Abinitio MO predictions for the geometries and energies of the Σ and II states of some conjugated free radicals

1980 ◽  
Vol 58 (7) ◽  
pp. 733-738 ◽  
Author(s):  
N. Colin Baird ◽  
Kathleen F. Taylor
Keyword(s):  
Free Radicals ◽  
Nitrogen Atom ◽  
Configuration Interaction ◽  
Open Shell ◽  
Basis Set ◽  
Basis Sets ◽  
Energy Separation ◽  
Mo Calculations ◽  
Geometric Variables

Abinitio MO calculations are reported for the lowest Σ and Π doublet states of the radicals 1–10. Configuration interaction is included, using the restricted open-shell MO's as a base. The important geometric variables are optimized in calculations using the STO-3G basis set; in some cases energies and geometries are redetermined using the 4-31G basis. In most cases the energy separation between the Σ and Π doublet states is calculated to be small; the results are very dependent upon the basis set whenever the radical is localized upon a nitrogen atom. For radicals XYZ with different terminal groups, a valid prediction of the structure is obtained only if the basis set properly distinguishes between the π energies of X=Y and Y=Z bonds. Small basis sets appear to underestimate the π energy of carbonyl bonds, and thus are overbiased in favour of X=C—O••structures. Unsymmetrical structures are predicted for the Σ states of some radicals, even when the terminal groups are identical.

The structure and energetics of low-lying states of RCO and RNN free radicals

1977 ◽  
Vol 55 (5) ◽  
pp. 863-868 ◽  
Author(s):  
N. Colin Baird ◽  
Harish B. Kathpal
Keyword(s):  
Experimental Data ◽  
Free Radicals ◽  
Hydrogen Atom ◽  
Ab Initio ◽  
Open Shell ◽  
Basis Set ◽  
Basis Sets ◽  
Mo Calculations ◽  
Decomposition Products ◽  
Ab Initio Mo Calculations

The important geometrical variables in the structures of the lowest 2A′ and 2A′′ states of the free radicals HCO, CH3CO, NH2CO, HNN, and CH3NN have been determined by ab initio MO calculations using the STO-3G basis set. The energy differences between the states, and the energies of the radicals relative to their decomposition products and relative to their hydrogen atom addition products, are reported using both STO-3G and 4-31G basis sets in the restricted open-shell calculations. The trends in these results and their relation to available experimental data are discussed.

Electron Affinities of BN, NO and NF: Coupled Cluster and Multireference Configuration Interaction Calculations

2005 ◽  
Vol 70 (7) ◽  
pp. 923-940 ◽  
Author(s):  
Jiří Fišer ◽  
Rudolf Polák
Keyword(s):  
Configuration Interaction ◽  
Relativistic Effects ◽  
Basis Set ◽  
Basis Sets ◽  
Spectroscopic Constants ◽  
Coupled Cluster ◽  
Electron Affinities ◽  
Complete Basis Set ◽  
Correlation Consistent ◽  
Consistent Basis

The accurate adiabatic electron affinities (EA) of the BN, NO and NF molecules have been determined using the coupled cluster approach and multireference configuration interaction methods. By combining large doubly augmented correlation-consistent basis sets (through the sextuple zeta) and complete basis set extrapolations with corrections for core-valence correlation and relativistic effects, we find that the RCCSD(T) method gives EA(BN) = 3.153 eV in very close agreement with experiment and predicts EA(NF) = 0.247 eV. The RCCSD(T) and UCCSD(T) EA(NO) results, 0.008 and 0.031 eV, bracket the experimental value. For both the neutral and anionic ground state species the usual spectroscopic constants were derived.

scholarly journals Chemically Accurate Excitation Energies With Small Basis Sets

2019 ◽  
Author(s):  
Emmanuel Giner ◽  
Anthony Scemama ◽  
Julien Toulouse ◽  
Pierre-Francois Loos
Keyword(s):  
Excited States ◽  
Configuration Interaction ◽  
Density Functional ◽  
Chem Phys ◽  
Basis Set ◽  
Basis Sets ◽  
Excitation Energies ◽  
Functional Correction ◽  
The One ◽  
Range Density

