One- and three-parameter density functional theory and high level ab initio theory study of the CH+CH disproportionation reaction

Complete basis set (CBS) ab initio computational studies were performed with the target being to explore the CH+CH potential energy surface. Several closed and open shell intermediates were located on the potential energy surface. Computed enthalpies for the branching reactions, as well as heats of formation are in excellent agreement. Although CBS computed energies are of high quality, this computational study is not capable of predicting the branching product ratio due to fact that neither the MP2 nor the 6-311G(2d,2p) basis set are sufficient to locate the reactant complexes and the transition state structures for the hydrogen and carbon transfer reactions in the reaction complexes. To properly explore the CH+CH potential energy surface a much higher ab initio theory level is required.     

H2CO和NO2反应机理的密度泛函理论计算研究

用密度泛函理论方法在UB3LYP/ 6-311++G(d,p)并包含零点能水平上计算得到了H2CO和NO2反应的势能面.在势能面上找到了由H2CO和NO2反应生成HCO和trans-HONO的两条反应通道.直接H迁移反应通道的势垒只有90.54 kJ*mol-1,是主要的反应通道,其TST速率是7.9 cm3*mol-1*s-1,与文献值相符;另一条通道是H2CO异构化为trans-HCOH,然后C位H迁移,最后生成的HOC分子异构化为HCO,这条通道反应势垒高达348.03 kJ*mol-1,是一条次要反应通道.     

Ionic dissociations of chlorosulfonic acid in microsolvated clusters: A density functional theory and ab initio MO study

Ionic dissociation of chlorosulfonic acid (HSO3Cl) in the molecular clusters HSO3Cl-(H2O)n (n = 1-4) and HSO3Cl-NH3-(H2O)n (n = 0-3) was investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and thermodynamic properties, such as relative enthalpy and relative Gibbs free energy, and were calculated using the hybrid density func- tional (B3LYP) method and the second order M?ller-Plesset approximation (MP2) method with the 6-311 G** basis set. Chlorosulfonic acid was found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules were found to be strongly hydrogen-bonded. The related properties and acid strength of chlorosulfonic acid were discussed and compared to the acid strengths of perchloric acid and sulfuric acid in the context of clusters with ammonia and water. The relative stabilities of these clusters were also investigated.     

Comparison of some representative density functional theory and wave function theory methods for the studies of amino acids

Energies of different conformers of 22 amino acid molecules and their protonated and deprotonated species were calculated by some density functional theory (DFT; SVWN, B3LYP, B3PW91, MPWB1K, BHandHLYP) and wave function theory (WFT; HF, MP2) methods with the 6-311++G(d,p) basis set to obtain the relative conformer energies, vertical electron detachment energies, deprotonation energies, and proton affinities. Taking the CCSD/6-311++G(d,p) results as the references, the performances of the tested DFT and WFT methods for amino acids with various intramolecular hydrogen bonds were determined. The BHandHLYP method was the best overall performer among the tested DFT methods, and its accuracy was even better than that of the more expensive MP2 method. The computational dependencies of the five DFT methods and the HF and MP2 methods on the basis sets were further examined with the 6-31G(d,p), 6-311++G(d,p), aug-cc-pVDZ, 6-311++G(2df,p), and aug-cc-pVTZ basis sets. The differences between the small and large basis set results have decreased quickly for the hybrid generalized gradient approximation (GGA) methods. The basis set convergence of the MP2 results has been, however, very slow. Considering both the cost and the accuracy, the BHandHLYP functional with the 6-311++G(d,p) basis set is the best choice for the amino acid systems that are rich in hydrogen bonds.     

An ab initio and density functional theory study on the mechanism for the reaction of OH with 2-ethylfuran

The mechanism of the reaction of OH + 2-ethylfuran has been investigated using the G3MP2 and G3MP2B3 levels of theory. The geometric parameters of all species involved in the reaction have been optimized at the MP2 and B3LYP levels of theory with 6-311G(d,p) basis set. The overall profile of doublet potential energy surface (PES) for the OH + 2-ethylfuran reaction has been constructed using the G3MP2 and G3MP2B3 methods. The results show that the addition-elimination mechanism dominates the OH + 2-ethylfuran reaction and the major products are CH₃CH₂C(OH)CHCHCHOH (P8) and CH₃CH₂COCHCHCHOH (P6).     

