REDUCED MULTIREFERENCE COUPLED-CLUSTER METHOD AND ITS APPLICATION TO THE PYRIDYNE DIRADICALS

2008 ◽  
Vol 07 (04) ◽  
pp. 805-820 ◽  
Author(s):  
XIANGZHU LI ◽  
JOSEF PALDUS
Keyword(s):  
Molecular Geometry ◽  
External Source ◽  
Cluster Method ◽  
Coupled Cluster ◽  
Triplet States ◽  
Correlation Effects ◽  
Spin Multiplicity ◽  
Triplet Splitting ◽  
Singlet And Triplet States

The reduced multireference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that employs a modest-size MR CISD wave function as an external source for the most important (primary) triples and quadruples in order to account for the nondynamic correlation effects in the presence of quasidegeneracy, and which is further perturbatively corrected for the remaining (secondary) triples, RMR CCSD(T), is employed to compute the molecular geometry and the energy of the lowest-lying singlet and triplet states, as well as the corresponding singlet–triplet splitting, for all possible isomers of the m, n-pyridyne diradicals. A comparison is made with earlier results that were obtained by other authors, and the role of the multireference effects for both the geometry and the spin multiplicity of the lowest state, as described by the RMR-type methods, is demonstrated on the example of 2,6- and 3,5-pyridynes.

Singlet-Triplet Splitting in Methylene: An Accurate Description of Dynamic and Nondynamic Correlation by Reduced Multireference Coupled Cluster Method

1998 ◽  
Vol 63 (9) ◽  
pp. 1381-1393 ◽  
Author(s):  
Xiangzhu Li ◽  
Josef Paldus
Keyword(s):  
Basis Sets ◽  
Cluster Method ◽  
Coupled Cluster ◽  
Triplet States ◽  
Correlation Effects ◽  
Triplet Splitting ◽  
Double Zeta ◽  
Reference Space ◽  
Singlet And Triplet States ◽  
Atomic Natural Orbital

A classical multireference problem - the singlet-triplet separation in methylene - is examined by the recently introduced reduced multireference (RMR) singles and doubles coupled cluster (CCSD) method, using both double zeta plus polarization (DZP) and large atomic natural orbital (ANO) basis sets. In the former case, the performance of the RMR CCSD as well as of other approaches is assessed by a comparison with the full configuration interaction (FCI) result that represents the exact solution for this basis, while in the latter case a comparison is made with the experiment. It is shown that using a minimal two-configuration reference space, the RMR CCSD result compares well with either FCI or experiment; and is of the same quality as that provided by the two-reference state universal MR CCSD theory. Both MR CCSD approaches give a balanced description for the singlet and triplet states involved and correct the shortcomings of the single reference CCSD approach that is lacking in the presence of nondynamical correlation effects.

Energetics of naphthynes — Performance of reduced multi-reference coupled-cluster methods for diradicals

2009 ◽  
Vol 87 (7) ◽  
pp. 917-926 ◽  
Author(s):  
Xiangzhu Li ◽  
Josef Paldus
Keyword(s):  
External Source ◽  
Basis Set ◽  
Coupled Cluster ◽  
Triplet States ◽  
Spin Multiplicity ◽  
Doubly Excited ◽  
Diradical Character ◽  
Singlet And Triplet States ◽  
Cluster Methods ◽  
The Relationship

The energetics of all the possible isomers of the didehydronaphthalene (naphthalyne or naphthyne) diradical has been examined using the reduced multi-reference (RMR) coupled-cluster (CC) method with singles and doubles (RMR CCSD) that employs a modest size MR CISD wave function as an external source of the most important (primary) triples and quadruples to account for nondynamic correlation effects in the presence of quasi-degeneracy, as well as by its perturbatively corrected version for the secondary triples, the RMR CCSD(T) method. The resulting energies are compared with those obtained by the standard single-reference (SR) CCSD and CCSD(T) approaches. In all cases, we used both the cc-pVDZ basis set, as well as its restricted version cc-pVDZ* with deleted p-functions on hydrogens. Once the optimal geometry for each isomer was found, we computed the energy of the lowest-lying singlet and triplet states using the above mentioned CC methods, as well as the implied singlet–triplet splittings. These results enabled us to classify the isomers into three groups according to their stability and to determine, whenever possible, the spin multiplicity of the ground state. Finally, we point out the relationship between the extent of the diradical character of naphthyne isomers, the degree of their MR nature, the distance separating the radical centers, and, finally, the size of the largest doubly-excited cluster amplitude in their CC wave functions.

Linear Dichroism Spectroscopy of Retinal with Picosecond Time Resolution

1985 ◽  
Vol 40 (11-12) ◽  
pp. 880-885
Author(s):  
M. E. Lippitsch ◽  
M. Riegler ◽  
F. R. Aussenegg ◽  
N. Friedman ◽  
M. Sheves ◽  
...  
Keyword(s):  
Transient Absorption ◽  
Molecular Geometry ◽  
Linear Dichroism ◽  
Triplet States ◽  
Polyethylene Films ◽  
Transient Absorption Spectra ◽  
Time Regime ◽  
Singlet And Triplet States ◽  
First Time ◽  
Time Linear

Abstract For the first time linear dichroism spectroscopy has been extended to the picosecond time regime. 11-cis retinal, all-trans retinal and 1,8-diphenyl-1,3,5,7-octatetraene (DPOT) are incorporated into polyethylene films and oriented by stretching the films. By measuring picosecond transient absorption spectra polarized parallel and perpendicular to the stretching direction and calculating the dichroic ratio we get informations about the molecular geometry in excited singlet and triplet states. The results may have relevance to vision.

Externally Corrected Coupled-Cluster Approaches: Energy versus Amplitude Corrected CCSD

2003 ◽  
Vol 68 (3) ◽  
pp. 554-586 ◽  
Author(s):  
Josef Paldus ◽  
Xiangzhu Li
Keyword(s):  
Wave Function ◽  
Vibrational Energy ◽  
Energy Levels ◽  
External Source ◽  
Coupled Cluster ◽  
Correlation Effects ◽  
Hartree Fock ◽  
Versus Amplitude ◽  
Vibrational Energy Levels ◽  
Cluster Methods

The externally corrected coupled-cluster methods with singles and doubles (ecCCSD), which exploit some independently available wave function as a source of higher-than-pair clusters, are considered. The focus is on methods that employ a modest-size multireference (MR) configuration interaction (with singles and doubles, CISD) wave function as the external source. Both the amplitude- and energy-corrected CCSD methods are employed, the former correcting the standard single reference (SR) CCSD equations for triples and quadruples, while the latter accounts for the nondynamic correlation effects when evaluating the energy by employing the MR CISD wave function in lieu of the single determinantal (usually Hartree-Fock) reference in the asymmetric energy formula. The performance and relationship of both types of approaches is illustrated by computing the rotational and vibrational energy levels using the potential generated by these various methods and by comparing the calculated spectra with the experimental ones for the simplest first-row hydride, namely the LiH molecule. A special attention is paid to the role of core-correlation effects, in which case we also consider the HF molecule.

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