<p>By combining extrapolated selected configuration interaction (sCI) energies obtained with the CIPSI (Configuration Interaction using a Perturbative Selection made Iteratively) algorithm with the recently proposed short-range density-functional correction for basis-set incompleteness [Giner et al., J. Chem. Phys. 2018, 149, 194301], we show that one can get chemically accurate vertical and adiabatic excitation energies with, typically, augmented double-ζ basis sets. We illustrate the present approach on various types of excited states (valence, Rydberg, and double excitations) in several small organic molecules (methylene, water, ammonia, carbon dimer and ethylene). The present study clearly evidences that special care has to be taken with very diffuse excited states where the present correction does not catch the radial incompleteness of the one-electron basis set.</p>

Electronic Structure and Bonding Nature of the Ground State Monocarbide Cations, ScC+, TiC+, VC+, and CrC+

2003 ◽  
Vol 68 (2) ◽  
pp. 387-404 ◽  
Author(s):  
Ioannis S. K. Kerkines ◽  
Aristides Mavridis
Keyword(s):  
Ground State ◽  
Ground States ◽  
Relativistic Effects ◽  
Zero Point ◽  
Basis Set ◽  
Basis Sets ◽  
Energy Separation ◽  
Full Potential ◽  
Complete Basis Set ◽  
Correlation Consistent

The ground states of the transition-metal diatomic carbide cations, MC+ (M = Sc, Ti, V, and Cr), are studied using multireference configuration interaction (MRCI) methods in conjunction with quantitative basis sets. Full potential energy curves are calculated for all four systems. When 3s23p6 core/valence correlation contributions and scalar relativistic effects are taken into account, our best estimates for the zero-point-corrected dissociation energies of the MC+ series are in good agreement with relevant experimental results. For TiC+, the recent correlation-consistent-type basis sets for Ti of Bauschlicher are also exploited to extract complete basis set limits of selected properties. The ground states of VC+(X 3∆) and CrC+(X 2∆) are reported for the first time in the literature. For CrC+ an interesting competition is revealed between the 2∆ and 4Σ- states; although 4Σ- is formally the ground state at the MRCI level of theory, when core/valence and/or relativistic effects are included, the ground state of CrC+ becomes of 2∆ symmetry, with a calculated energy separation (a 4Σ- ← X 2∆) of 2.3 kcal/mol.

scholarly journals Chemically Accurate Excitation Energies With Small Basis Sets

2019 ◽  
Author(s):  
Emmanuel Giner ◽  
Anthony Scemama ◽  
Julien Toulouse ◽  
Pierre-Francois Loos
Keyword(s):  
Excited States ◽  
Configuration Interaction ◽  
Density Functional ◽  
Chem Phys ◽  
Basis Set ◽  
Basis Sets ◽  
Excitation Energies ◽  
Functional Correction ◽  
The One ◽  
Range Density

<p>By combining extrapolated selected configuration interaction (sCI) energies obtained with the CIPSI (Configuration Interaction using a Perturbative Selection made Iteratively) algorithm with the recently proposed short-range density-functional correction for basis-set incompleteness [Giner et al., J. Chem. Phys. 2018, 149, 194301], we show that one can get chemically accurate vertical and adiabatic excitation energies with, typically, augmented double-ζ basis sets. We illustrate the present approach on various types of excited states (valence, Rydberg, and double excitations) in several small organic molecules (methylene, water, ammonia, carbon dimer and ethylene). The present study clearly evidences that special care has to be taken with very diffuse excited states where the present correction does not catch the radial incompleteness of the one-electron basis set.</p>

Ab Initio Studies on Amides: Cyanamide, Dicyanamide and Tricyanamide

1985 ◽  
Vol 38 (6) ◽  
pp. 835 ◽  
Author(s):  
NV Riggs ◽  
L Radom
Keyword(s):  
Nitrogen Atom ◽  
Ab Initio ◽  
Basis Set ◽  
Basis Sets ◽  
Pyramidal Structure ◽  
Bond Lengths ◽  
Planar Form ◽  
Planar Molecules ◽  
Experimental Values ◽  
Experimental Geometry

