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.

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.

The Fock-Space Coupled-Cluster Method in the Calculation of Excited State Properties

2005 ◽  
Vol 70 (6) ◽  
pp. 811-825 ◽  
Author(s):  
Monika Musiał ◽  
Leszek Meissner
Keyword(s):  
Experimental Data ◽  
Excited States ◽  
Fock Space ◽  
Hamiltonian Formulation ◽  
Basis Sets ◽  
Cluster Method ◽  
Coupled Cluster ◽  
Coupled Cluster Method ◽  
Excitation Energies ◽  
Equilibrium Geometries

Our recently developed intermediate Hamiltonian Fock-space coupled-cluster (FS-CC) method with singles and doubles is applied to calculation of equilibrium geometries, harmonic frequencies and adiabatic excitation energies for some excited states of N2 and CO. Due to the intermediate Hamiltonian formulation, which provides a robust computational scheme for solving the FS-CC equations, and the efficient factorization strategy, relatively large basis sets and reference spaces are employed permitting a comparison of calculated properties with experimental data.

Singlet and Triplet States of Aminonitrenes: An Ab Initio Molecular Orbital Study

1973 ◽  
Vol 51 (20) ◽  
pp. 3303-3308 ◽  
Author(s):  
N. C. Baird ◽  
R. F. Barr
Keyword(s):  
Ab Initio ◽  
Molecular Orbital ◽  
Molecular Orbital Calculations ◽  
Triplet States ◽  
Density Distributions ◽  
Molecular Orbital Study ◽  
Triplet Splitting ◽  
Optimum Geometry ◽  
Singlet And Triplet States ◽  
Carbonyl Substitution

Ab initio STO-3G molecular orbital calculations are reported for the lowest singlet and the lowest triplet state of the aminonitrenes H2NN, CH3(H)NN, F(H)NN, and HCO(H)NN. The optimum geometry calculated for the H2NN triplet is nonplanar (Cs symmetry), with N—N and N—H bond distances of 1.436 and 1.034 Å, and HNN and HNH angles both of 107.5°. The optimum singlet is predicted to be planar (C2v symmetry), with N—N and N—H distances of 1.276 and 1.041 Å respectively, and an HNN angle of 124°. An analysis of the calculated geometries and electron density distributions indicates that the "idealized" representations 3 and 5 are adequate to explain the bonding in the triplet and singlet respectively. The effects of methyl, fluoro, and carbonyl substitution upon the singlet–triplet splitting and upon the charge distribution are discussed. The optimum-geometry triplet is predicted to be slightly more stable than the optimum-geometry singlet in all cases.

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.

COUPLED-CLUSTER APPROACH TO CORRELATION IN SMALL MOLECULES: ENERGY VERSUS AMPLITUDE CORRECTED METHODS

2003 ◽  
Vol 17 (28) ◽  
pp. 5379-5391 ◽  
Author(s):  
J. PALDUS ◽  
X. LI
Keyword(s):  
Small Molecules ◽  
Cluster Method ◽  
Coupled Cluster ◽  
Correlation Effects ◽  
Cluster Approach ◽  
Fertile Ground ◽  
Electron Correlation Effects ◽  
Molecular Electronic Structure ◽  
Versus Amplitude ◽  
The Many

The coupled cluster method, which is based on the exponential Ansatz for the wave operator, found a particularly fertile ground in quantum chemical investigations of the molecular electronic structure, where the many-electron correlation effects play a crucial role and where this approach became the method of choice, particularly when handling nondegenerate ground states. The objective of this contribution is to provide a brief historical perspective on some current challenges of the coupled cluster methodology and to outline some most recent attempts at their solution.

The relativistic coupled-cluster method: transition energies of bismuth and eka-bismuth

1998 ◽  
Vol 94 (1) ◽  
pp. 181-187 ◽  
Author(s):  
EPHRAIM ELIAV ◽  
UZI KALDOR ◽  
YASUYUKI ISHIKAWA
Keyword(s):  
Cluster Method ◽  
Coupled Cluster ◽  
Coupled Cluster Method ◽  
Transition Energies
Sign in / Sign up

Export Citation Format

Share Document