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.

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.

Ab initio molecular orbital studies of the potential energy curves for the low lying triplet states of Be2 and Be3

1988 ◽  
Vol 66 (8) ◽  
pp. 2034-2040 ◽  
Author(s):  
Ratnakar K. Gosavi ◽  
Otto P. Strausz
Keyword(s):  
Potential Energy ◽  
Triplet State ◽  
Ab Initio ◽  
Potential Energy Curves ◽  
Basis Set ◽  
Triplet States ◽  
Diradical Character ◽  
State Formula ◽  
State Potential ◽  
Gaussian Basis Set

Ab initio calculations with uniform quality gaussian basis set were carried out at the RHF-SCF and CI level on the potential energy curves of the low lying triplet states of Be2 and Be3. The lowest excited state, the [Formula: see text] state of Be2 is 26.0 kcal/mol higher in energy than the ground [Formula: see text] state, and 39.4 kcal/mol lower than the separated Be(1S0) + Be(3P) atoms, with the s, p, d basis set. The next higher triplet state, the 3Πg, is only 8.7 kcal/mol above the lowest [Formula: see text] state. The [Formula: see text] and 3Πu states lie comparatively much higher than the [Formula: see text] state. All the triplet state potential curves have a bonding nature. The lowest triplet state [Formula: see text] in Be3 with D∞h symmetry lies ~15 kcal/mol above the ground [Formula: see text] state, and 48.2 kcal/mol lower than the separated atoms, 2Be(1S0) + Be(3P). The [Formula: see text] state has 1,3-diradical character and in all the higher triplet states α spin electrons are delocalized among the three Be atoms. The next higher triplet state 3Πu is 7.7 kcal/mol above the lowest [Formula: see text] state. The [Formula: see text] and 3Πg states lie much higher than the [Formula: see text] and 3Πu states. Like the triplet states of Be2, all Be3 triplet states have a bonding nature. Reaction path studies on the Be(3P) energy transfer reactions, [Formula: see text] and [Formula: see text] show that these reactions do not feature any activation energy barrier.

Molecular orbital studies of the singlet and triplet states of formylmethylene conformers

1991 ◽  
Vol 69 (11) ◽  
pp. 1630-1635 ◽  
Author(s):  
Ratnakar K. Gosavi ◽  
Manuel Torres ◽  
Otto P. Strausz
Keyword(s):  
Singlet State ◽  
Closed Shell ◽  
Basis Set ◽  
Planar Geometry ◽  
Present Calculation ◽  
Triplet States ◽  
Excited Singlet ◽  
Singlet And Triplet States ◽  
Ci Calculations ◽  
Single Configuration

The energies and geometries of the low-lying electronic states of formylmethylene have been calculated at the SCF and CI levels using 6-31G** basis set. In agreement with previous reports and accumulated experimental observations, the ground state is the carbenoid triplet with a planar geometry. Also, in agreement with all previous single configuration SCF and CI calculations, the lowest excited singlet state is computed to be the nonplanar closed shell carbenoid structure. In contrast, accumulated experimental evidence along with previously reported MC–SCF results require this state to be planar. The present calculation predicts the existence of a (σ–σ) 1,3-diradical 1A′ state, which appears to be identical to the MC–SCF lowest singlet state, but this state lies some 11 kcal/mol above the closed shell carbenoid 1A state. Apparently, single configuration SCF methods are inadequate for the correct description of the electronic manifold of formylmethylene. Key words: formylmethylene, singlet and triplet states, ab initio MO calculations, conformers, molecular structure.

The X40x10 Halogen Bonding Benchmark Revisited: Surprising Importance of (n-1)d Subvalence Correlation

2018 ◽  
Author(s):  
Manoj Kumar Kesharwani ◽  
Nitai Sylvetsky ◽  
Debashree Manna ◽  
Jan M.L. Martin
Keyword(s):  
Noncovalent Interactions ◽  
Halogen Bonding ◽  
Basis Set ◽  
Basis Sets ◽  
Coupled Cluster ◽  
Dissociation Energies ◽  
Explicitly Correlated ◽  
Two Factors ◽  
High Level ◽  
Cluster Methods