Density functional theory and quadratic complete basis set ab initio studies of the HNSi to HSiN isomerization and hydrogen insertion reactions with further isomerizations of the insertion products

Ab initio and density functional theory (DFT) computational studies were performed in order to determine the potential energy profiles for the inter-conversions of the small silicon–nitrogen built chemical systems: HSiN, HNSi, HSiNH₂, and HNSiH₂. Three hybrid DFT methods demonstrated high reliability in computing structures and potential energy profiles for these chemical transformations. The stability of some of these molecular systems and their interconversions was discussed and computational results were compared with experimental observations when available.     

A dual-level ab initio and hybrid density functional theory dynamics study on the unimolecular decomposition reaction C2H5O-->CH2O + CH3

We present a direct ab initio and hybrid density functional theory dynamics study of the thermal rate constants of the unimolecular decomposition reaction of C2H5O-->CH2O + CH3 at a high-pressure limit. MPW1K/6-31+G(d,p), MP2/6-31+G(d,p), and MP2(full)/6-31G(d) methods were employed to optimize the geometries of all stationary points and to calculate the minimum energy path (MEP). The energies of all the stationary points were refined at a series of multicoefficient and multilevel methods. Among all methods, the QCISD(T)/aug-cc-pVTZ energies are in good agreement with the available experimental data. The rate constants were evaluated based on the energetics from the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory using both microcanonical variational transition state theory (microVT) and RRKM theory with the Eckart tunneling correction in the temperature range of 300-2500 K. The calculated rate constants at the QCISD(T)/aug-cc-pVTZ/MPW1K/6-31+G(d,p) level of theory are in good consistent with experimental data. The fitted three-parameter Arrhenius expression from the microVT/Eckart rate constants in the temperature range 200-2500 K is k = 2.52 x 10(12)T(0.41)e(-8894.0/T) s(-1). The falloff curves of pressure-dependent rate constants are performed using master-equation method within the temperature range of 391-471 K. The calculated results are in good agreement with the available experimental data.     

Complete basis set ab initio computational study of ionization potential, electron affinity and the C-F bond dissociation energy for perfluorinated methane derivatives

A computational study of perfluorinated methane derivatives was performed with complete basis set ab initio methods. The total energies for their neutral, cation, and anionic states were computed. From these values, the energy gaps between different electronic states, ionization potentials, electron affinities, and C-F bond dissociation energies were calculated. The computed values are compared with experimental data and the reliability of complete basis set ab initio methods is discussed. New values for C-F bond dissociation energies are suggested. Received: 12 January 1998 / Accepted: 2 April 1998 / Published online: 29 July 1998     

Complete basis set, the MP2 ab initio, and hybrid density functional theory evaluation of ionization potential and electron affinity for PH, PH2, PHF, PF, and PF2

A systematic study was performed on the small molecular systems built from phosphor, hydrogen and fluorine with the target being to evaluate accurately their ionization potentials and electron affinities, as well as influence fluorine on the ionization potential of phosphor as a central atom. To determine the accuracy of hybrid density functional methods for computing those energies, ionization energies for hydrogen, fluorine and phosphor were calculated and compared with the experimental and CBSQ values. To demonstrate the accuracy of this method, both the ionization potential and the electron affinity for phosphorus and fluorine atoms were calculated and compared with the experimental data. For both PF and PF₂, an identical electron affinity of 0.72 eV and for PH and PHF 1.0 eV were suggested.     

Complete basis set ab initio and hybrid density functional theory exploration of the potential energy surface in the reaction between an amino radical and nitrogen oxide

The potential energy surface for the reaction involving NH₂ plus NO was explored with a quadratic complete basis set ab initio approach and three hybrid density functional theory methods, the target being to accurately estimate activation barriers and the relative stability of the nitrogen–oxygen isomers. The computational results were compared with previously performed ab initio calculations and new, more accredited values for the NH₂NO rearrangement to HNNOH and for the HNNOH decomposition reaction were suggested. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 409–414, 1998     

Theoretical ab initio study of NO and CO depollution reaction catalyzed by copper

