Multireference Brillouin-Wigner Coupled Cluster Singles and Doubles (MRBWCCSD) and Multireference Doubles Configuration Interaction (MRD-CI) Calculations for the Bergman Cyclization Reaction

2003 ◽  
Vol 68 (12) ◽  
pp. 2309-2321 ◽  
Author(s):  
Oscar Rey Puiggros ◽  
Jiří Pittner ◽  
Petr Čársky ◽  
Philipp Stampfuss ◽  
Wolfgang Wenzel
Keyword(s):  
Cluster Expansion ◽  
Configuration Interaction ◽  
Cyclization Reaction ◽  
Basis Set ◽  
Coupled Cluster ◽  
Bergman Cyclization ◽  
Lower Accuracy ◽  
Benchmark System ◽  
Ci Calculations ◽  
Good Agreement

We used the Bergman cyclization reaction of hex-3-ene-1,5-diyne to 1,4-didehydrobenzene (p-benzyne) as a benchmark system to assess the accuracy of recently developed multireference Brillouin-Wigner coupled cluster singles and doubles method (MRBWCCSD) in comparison with the multireference doubles configuration interaction (MRD-CI) calculations using the same geometry and basis set. Activation and reaction enthalpies were calculated. We found good agreement between experiment and theory at the MRCI+Q/CCSD(T) level of theory, provided a sufficiently large polarized basis set is used (cc-pVTZ). The MRBWCCSD theory gives results of a somewhat lower accuracy, presumably because of the absence of T3 clusters in the cluster expansion.

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 A test of composite natural orbitals for benzene

1992 ◽  
Vol 70 (2) ◽  
pp. 532-536 ◽  
Author(s):  
Yuichi Yamamoto ◽  
Takeshi Noro ◽  
Kimio Ohno
Keyword(s):  
Excited State ◽  
Configuration Interaction ◽  
Benzene Molecule ◽  
Natural Orbital ◽  
Excitation Energies ◽  
Natural Orbitals ◽  
Experimental Values ◽  
Ci Calculations ◽  
Good Agreement

Approximate natural orbitals (NO's) of a larger system can be constructed from the NO's of smaller fragment systems. These orbitals, called composite NO's (CNO's) are expected to be useful in configuration interaction (CI) calculations. The effectiveness of these NO's is shown for the benzene molecule. This molecule is considered a combination of three ethylenes. The CI calculations were carried out for the S1 – S3 and T1 – T3 states. We take into account single and double excitations from σ and π electrons in the CI calculations. The calculated excitation energies are in good agreement with the experimental values Keywords: benzene, π–π* excited state, composite natural orbital, ionic and covalent, SDCI.

Computational Study on the Kinetics and Mechanisms of Unimolecular Reactions of (Z)-(Z)-N-(λ5-phosphanylidene) Formohydrazonic Formic Anhydride: Intramolecular aza-Wittig Reaction in Competition with Mumm Rearrangement

2020 ◽  
Vol 17 (11) ◽  
pp. 884-889
Author(s):  
Somayeh Mirdoraghi ◽  
Hamed Douroudgari ◽  
Farideh Piri ◽  
Morteza Vahedpour
Keyword(s):  
Rate Constants ◽  
Density Functional ◽  
Wittig Reaction ◽  
Computational Study ◽  
Single Step ◽  
Low Energy ◽  
Coupled Cluster ◽  
Unimolecular Reaction ◽  
Single Step Reaction ◽  
Good Agreement

For (Z)-(Z)-N-(λ5-phosphanylidene) formohydrazonic formic anhydride, Aza-Wittig reaction and Mumm rearrangement are studied using both density functional and coupled cluster theories. For this purpose, two different products starting from one substrate are considered that are competing with each other. The obtained products, P1 and P2, are thermodynamically favorable. The product of the aza-Wittig reaction, P1, is more stable than the product of Mumm rearrangement (P2). For the mentioned products, just one reliable pathway is separately proposed based on unimolecular reaction. Therefore, the rate constants based on RRKM theory in 300-600 K temperature range are calculated. Results show that the P1 generation pathway is a suitable path due to low energy barriers than the path P2. The first path has three steps with three transition states, TS1, TS2, and TS3. The P2 production path is a single-step reaction. In CCSD level, the computed barrier energies are 14.55, 2.196, and 10.67 kcal/mol for Aza-Wittig reaction and 42.41 kcal/mol for Mumm rearrangement in comparison with the corresponding complexes or reactants. For final products, the results of the computational study are in a good agreement with experimental predictions.

scholarly journals Potential Functions and Thermodynamic Properties of UC, UN, and UH

2020 ◽  
Vol 2020 ◽  
pp. 1-6
Author(s):  
Shuang-Ling Tang ◽  
Yu Wang ◽  
Qi-Ying Xia ◽  
Xue-Hai Ju
Keyword(s):  
Configuration Interaction ◽  
Density Functional ◽  
Least Square Method ◽  
Potential Functions ◽  
Least Square ◽  
Basis Set ◽  
Dissociation Energies ◽  
Anharmonicity Constant ◽  
Different Temperatures ◽  
Relationship Of

Potential energy surface scanning for UC, UN, and UH was performed by configuration interaction (CI), coupled cluster singles and doubles (CCSD) excitation, quadratic configuration interaction (QCISD (T)), and density functional theory PBE1 (DFT-PBE1) methods in coupling with the ECP80MWB_AVQZ + 2f basis set for uranium and 6 − 311 + G∗ for carbon, hydrogen, and nitrogen. The dissociation energies of UC, UN, and UH are 5.7960, 4.5077, and 2.6999 eV at the QCISD (T) levels, respectively. The calculated energy was fitted to the potential functions of Morse, Lennard-Jones, and Rydberg by using the least square method. The anharmonicity constant of UC is 0.0047160. The anharmonic frequency of UC is 780.27 cm−1 which was obtained based on the PBE1 results. For UN, the anharmonicity constant is 0.0049827. The anharmonic frequency is 812.65 cm−1 which was obtained through the PBE1 results. For UH, the anharmonicity constant is 0.017300. The anharmonic frequency obtained via the QCISD (T) results is 1449.8 cm−1. The heat capacity and entropy in different temperatures were calculated using anharmonic frequencies. These properties are in good accordance with the direct DFT-UPBE1 results (for UC and UN) and QCISD (T) results (for UH). The relationship of entropy with temperature was established.

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