Optimization of the geometry of cyanamide with the 3-21G basis set leads to a planar (C2v) structure, whereas the STO-3G, 6-31G* and 6- 31G** basis sets lead to a pyramidal structure at the amido -nitrogen atom. The 6-31G* and 6-31G** geometries are reasonably close to the experimental geometry, and the calculated barriers to inversion (4.5 and 3.4 kJ mol-1, respectively) are also close to experimental values (5.4-5.6 kJ mol-1), but the STO-3G value (13.9 kJ mol-1) is much too high. Dicyanamide is far from planar at the STO-3G level but planar at the more definitive 6-31G* level, whereas tricyanamide is planar even at the STO-3G level. The prediction is that, in contrast to cyanamide itself, dicyanamide and tricyanamide are planar molecules. Calculated increases in N-C bond-lengths and decreases in C≡N bond-lengths for dicyanamide and tricyanamide as compared with the planar form of cyanamide are used to predict experimental lengths for these bonds in di - and tri- cyanamide.

Charge distributions and chemical effects. XXXVI. Charge analysis involving configuration interaction: application to alkanes

1985 ◽  
Vol 63 (7) ◽  
pp. 1741-1745 ◽  
Author(s):  
C. Mijoule ◽  
J.-M. Leclercq ◽  
S. Odiot ◽  
S. Fliszár
Keyword(s):  
Empirical Result ◽  
Configuration Interaction ◽  
Absolute Magnitude ◽  
Basis Set ◽  
Basis Sets ◽  
Atomic Charges ◽  
Net Charge ◽  
Chemical Effects ◽  
Charge Analysis ◽  
Abinitio Calculations

An analysis of atomic charges is presented for simple alkanes. Basically, Mulliken's scheme is followed, except for the partitioning of CH overlap populations. This achieves a relative ordering of atomic charges which is independent of the basis sets used in abinitio calculations. The absolute magnitude of atomic charges, however, is basis set dependent. Extensive geometry and scale factor optimizations yield the following results (in 10−3 e units) for the carbon net charge in ethane: 69.4 (STO-3G), 55.1 (STO-3G + CI), 42.8 (4-31G), and 37.8 (4-31G + CI). It appears that charge analyses converge toward the empirical result, 35.1 × 10−3 e, provided they are carried out after configuration interaction involving reasonably large optimized basis sets.

A Systematic DFT Study of Two Elementary Steps Involving Hydrogen Peroxide and the Hydroxyl Radical in the Fenton Reaction

2018 ◽  
Author(s):  
Danilo Carmona ◽  
David Contreras ◽  
Oscar A. Douglas-Gallardo ◽  
Stefan Vogt-Geisse ◽  
Pablo Jaque ◽  
...  
Keyword(s):  
Hydrogen Peroxide ◽  
Hydroxyl Radical ◽  
Ab Initio ◽  
Fenton Reaction ◽  
Basis Set ◽  
Basis Sets ◽  
Dft Methods ◽  
Elementary Steps ◽  
Oxygen Oxygen ◽  
Complete Basis Set

The Fenton reaction plays a central role in many chemical and biological processes and has various applications as e.g. water remediation. The reaction consists of the iron-catalyzed homolytic cleavage of the oxygen-oxygen bond in the hydrogen peroxide molecule and the reduction of the hydroxyl radical. Here, we study these two elementary steps with high-level ab-initio calculations at the complete basis set limit and address the performance of different DFT methods following a specific classification based on the Jacob´s ladder in combination with various Pople's basis sets. Ab-initio calculations at the complete basis set limit are in agreement to experimental reference data and identified a significant contribution of the electron correlation energy to the bond dissociation energy (BDE) of the oxygen-oxygen bond in hydrogen peroxide and the electron affinity (EA) of the hydroxyl radical. The studied DFT methods were able to reproduce the ab-initio reference values, although no functional was particularly better for both reactions. The inclusion of HF exchange in the DFT functionals lead in most cases to larger deviations, which might be related to the poor description of the two reactions by the HF method. Considering the computational cost, DFT methods provide better BDE and EA values than HF and post--HF methods with an almost MP2 or CCSD level of accuracy. However, no systematic general prediction of the error based on the employed functional could be established and no systematic improvement with increasing the size in the Pople's basis set was found, although for BDE values certain systematic basis set dependence was observed. Moreover, the quality of the hydrogen peroxide, hydroxyl radical and hydroxyl anion structures obtained from these functionals was compared to experimental reference data. In general, bond lengths were well reproduced and the error in the angles were between one and two degrees with some systematic trend with the basis sets. From our results we conclude that DFT methods present a computationally less expensive alternative to describe the two elementary steps of the Fenton reaction. However, choice of approximated functionals and basis sets must be carefully done and the provided benchmark allows a systematic validation of the electronic structure method to be employed