The reaction between NO and CO leading to N₂ and CO₂ is the most studied depollution process of the former molecules. An ab initio study of a multistage mechanism of this reaction catalyzed by copper was performed at SCF level. Many intermediates intervene in the proposed mechanism, such as CuCO, CuNO, CuO, and NCO. Geometrical parameters, atomic charge, dipole moment, vibrational normal mode wave number, and dissociation energy of intervening molecules were calculated. Thermochemistry parameters (ΔH, ΔG, ΔS) were also obtained. Transition state has also been determined and has allowed us to discuss the reaction mechanism. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Study of the formamide–methanol dimer with ab initio and density functional theory methods

For the first time, the structures and energies for the hydrogen bonding of a 1:1 complex formed between formamide and methanol molecules have been computed with various pure and hybrid density functional theory (DFT) and ab initio methods at varied basis set levels from 6‐31g to 6‐31+g(d,p). Five reasonable geometries on the potential energy surface of methanol and formamide system are considered and their relative stability is discussed. The infrared (IR) spectrum frequencies, IR intensities, and vibrational frequency shifts are reported. From the systematic studies, it is found that all the DFT methods selected here correctly compute the dimerization energies and geometries, with the B3P86 method predicting the hydrogen bond lengths relatively shorter and BPW91 yielding the interaction energies relatively lower. Finally, the solvent effects on the geometries of the formamide–methanol complexes have also been investigated using self‐consistent reaction field (SCRF) calculations with five different DFT methods at the 6‐31+g(d,p) basis set level. The results indicate that the polarity of the solvent has played an important role on the structures and relative stabilities of different isomers. Moreover, the basis set superposition error correction is critical to the interaction energies in the polar solvents. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Comparison of ab initio and density functional methods for vibrational analysis of TeCl4

Vibrational analysis of tellurium tetrachloride, TeCl₄, was performed with Hartree–Fock (HF), MP2, and generalized gradient approximation density functional theory (DFT) methods supplemented with polarized double-zeta split valence (DZVP) basis sets and relativistic effective core potentials (RECP) of Hay and Wadt. The molecular geometry is best reproduced at the HF and MP2/RECP+DZVP [polarized Hay and Wadt RECP for Te and 6–31G(d) basis set for Cl] levels of theory. The DFT methods gave rise to poorer results, especially those using Becke's 1988 exchange functional. Generally, the vibrational frequencies calculated by the MP2 and B3-type DFT methods with the all electron and RECP+DZVP basis sets as well as at the HF/RECP level were in satisfactory accord with the experimental data. The agreement was good enough to assist the assignment of the measured vibrational spectra. The best agreement with the experimental vibrational frequencies was achieved with the scaled HF/RECP force field. Consistent results were obtained for the unobserved A₂ (ν₄) fundamental, where the results of the best methods were within 4 cm⁻¹. The best force fields were obtained with the following methods: Becke3–Lee–Yang–Parr and Becke3–Perdew/all electron basis, MP2 and Becke3-Perdew/RECP+DZVP, and HF/RECP. The methods using RECPs are advantageous for large-scale computations. The RECP basis set effectively compensates the errors of the HF method for TeCl₄; however, it provides poor results with correlated methods. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 308–318, 1998     

基于密度泛函理论计算的CO2在SrTiO3(100)表面的吸附

通过密度泛函理论的第一性原理,模拟了CO2分子在SrTiO3(100)表面TiO2-和SrO-位点上的吸附行为,获得了CO2在几种不同吸附模型下的结构参数及表面吸附能,进而研究了吸附机理和结构稳定性.计算结果表明,当CO2的C原子吸附在SrTiO3(100)表面SrO-及TiO2-位点的氧原子上时,吸附结构较稳定,尤其是C、O原子共吸附在TiO2-位点时最稳定,而其余吸附模型则不稳定.对吸附稳定模型的Mulliken布局数及态密度分析显示:CO2分子在SrTiO3(100)表面吸附主要是由于SrTiO3(100)面的电子跃迁至CO2分子,CO2分子得到电子形成弯曲的CO2-阴离子结构,并伴随着C-O键的伸长,从而达到吸附活化CO2的目的.     

