Search results for "Quantum"
showing 10 items of 9714 documents
Analytic calculation of the diagonal Born-Oppenheimer correction within configuration-interaction and coupled-cluster theory
2006
Schemes for the analytic calculation of the diagonal Born-Oppenheimer correction (DBOC) are formulated and implemented for use with general single-reference configuration-interaction and coupled-cluster wave function models. Calculations are reported to demonstrate the convergence of the DBOC with respect to electron-correlation treatment and basis set as well as to investigate the size-consistency error in configuration-interaction calculations of the DBOC. The importance of electron-correlation contributions to the DBOC is illustrated in the computation of the corresponding corrections for the reaction energy and activation barrier of the F + H2 --FH + H reaction as well as of the atomiza…
Analytic first derivatives for a spin-adapted open-shell coupled cluster theory: Evaluation of first-order electrical properties
2014
An analytic scheme is presented for the evaluation of first derivatives of the energy for a unitary group based spin-adapted coupled cluster (CC) theory, namely, the combinatoric open-shell CC (COSCC) approach within the singles and doubles approximation. The widely used Lagrange multiplier approach is employed for the derivation of an analytical expression for the first derivative of the energy, which in combination with the well-established density-matrix formulation, is used for the computation of first-order electrical properties. Derivations of the spin-adapted lambda equations for determining the Lagrange multipliers and the expressions for the spin-free effective density matrices for…
A MRCI PS and CASSCF study of the ground state MgO dissociation energy
1991
Abstract Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1 Σ + state of MgO, which adiabatically dissociates to the ground state 1 S g of magnesium and to the excited 1 D g state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) an…
CASSCF/CASPT2 analysis of the fragmentation of H2 on a Pd4 cluster
2009
Two reactive pathways are described for the dissociative adsorption of a hydrogen molecule on a Pd4 pyramidal cluster, by using DFT/B3LYP and CASSCF/CASPT2 computational methods. Because of the different spin multiplicities of the cluster in the initial and final states, the reaction proceeds through a spin multiplicity change, which occurs close to the transition states. The activation energy values are very similar when the DFT method is used, whereas the CASPT2 calculations predict that the concerted mechanism is disfavored. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010
On the relaxation mechanisms of 6-azauracil
2011
The nonadiabatic photochemistry of 6-azauracil has been studied by means of the CASPT2//CASSCF protocol and double-ζ plus polarization ANO basis sets. Minimum energy states, transition states, minimum energy paths, and surface intersections have been computed in order to obtain an accurate description of several potential energy hypersurfaces. It is concluded that, after absorption of ultraviolet radiation (248 nm), two main relaxation mechanisms may occur, via which the lowest (3)(ππ*) state can be populated. The first one takes place via a conical intersection involving the bright (1)(ππ*) and the lowest (1)(nπ*) states, ((1)ππ*/(1)nπ*)(CI), from which a low-energy singlet-triplet crossin…
Magnetic exchange interaction in clusters of orbitally degenerate ions. I. Effective Hamiltonian
2001
Abstract A new effective Hamiltonian is reported for the kinetic exchange between two arbitrary terms 2S A +1 Λ A and 2S B +1 Λ B that can be ground or excited in octahedrally coordinated transition metal ions. This Hamiltonian is applicable to both homo- and heterometallic clusters. For the homonuclear cluster the resonance part of the effective Hamiltonian is also presented for the case when one of the ions is excited. The operator part of the exchange Hamiltonian contains symmetry adapted products of the cubic irreducible tensors acting in orbital spaces ΛA and ΛB and scalar product of site spin operators. The parameters of the Hamiltonian are defined by the relevant intercenter transfer…
On a topological interpretation of electronic and vibrational molecular energies
1998
Abstract A relationship between Randic's connectivity index and various quantum mechanical parameters derived from the Huckel Molecular Orbital (HMO) approach is demonstrated. When applied to conjugated hydrocarbons, this index represents the measure of the global π electron molecular energy and, therefore, of the resonance energy. Moreover, the development of the procedure, allows the introduction of a new definition of the bond order which, in turn, makes possible a better prediction not only for bond lengths of naphtalene but also for the resonance integral and conjugation energy for butadiene. Also, a corrected value for the Randic index is deduced, which allows for the reduction of the…
Theoretical Study on the Structures and Electronic Spectra of TCNE2−
2006
Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) compiles have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE 2- has D 2d symmetry in vacuum as well as in the solvents dichloromethane and cicetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoreitcal results, are compared to the experimental data and good agreement is achieved.
Excitation of Nucleobases from a Computational Perspective I: Reaction Paths
2013
The main intrinsic photochemical events in nucleobases can be described on theoretical grounds within the realm of non-adiabatic computational photochemistry. From a static standpoint, the photochemical reaction path approach (PRPA), through the computation of the respective minimum energy path (MEP), can be regarded as the most suitable strategy in order to explore the electronically excited isolated nucleobases. Unfortunately, the PRPA does not appear widely in the studies reported in the last decade. The main ultrafast decay observed experimentally for the gas-phase excited nucleobases is related to the computed barrierless MEPs from the bright excited state connecting the initial Franck…
Superradiance and Exciton (De)localization in Light-Harvesting Complex II from Green Plants?
2002
Fluorescence quantum yield and fluorescence lifetime measurements were performed on trimeric light-harvesting complex II (LHCII) from spinach in the temperature range 7−293 K. From the results the radiative rate was calculated, which is related to the amount of delocalization of excitations over different pigments because of intermolecular interactions. The emitting dipole strength of LHCII is very similar to that of unbound Chl a, and it appears to be almost independent of temperature. The apparent increase of the radiative rate upon lowering the temperature can largely be explained by the shrinking of the sample. It is concluded that at all temperatures the amount of exciton delocalizatio…