scholarly journals Ab Initio Investigations of the Electronic Structure of HeNH + and HeNH 2+

1998 ◽  
Vol 51 (1) ◽  
pp. 57 ◽  
Author(s):  
Jason M. Hughes ◽  
Ellak I. von ◽  
Nagy-Felsobuki
Keyword(s):  
Electron Correlation ◽  
Electronic Structures ◽  
Structural Parameters ◽  
Minimum Energy ◽  
Electronic States ◽  
Basis Set ◽  
Coupled Cluster ◽  
Geometrical Parameters ◽  
Correlation Consistent ◽  
Triple Excitation

Electronic structures of HeNH+ and HeNH2+ were investigated using the all-electron coupled cluster single, double and triple excitation (CCSD(T)_AE) method coupled with an augmented correlation-consistent polarised core valence triple zeta basis set (aug-cc-pCVTZ). For HeNH+ and HeNH2+, the CCSD(T)_AE/aug-cc-pCVTZ model yielded cations of Cs symmetry with 2 A″ and 1 A′ ground electronic states possessing optimised geometrical parameters {rN-H; rN-He; θH-N-He} of {1·064 Å, 1·517 Å, 88·5°} and {1·192 Å, 1·309 Å, 96·2°} respectively. Of all the linear isomeric forms possible for these two cations, the only converged minimum energy structures were for the 4Σ- states of HeHN+ and HeNH+ yielding optimised structural parameters of {1·093 Å, 2·403 Å, 180·0°} and {1·117 Å, 1·364 Å, 180·0°} respectively. These two linear structures were calculated to be 9·6 and 18·6 kJ mol-1 respectively above the Cs structure. The calculated harmonic frequencies were real for all these states and their magnitudes were sensitive to the incorporation of electron correlation.

Inelastic scattering of interstellar silyl cyanide (SiH3CN) by helium atoms : cross-sections and rate coefficients for A- and E-SiH3CN

2021 ◽  
Vol 507 (4) ◽  
pp. 5264-5271
Author(s):  
Manel Naouai ◽  
Abdelhak Jrad ◽  
Ayda Badri ◽  
Faouzi Najar
Keyword(s):  
Inelastic Scattering ◽  
Total Energy ◽  
Cross Sections ◽  
Three Dimensional ◽  
Basis Set ◽  
Rate Coefficients ◽  
Close Coupling ◽  
Explicitly Correlated ◽  
Correlation Consistent ◽  
Triple Excitation

ABSTRACT Rotational inelastic scattering of silyl cyanide (SiH3CN) molecule with helium (He) atoms is investigated. Three-dimensional potential energy surface (3D-PES) for the SiH3CN–He interacting system is carried out. The ab initio 3D-PES is computed using explicitly correlated coupled cluster approach with single, double, and perturbative triple excitation CCSD(T)-F12a connected to augmented-correlation consistent-polarized valence triple zeta Gaussian basis set. A global minimum at (R = 6.35 bohr; θ = 90○; ϕ = 60○) with a well depth of 52.99 cm−1 is pointed out. Inelastic rotational cross-sections are emphasized for the 22 first rotational levels for total energy up to 500 cm−1 via close coupling (CC) approach in the case of A-SiH3CN and for the 24 first rotational levels for total energy up to 100 cm−1 via CC and from 100 to 500 cm−1 via coupled states (CS) in the case of E-SiH3CN. Rate coefficients are derived for temperature until 80 K for both A- and E-SiH3CN–He systems. Propensity rules are obtained for |ΔJ| = 2 processes with broken parity for A-SiH3CN and for |ΔJ| = 2 processes with |ΔK| = 0 and unbroken parity for E-SiH3CN.

scholarly journals Ab initio study of the mechanism of the reaction ClO + O --> Cl + O2

2021 ◽  
Vol 66 (1) ◽  
Author(s):  
S. Naskar ◽  
G. Nandi ◽  
T. K. Ghosh
Keyword(s):  
Reaction Mechanism ◽  
Ab Initio ◽  
Minimum Energy ◽  
Transition States ◽  
Basis Set ◽  
Heat Of Reaction ◽  
Minimal Basis ◽  
Dissociation Energies ◽  
Correlation Consistent ◽  
Equilibrium Geometries

Abstract. Ab initio investigation on the reaction mechanism of ClO + O --> Cl + O2 reaction has been performed using correlation consistent triple zeta basis set. The geometry and frequency of the reactants, products, minimum energy geometries and transition states are obtained using MP2 method and energetics are obtained at the QCISD(T)//MP2 level of theory. Primarily, a possible reaction mechanism is obtained on the basis on IRC calculations using MP2 level of theory. To obtain true picture of the reaction path, we performed IRC calculations using CASSCF method with a minimal basis set 6-31G**. Some new equilibrium geometries and transition states have been identified at the CASSCF level. Energetics are also obtained at the QCISD(T)//CASSCF method. Possible reaction paths have been discussed, which are new in literature. Heat of reaction is found to be consistent with the experimental data. Bond dissociation energies to various dissociation paths are also reported.

