Search results for "Molecular Orbital Method"

showing 3 items of 3 documents

Theoretical study of the low‐lying states of trans‐1,3‐butadiene

1992

We present extensive ab initio calculations on the low‐lying electronic states of trans‐1,3‐butadiene within the multireference configuration interaction (MRCI) framework by selecting the configurations with a perturbative criterion. The X 1Ag ground state and 1 3Bu, 1 3Ag, 2 1Ag, and 1 1Bu valence excited states have been calculated at a fixed geometry. The results obtained are in good agreement with previous experimental and calculated values, and could help to understand polyene spectroscopy, photochemistry, and photophysics. The advantages of a MRCI method where the most important contributions to the total MRCI wave function, perturbatively selected, are treated variationally, and the …

ButadieneElectronic correlationChemistryGeneral Physics and AstronomyMultireference configuration interactionPolyenesConfiguration interactionChromophoresUNESCO::FÍSICA::Química físicaConfiguration InteractionComputational chemistryAb initio quantum chemistry methodsExcited stateButadiene ; Ab Initio Calculations ; Configuration Interaction ; Perturbation Theory ; Energy Levels ; Molecular Orbital Method ; Polyenes ; Biology ; ChromophoresEnergy LevelsPerturbation TheoryPerturbation theory (quantum mechanics)Molecular Orbital MethodPhysical and Theoretical ChemistryAtomic physicsGround stateWave function:FÍSICA::Química física [UNESCO]Ab Initio CalculationsBiology
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The integral‐direct coupled cluster singles and doubles model

1996

An efficient and highly vectorized implementation of the coupled cluster singles and doubles (CCSD) model using a direct atomic integral technique is presented. The minimal number of n6processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of integral prescreening. © 1996 American Institute of Physics.

Computer scienceClose Coupling ApproximationSymmetry GroupsGeneral Physics and AstronomyBasis functionSymmetry groupUNESCO::FÍSICA::Química físicaComputational scienceCluster ModelClose Coupling Approximation ; Algorithms ; Cluster Model ; Electronic Structure ; Molecular Orbital Method ; Symmetry GroupsPhysics and Astronomy (all)Range (mathematics)Coupled clusterElectronic StructureComputational chemistryCluster (physics)Molecular symmetryMolecular Orbital MethodPhysical and Theoretical Chemistry:FÍSICA::Química física [UNESCO]Direct-coupled amplifierAlgorithmsThe Journal of Chemical Physics
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Conformational investigation of alpha,beta-dehydropeptides. IX. N-Acetyl-(E)-alpha,beta-methylamide: stereoelectronic properties from infrared and th…

1998

The Fourier transform infrared spectra of Ac-(E)-deltaAbu-NHMe were analyzed to determine the predominant solution conformation(s) of this (E)-alpha,beta-dehydropeptide-related compound and the electron density perturbation in its amide groups. The measurements were performed in dichloromethane and acetonitrile in the region of mode vs (N-H), amide I, amide II and vs (C(alpha)=Cbeta). The equilibrium geometrical parameters, calculated by a method based on the density functional theory with the B3LYP functional and the 6-31G* basis set, were used to support spectroscopic interpretation and gain some deeper insight into the molecule. The experimental and theoretical data were compared with th…

β-dehydroamino acidsFTIR spectroscopyα(E)-αMolecular orbital methodAmidic resonanceC5 hydrogen bondβ- dehydrobutyrineC5 conformationSolution peptide conformationDensity functionalsJOURNAL OF PEPTIDE RESEARCH
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