interacts with Zn2+. The most stable isomers consist on a coordinated covalent bond between the lone pair of unprotonated nitrogen and the vacant 4s orbital of Zn2+. The roles of covalent interactions within these complexes are discussed after vibrational, NBO, NPA charges and orbital analyses. The bonding is dominated by charge transfer from Zn2+ to Tz and intramolecular charge transfer, which plays a vital role for the catalytic activity of these complexes. These findings are important to understand, at the microscopic level, the structure and the bonding within triazolate based macromolecular porous materials and Zn-enzymes.

Assessing the role of Hartree-Fock exchange, correlation energy and long range corrections in evaluating ionization potential, and electron affinity in density functional theory.

Accurate determination of ionization potentials (IPs), electron affinities (EAs), fundamental gaps (FGs), and HOMO, LUMO energy levels of organic molecules play an important role in modeling and predicting the efficiencies of organic photovoltaics, OLEDs etc. In this work, we investigate the effects of Hartree Fock (HF) Exchange, correlation energy, and long range corrections in predicting IP and EA in Hybrid Functionals. We observe increase in percentage of HF exchange results in increase of IPs and decrease in EAs. Contrary to the general expectations inclusion of both HF exchange and correlation energy (from the second order perturbation theory MP2) leads to poor prediction. Range separated Hybrid Functionals are found to be more reliable among various DFT Functionals investigated. DFT Functionals predict accurate IPs whereas post HF methods predict accurate EAs. © 2017 Wiley Periodicals, Inc.

Electronic structure, stability and spectroscopy of low-lying states of NO−, HNO− and HON− molecular anions

Accurate post Hartree-Fock methods have been used to investigate electronic structure, spectroscopy, stability and metastability of low-lying electronic states of HNO, HON, NO, NO−, HNO− and HON−. Accurate vertical transition energies between lowest electronic states of HNO and HON isomers have been determined. The HNO vertical transition energy of the X∼1A′-ã3A″ transition is shown to be 0.87eV. Core-valence correlations and relativistic corrections have been taken into account to determine accurately spectroscopic constants of electronic states of NO− and NO. Adiabatic excitation energies and dipole moments of low-lying electronic states of NO− have also been calculated. The adiabatic electron affinity of NO has been deduced to be 0.0289eV, which is in excellent agreement with the experimental value of 0.026±0.05eV. The comparison of potential energy curves of low-lying states of NO/NO−, HNO/HNO− and HON/HON− reveals short-lived excited electronic states. The HON− anion appears as an unbound anion unlike to HNO− which is a bound anion. Three-dimensional potential energy surfaces (3D-PESs) for both HNO (X∼1A′) and HNO−(X2A″) ground states, have been generated and fitted by second order perturbation theory to produce accurate spectroscopic constants. Spectroscopic data of HNO−(X2A″) deduced from this work is computed for the first time.