DFT Benchmark Study of the O--O Bond Dissociation Energy in Peroxides Validated with High-Level Ab-Initio Calculations

2019 ◽  
Author(s):  
Danilo Carmona ◽  
Pablo Jaque ◽  
Esteban Vöhringer-Martinez
Keyword(s):  
Reference Value ◽  
Basis Set ◽  
Basis Sets ◽  
Mean Deviation ◽  
Bond Dissociation ◽  
Dissociation Energies ◽  
The Mean ◽  
Experimental Values ◽  
High Level ◽  
Almost All

<div><div><div><p>Peroxides play a central role in many chemical and biological pro- cesses such as the Fenton reaction. The relevance of these compounds lies in the low stability of the O–O bond which upon dissociation results in radical species able to initiate various chemical or biological processes. In this work, a set of 64 DFT functional-basis set combinations has been validated in terms of their capability to describe bond dissociation energies (BDE) for the O–O bond in a database of 14 ROOH peroxides for which experimental values ofBDE are available. Moreover, the electronic contributions to the BDE were obtained for four of the peroxides and the anion H2O2− at the CBS limit at CCSD(T) level with Dunning’s basis sets up to triple–ζ quality provid- ing a reference value for the hydrogen peroxide anion as a model. Almost all the functionals considered here yielded mean absolute deviations around 5.0 kcal mol−1. The smallest values were observed for the ωB97 family and the Minnesota M11 functional with a marked basis set dependence. Despite the mean deviation, order relations among BDE experimental values of peroxides were also considered. The ωB97 family was able to reproduce the relations correctly whereas other functionals presented a marked dependence on the chemical nature of the R group. Interestingly, M11 functional did not show a very good agreement with the established order despite its good performance in the mean error. The obtained results support the use of similar validation strategies for proper prediction of BDE or other molecular properties by DF Tmethods in subsequent related studies.</p></div></div></div>

Benchmark of Simplified Time-Dependent Density Functional Theory for UV-Vis Spectral Properties of Porphyrinoids

2019 ◽  
Author(s):  
Kamal Batra ◽  
Stefan Zahn ◽  
Thomas Heine
Keyword(s):  
Density Functional Theory ◽  
Density Functional ◽  
Large Scale ◽  
Absolute Error ◽  
Time Dependent ◽  
Basis Set ◽  
Basis Sets ◽  
Functional Theory ◽  
Framework Materials ◽  
Dependent Density

<p>We thoroughly benchmark time-dependent density- functional theory for the predictive calculation of UV/Vis spectra of porphyrin derivatives. With the aim to provide an approach that is computationally feasible for large-scale applications such as biological systems or molecular framework materials, albeit performing with high accuracy for the Q-bands, we compare the results given by various computational protocols, including basis sets, density-functionals (including gradient corrected local functionals, hybrids, double hybrids and range-separated functionals), and various variants of time-dependent density-functional theory, including the simplified Tamm-Dancoff approximation. An excellent choice for these calculations is the range-separated functional CAM-B3LYP in combination with the simplified Tamm-Dancoff approximation and a basis set of double-ζ quality def2-SVP (mean absolute error [MAE] of ~0.05 eV). This is not surpassed by more expensive approaches, not even by double hybrid functionals, and solely systematic excitation energy scaling slightly improves the results (MAE ~0.04 eV). </p>

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