<p>We have re-evaluated the X40x10 benchmark for halogen bonding using conventional and explicitly correlated coupled cluster methods. For the aromatic dimers at small separation, improved CCSD(T)–MP2 “high-level corrections” (HLCs) cause substantial reductions in the dissociation energy. For the bromine and iodine species, (n-1)d subvalence correlation increases dissociation energies, and turns out to be more important for noncovalent interactions than is generally realized; ; (n-1)sp subvalence correlation is much less important. The (n-1)d subvalence term is dominated by core-valence correlation; with the smaller cc-pVDZ-F12-PP and cc-pVTZ-F12-PP basis sets, basis set convergence for the core-core contribution becomes sufficiently erratic that it may compromise results overall. The two factors conspire to generate discrepancies of up to 0.9 kcal/mol (0.16 kcal/mol RMS) between the original X40x10 data and the present revision.</p>

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.

scholarly journals Can Popular DFT Approximations and Truncated Coupled Cluster Theory Describe the Potential Energy Surface of the Beryllium Dimer?

2018 ◽  
Vol 71 (10) ◽  
pp. 804 ◽  
Author(s):  
Amir Karton ◽  
Laura K. McKemmish
Keyword(s):  
Potential Energy ◽  
Potential Energy Surface ◽  
Density Functional ◽  
Energy Surface ◽  
Basis Set ◽  
Coupled Cluster ◽  
Good Representation ◽  
Coupled Cluster Methods ◽  
Cluster Methods ◽  
Beryllium Dimer

The potential energy surface (PES) of the ground state of the beryllium dimer poses a significant challenge for high-level ab initio electronic structure methods. Here, we present a systematic study of basis set effects over the entire PES of Be2 calculated at the full configuration interaction (FCI) level. The reference PES is calculated at the valence FCI/cc-pV{5,6}Z level of theory. We find that the FCI/cc-pV{T,Q}Z basis set extrapolation reproduces the shape of the FCI/cc-pV{5,6}Z PES as well as the binding energy and vibrational transition frequencies to within ~10 cm−1. We also use the FCI/cc-pV{5,6}Z PES to evaluate the performance of truncated coupled cluster methods (CCSD, CCSD(T), CCSDT, and CCSDT(Q)) and contemporary density functional theory methods (DFT) methods for the entire PES of Be2. Of the truncated coupled cluster methods, CCSDT(Q)/cc-pV{5,6}Z provides a good representation of the FCI/cc-pV{5,6}Z PES. The GGA functionals, as well as the HGGA and HMGGA functionals with low percentages of exact exchange tend to severely overbind the Be2 dimer, whereas BH&HLYP and M06-HF tend to underbind it. Range-separated DFT functionals tend to underbind the dimer. Double-hybrid DFT functionals show surprisingly good performance, with DSD-PBEP86 being the best performer. Møller–Plesset perturbation theory converges smoothly up to fourth order; however, fifth-order corrections have practically no effect on the PES.

The X40x10 Halogen Bonding Benchmark Revisited: Surprising Importance of (n-1)d Subvalence Correlation

2018 ◽  
Author(s):  
Manoj Kumar Kesharwani ◽  
Nitai Sylvetsky ◽  
Debashree Manna ◽  
Jan M.L. Martin
Keyword(s):  
Noncovalent Interactions ◽  
Halogen Bonding ◽  
Basis Set ◽  
Basis Sets ◽  
Coupled Cluster ◽  
Dissociation Energies ◽  
Explicitly Correlated ◽  
Two Factors ◽  
High Level ◽  
Cluster Methods

<p>We have re-evaluated the X40x10 benchmark for halogen bonding using conventional and explicitly correlated coupled cluster methods. For the aromatic dimers at small separation, improved CCSD(T)–MP2 “high-level corrections” (HLCs) cause substantial reductions in the dissociation energy. For the bromine and iodine species, (n-1)d subvalence correlation increases dissociation energies, and turns out to be more important for noncovalent interactions than is generally realized; ; (n-1)sp subvalence correlation is much less important. The (n-1)d subvalence term is dominated by core-valence correlation; with the smaller cc-pVDZ-F12-PP and cc-pVTZ-F12-PP basis sets, basis set convergence for the core-core contribution becomes sufficiently erratic that it may compromise results overall. The two factors conspire to generate discrepancies of up to 0.9 kcal/mol (0.16 kcal/mol RMS) between the original X40x10 data and the present revision.</p>

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