Assessment of density functional methods for reaction energetics: Iridium‐catalyzed water oxidation as case study

We investigate basis set convergence for a series of density functional theory (DFT) functionals (both hybrid and nonhybrid) and compare to coupled‐cluster with single and double excitations and perturbative triples [CCSD(T)] benchmark calculations. The case studied is the energetics of the water oxidation reaction by an iridium‐oxo complex. Complexation energies for the reactants and products complexes as well as the transition state (TS) energy are considered. Contrary to the expectation of relatively weak basis set dependence for DFT, the basis set effects are large, for example, more than 10 kcal mol^?1 difference from converged basis for the activation energy with “small” basis sets (DZ/6‐31G** for Ir/other atoms, or SVP) and still more than 6 kcal mol^?1 for def2‐TZVPP/6‐31G**. Inclusion of the dispersion correction in DFT‐D3 schemes affects the energies of reactant complex (RC), TS, and product complex (PC) by almost the same amount; it significantly improves the complexation energy (the formation of RC), but has little effect on the activation energy with respect to RC. With converged basis, some pure GGAs (PBE‐D3, BP86‐D3) as well as the hybrid functional B3LYP‐D3 are very accurate compared to benchmark CCSD(T) calculations. © 2012 Wiley Periodicals, Inc.     

Insight into the reaction mechanism of CH2SH with HO2: A density functional theory study

Density functional theory was adopted in this work to reveal the reaction mechanism of CH₂SH with HO₂. Reaction rate constants were computed from 200 to 2000 K using the transition state theory combined with Wigner and Eckart tunneling correction. Moreover, localized orbital locator, atoms in molecules and Mayer bond order analyses were used to study the essence of chemical bonding evolution. Eleven singlet paths and three triplet ones are located on the potential surface (PES). The results show that the main products on the singlet PES are ¹CH₂S and H₂O₂, whereas on the triplet PES they are CH₃SH + ³O₂, which are coincident with the similar reaction of CH₃S and HO₂. This conclusion is also supported by rate constant calculation results. Interestingly, all the possible paths are involved in the hydrogen transfer. The results have provided underlying insights to the analogous reactions and further experimental studies.     

The Gaussian and augmented-plane-wave density functional method for ab initio molecular dynamics simulations

A new algorithm for density-functional-theory-based ab initio molecular dynamics simulations is presented. The Kohn–Sham orbitals are expanded in Gaussian-type functions and an augmented-plane-wave-type approach is used to represent the electronic density. This extends previous work of ours where the density was expanded only in plane waves. We describe the total density in a smooth extended part which we represent in plane waves as in our previous work and parts localised close to the nuclei which are expanded in Gaussians. Using this representation of the charge we show how the localised and extended part can be treated separately, achieving a computational cost for the calculation of the Kohn–Sham matrix that scales with the system size N as O(NlogN). Furthermore, we are able to reduce drastically the size of the plane-wave basis. In addition, we introduce a multiple-cutoff method that improves considerably the performance of this approach. Finally, we demonstrate with a series of numerical examples the accuracy and efficiency of the new algorithm, both for electronic structure calculations and for ab initio molecular dynamics simulations. Received: 15 December 1998 /Accepted: 18 February 1999 /Published online: 14 July 1999     

Molecular structure and vibrational spectra of phenobaraitone by density functional theory and ab initio hartree-Fock calculations

Quantum chemistry calculations have been performed using Gaussian03 program to compute optimized geometry, harmonic vibrational frequency along with intensities in IR and Raman spectra at RHF/6-31++G** and B3LYP/6-31++G** levels for phenobarbitone (C₁₂H₁₂N₂O₃) in the ground state. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR and FT-Raman spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions (PEDs) using MOLVIB program. A detailed interpretation of the infrared spectra of the title compound is reported. On the basis of the agreement between the calculated and observed results, the assignments of fundamental vibrational modes of phenobarbitone were examined and some assignments were proposed. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title compound have been constructed.     

Computational studies of bond dissociation energies, ionization potentials, and heat of formation for NH and NH+. Are hybrid density functional theory methods as accurate as quadratic complete basis set and Gaussian-2 ab initio methods?

Ionization potentials, bond dissociation energies, and heat of formation for NH and NH⁺ molecular species as well as for their elements were computed with highly reliable quadratic complete basis set and Gaussian-2 ab initio methods. The results are compared with experimental results and the assurance of these ab initio approaches is assessed. The same studies were also performed with three hybrid density functional methods (B3LYP, B3P86, and B3PW91) in combination with variously sized basis sets. The computational results are discussed in light of density functional theory reliability for exploring the potential energy of small polar molecular systems. Received: 21 July 1997 / Accepted: 8 December 1997