Search results for "EXCITATION"
showing 10 items of 1290 documents
Coupled cluster calculations of the vertical excitation energies of tetracyanoethylene
2003
The vertical spectrum of tetracyanoethylene was studied using coupled cluster theory. It was found that the lowest singlet-singlet transition, which corresponds to the excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) excitation, occurs at 5.16 eV in the gas phase and is lowered approximately 0.1 eV due to solvent effects in acetonitrile. A parallel study on the ethene spectrum showed the quality of the basis sets and methods used, by placing the V state 7.92 eV above the ground state and giving an energy for the 0-0 transition of 5.42 eV to be compared with the experimental value of 5.50 eV.
Analytic second derivatives for general coupled-cluster and configuration-interaction models.
2004
Analytic second derivatives of energy for general coupled-cluster (CC) and configuration-interaction (CI) methods have been implemented using string-based many-body algorithms. Wave functions truncated at an arbitrary excitation level are considered. The presented method is applied to the calculation of CC and CI harmonic frequencies and nuclear magnetic resonance chemical shifts up to the full CI level for some selected systems. The present benchmarks underline the importance of higher excitations in high-accuracy calculations.
Ground and excited state geometries via Mukherjee’s multireference coupled-cluster method
2012
Abstract A comprehensive study of molecular equilibrium structures is conducted to benchmark the multireference coupled-cluster (CC) method suggested by Mukherjee and coworkers (Mk-MRCC). We determine equilibrium structures and adiabatic excitation energies by applying the Mk-MRCC method within the singles and doubles (SD) approximation to ground and excited states of various small and medium-sized molecules. The results are compared to those obtained using other multireference or single-reference CC methods. For most molecules with a multireference ground state, it is found that equilibrium structures and excitation energies computed at the Mk-MRCCSD, equation-of-motion CCSD, multireferenc…
Triple excitation effects in coupled cluster calculations of Verdet constants
2000
Abstract The CC3 approach has been employed to calculate the Verdet constants of N 2 ,C 2 H 2 , and CH 4 . For N 2 and C 2 H 2 , relatively large triples contributions are found which need to be included in order to reach close agreement with the experimental constants.
A simple scheme for the direct calculation of ionization potentials with coupled-cluster theory that exploits established excitation energy methods
1999
Vertical ionization potentials can be obtained from existing computer programs for the high-level treatment of excited states by simply including a continuum orbital in the basis set. Exploiting this feature easily allows final state energies for ionized states to be calculated at several previously untested levels of theory that go beyond the equation-of-motion coupled-cluster singles and doubles model. Values obtained for N2, CO, and F2 with the most theoretically complete approximations studied here (those based on the CCSDT-3 and CC3 parametrizations of the neutral ground state) are in excellent agreement with experiment when a large basis set is used.
Calculation of frequency-dependent polarizabilities using general coupled-cluster models
2006
Abstract An analytic scheme for the calculation of frequency-dependent polarizabilities within a response-theory approach has been implemented for the use within general coupled-cluster (CC) models with arbitrary excitations in the cluster operator. Calculations for CH + and CN demonstrate the fast convergence of the coupled-cluster approach when successively higher excitations are considered. Quadruple excitation effects on the frequency-dependent polarizabilities are found to be rather small except close to the poles.
Frequency-dependent polarizabilities and first hyperpolarizabilities of CO and H2O from coupled cluster calculations
1999
Abstract Frequency-dependent electronic polarizabilities and first hyperpolarizabilities for CO and H 2 O have been investigated in coupled-cluster response theory calculations. Triple excitation effects have been considered by means of the CC3 model which has recently been implemented for frequency-dependent polarizabilities and first hyperpolarizabilities. The final estimates for polarizabilities and first hyperpolarizabilities are in good agreement with experimental results.
The effect of triple excitations in coupled cluster calculations of frequency-dependent polarizabilities
1998
Abstract Frequency-dependent polarizabilities have been implemented within the CC3 model. Comparison with full configuration interaction results shows that the triple excitation effects included in CC3 lead to a significantly improved treatment for the dispersion of the polarizability of CH+. For the refractivity and polarizability anisotropy of N2, better agreement with experiment is found for the CC3 results than for those obtained with the coupled cluster singles and doubles model.
Linear-response theory for Mukherjee's multireference coupled-cluster method: Excitation energies
2012
The recently presented linear-response function for Mukherjee's multireference coupled-cluster method (Mk-MRCC) [T.-C. Jagau and J. Gauss, J. Chem. Phys. 137, 044115 (2012)] is employed to determine vertical excitation energies within the singles and doubles approximation (Mk-MRCCSD-LR) for ozone as well as for o-benzyne, m-benzyne, and p-benzyne, which display increasing multireference character in their ground states. In order to assess the impact of a multireference ground-state wavefunction on excitation energies, we compare all our results to those obtained at the single-reference coupled-cluster level of theory within the singles and doubles as well as within the singles, doubles, and…
Electronic Excitation Energy Transfer in Multichromophoric Assemblies: A Single Molecule Insight
2009
In recent years, single molecule spectroscopy has provided novel insights into the fundamentals of electronic excitation energy transfer in molecular aggregates. In order of increasing structural complexity, we have studied simple molecular dimers and multichromophoric dendrimers. It will be shown that the combination of frequency-selective single molecule spectroscopy and confocal fluorescence microscopy at 1.4 K is a unique tool to study energy transfer processes in these systems. In particular, from the line widths of single molecule excitation spectra, rate constants of energy transfer can be deduced directly. A detailed analysis shows that, for several cases, the mechanism of energy tr…