BO3 Molecular Structures: Examples of the Importance of Electron Correlation

2008 ◽  
Vol 73 (11) ◽  
pp. 1495-1508 ◽  
Author(s):  
Alexander Yu. Sokolov ◽  
Nathan J. Stibrich ◽  
Henry F. Schaefer
Keyword(s):  
Electron Correlation ◽  
Global Minimum ◽  
Isosceles Triangle ◽  
Molecular Structures ◽  
Basis Set ◽  
Oxygen Species ◽  
Higher Symmetry ◽  
Wide Range ◽  
Spectroscopic Identification ◽  
Correlation Consistent

In 1991 Burkholder and Andrews reported the spectroscopic identification of the boron- oxygen species BO, BO2, B2O2, B2O3, and BO2-. In addition, they tentatively identified two infrared features due to BO3. In this research, a wide range of possible BO3 structures is considered theoretically. The highest level of theory used involves the CCSD(T) method with an augmented correlation consistent quadruple zeta basis set. A planar structure O-B-O-O is predicted to be the global minimum, lying 4.2 kcal mol-1 below a higher symmetry (C2v) structure incorporating a BO2 isosceles triangle. Reasonable agreement is found between the theoretical vibrational frequencies and the two fundamentals reported by Burkholder and Andrews. The potentially important ozone adduct B-O3 is predicted to lie much higher in energy.

scholarly journals How Good Are Polarizable and Flexible Models for Water: Insights from a Many-Body Perspective

2020 ◽  
Author(s):  
Eleftherios Lambros ◽  
Francesco Paesani
Keyword(s):  
Energy Surface ◽  
Water Clusters ◽  
Minimum Energy ◽  
Basis Set ◽  
Coupled Cluster ◽  
Ambient Conditions ◽  
Many Body ◽  
Systematic Analysis ◽  
Cluster Data ◽  
Flexible Models

<div> <div> <div> <p>We present a systematic analysis of state-of-the-art polarizable and flexible water models from a many-body perspective, with a specific focus on their ability to represent the Born-Oppenheimer potential energy surface of water, from the gas to the liquid phase. Using coupled cluster data in the completed basis set limit as a reference, we examine the accuracy of the polarizable models in reproducing individual many-body contributions to interaction energies and harmonic frequencies of water clusters, and compare their performance with that of MB-pol, an explicit many-body model that has been shown to correctly predict the properties of water across the entire phase diagram. Based on these comparisons, we use MB-pol as a reference to analyze the ability of the polarizable models to reproduce the energy landscape of liquid water at ambient conditions. We find that, while correctly reproducing the energetics of minimum-energy structures, the polarizable models examined in this study suffer from inadequate representations of many-body effects for distorted configurations. To investigate the role played by geometry-dependent representations of 1-body charge distributions in reproducing coupled cluster data for both interaction and many-body energies, we introduce a simplified version of MB-pol that adopts fixed atomic charges and demonstrate that the new model retains the same accuracy as the original MB-pol model. Based on the analyses presented in this study, we believe that future developments of both polarizable and explicit many-body models should continue in parallel and would benefit from synergistic efforts aimed at integrating the best aspects of the two theoretical/computational frameworks. </p> </div> </div> </div>

scholarly journals Hyperfine excitation of NS+ due to para-H2(j = 0) impact

2019 ◽  
Vol 487 (4) ◽  
pp. 5685-5691 ◽  
Author(s):  
Cheikh T Bop
Keyword(s):  
Cross Sections ◽  
Basis Set ◽  
Rate Coefficients ◽  
Cluster Method ◽  
Close Coupling ◽  
Nitrogenous Compounds ◽  
Deep Well ◽  
Explicitly Correlated ◽  
Correlation Consistent ◽  
Triple Excitation

ABSTRACT Sulphur bearing nitrogenous compounds have been observed in space over this last decade. Modelling their abundances has been done using rate coefficients of isoelectronic molecules. In order to satisfy the astrophysical precision required, we report the actual rate coefficients of NS+ induced by collision with the most abundant interstellar species (para-H2). Considering the 23 low-lying rotational levels of NS+, we were able to compute the (hyperfine) rate coefficients up to 100 K. These latter were carried out by averaging cross-sections over the Maxwell–Boltzmann velocity distribution. The state-to-state inelastic cross-sections were determined in the quantum mechanical close coupling approach for total energies ranging up to 1400 cm−1. These dynamic data result from a four dimensional potential energy surface (4D-PES) which was spherically averaged over the H2 orientations. The 4D-PES was calculated using the explicitly correlated coupled cluster method with simple, double, and non-iterative triple excitation (CCSD(T)–F12) connected to the augmented–correlation consistent–polarized valence triple zeta Gaussian basis set (aug–cc–pVTZ). The so-averaged PES presents a very deep well of 596.72 cm−1 at R = 5.94 a0 and θ1 = 123.20°. Discussions on the propensity rules for the (hyperfine) rate coefficients were made and they are in favour of (Δj = ΔF) Δj = 1 transitions. The results presented here may be crucially needed in order to accurately model the NS+ abundance in space. In addition, we expect that this paper will encourage investigations on the sulphur bearing nitrogenous compounds.

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.

Low-lying states of MX2 (M = Ag, Au; X = Cl, Br and I) with coupled-cluster approaches: effect of the basis set, high level correlation and spin–orbit coupling

2020 ◽  
Vol 22 (45) ◽  
pp. 26178-26188
Author(s):  
Xingxing Pang ◽  
Minggang Guo ◽  
Zhifan Wang ◽  
Fan Wang
Keyword(s):  
Electron Correlation ◽  
Orbit Coupling ◽  
Basis Set ◽  
Coupled Cluster ◽  
Spin Orbit Coupling ◽  
Spin Orbit ◽  
Jahn Teller ◽  
High Level ◽  
Correlation Level

Spin–orbit coupling, electron correlation level and basis set are important in describing Renner–Teller and pseudo-Jahn–Teller effects and properties of MX2.

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