scholarly journals Towards the Search for Thallium Nuclear Schiff Moment in Polyatomic Molecules: Molecular Properties of Thallium Monocyanide (TlCN)

Atoms ◽  
2019 ◽  
Vol 7 (3) ◽  
pp. 62 ◽  
Author(s):  
A. Kudrin ◽  
A. Zaitsevskii ◽  
T. Isaev ◽  
D. Maison ◽  
L. Skripnikov
Keyword(s):  
Inner Core ◽  
Molecular Properties ◽  
Basis Set ◽  
Cluster Method ◽  
Coupled Cluster ◽  
The Core ◽  
Complete Basis Set ◽  
Core Electrons ◽  
Complete Basis Set Limit ◽  
Potential Energy Minimum

Molecular properties of the thallium monocyanide (Tl·CN) system in its ground electronic state are studied using high-precision ab initio relativistic two-component pseudopotential replacing 60 inner-core electrons of Tl. A relativistic coupled-cluster method with single, double and perturbative triple amplitudes is employed to account for electronic correlations. Extrapolation of results to the complete basis set limit is used for all studied properties. The global potential energy minimum of Tl·CN corresponds to the linear cyanide (TlCN) isomer, while the non-rigid isocyanide-like (TlNC) structure lies by approximately 11 kJ/mol higher in energy. The procedure of restoration of the wavefunction in the “core” region of Tl atom was applied to calculate the interaction of the Tl nuclear Schiff moment with electrons. The parameter X of the interaction of the Tl nuclear Schiff moment with electrons in the linear TlCN molecule equals 7150 a.u. The prospects of using the TlCN molecule for the experimental detection of the nuclear Schiff moment are discussed.

scholarly journals A Guide to Benchmarking Enzymatically Catalysed Reactions: The Importance of Accurate Reference Energies and the Chemical Environment

2020 ◽  
Author(s):  
Dominique A. Wappett ◽  
Lars Goerigk
Keyword(s):  
Active Site ◽  
Finite Basis ◽  
Chemical Environment ◽  
Basis Set ◽  
Coupled Cluster ◽  
Enzyme Active Site ◽  
Complete Basis Set ◽  
Complete Basis Set Limit ◽  
The Common ◽  
Cluster Values

We explore two significant factors on the outcomes of benchmark studies for enzymatically catalysed reactions, namely the level of theory of the benchmarks and the size of the model system used to represent the enzyme active site. For the benchmarks, we compare two potential alternatives to canonical coupled cluster results for situations where CCSD(T) is computationally too demanding: a strategy to estimate finite basis set coupled cluster values and the local-correlation DLPNO-CCSD(T) method at the complete basis set limit. We confirm the high accuracy of DLPNO-CCSD(T) used with tight thresholds. We also show that notable differences can be seen when using both sets of references for a benchmark study, with absolute deviations from the higher quality references generally smaller than those from lowerquality ones as well as changes in the ranking of the assessed methods. For geometries, we test three models for the active site of 4-oxalocrotonate tautomerase: one typical of the QM region that may be used in QM/MM studies, and two smaller variants that neglect the surrounding chemical environment. Benchmarking of 12 density functionals known to perform well on enzymatically catalysed reactions shows inconsistent performance of each method across the three models, contradicting the common idea that small representative systems can be used to accurately assess the applicability of low-level methods for larger biochemical applications. Our findings shall serve as a reminder on the standards that should be adhered to in benchmark studies, and as a guide for future studies, both on enzyme-related and other chemical problems.

scholarly journals A Guide to Benchmarking Enzymatically Catalysed Reactions: The Importance of Accurate Reference Energies and the Chemical Environment

2020 ◽  
Author(s):  
Dominique A. Wappett ◽  
Lars Goerigk
Keyword(s):  
Active Site ◽  
Finite Basis ◽  
Chemical Environment ◽  
Basis Set ◽  
Coupled Cluster ◽  
Enzyme Active Site ◽  
Complete Basis Set ◽  
Complete Basis Set Limit ◽  
The Common ◽  
Cluster Values

We explore two significant factors on the outcomes of benchmark studies for enzymatically catalysed reactions, namely the level of theory of the benchmarks and the size of the model system used to represent the enzyme active site. For the benchmarks, we compare two potential alternatives to canonical coupled cluster results for situations where CCSD(T) is computationally too demanding: a strategy to estimate finite basis set coupled cluster values and the local-correlation DLPNO-CCSD(T) method at the complete basis set limit. We confirm the high accuracy of DLPNO-CCSD(T) used with tight thresholds. We also show that notable differences can be seen when using both sets of references for a benchmark study, with absolute deviations from the higher quality references generally smaller than those from lowerquality ones as well as changes in the ranking of the assessed methods. For geometries, we test three models for the active site of 4-oxalocrotonate tautomerase: one typical of the QM region that may be used in QM/MM studies, and two smaller variants that neglect the surrounding chemical environment. Benchmarking of 12 density functionals known to perform well on enzymatically catalysed reactions shows inconsistent performance of each method across the three models, contradicting the common idea that small representative systems can be used to accurately assess the applicability of low-level methods for larger biochemical applications. Our findings shall serve as a reminder on the standards that should be adhered to in benchmark studies, and as a guide for future studies, both on enzyme-related and other chemical problems.

Calculation of static molecular properties in the framework of the unitary group based coupled cluster approach

1996 ◽  
Vol 74 (6) ◽  
pp. 918-930 ◽  
Author(s):  
Josef Paldus ◽  
Xiangzhu Li
Keyword(s):  
Dipole Moment ◽  
Finite Field ◽  
Ground State ◽  
Unitary Group ◽  
Linear Response ◽  
Molecular Properties ◽  
Basis Set ◽  
Cluster Method ◽  
Coupled Cluster ◽  
Static Properties

The recently developed and implemented state selective, fully spin-adapted coupled cluster (CC) method that employs a single, yet effectively multiconfigurational, spin-free reference and the formalism of the unitary group approach (UGA) to the many-electron correlation problem, has been employed to calculate static electric properties of various open-shell (OS) systems using the finite field (FF) technique. Starting with the lithium atom, the method was applied at the first-order interacting space single and double excitation level (CCSD(is)) to several first- and second-row hydrides having OS ground state, namely to the CH, NH, OH, SiH, PH, and SH radicals. In the case of NH we also considered three OS excited states. In all cases the dipole moment and polarizability were determined using a high quality basis set and compared with the experiment, whenever available, as well as with various configuration interaction results and other theoretical results that are available from the literature. The agreement of our CCSD(is) values with experiment is very satisfactory except for the 3Σ− ground state of the NH radical, where the experimentally determined dipole moment is too small. No experimental data are available for the corresponding polarizabilities. It is also shown that the FF technique is not suitable for calculations of higher order static properties, such as the hyperpolarizability β and γ tensors. For this reason we formulate the linear response version of our UGA-based CCSD approach and discuss the aspects of its future implementation. Key words: static molecular properties, dipole moments, polarizabilities, free radicals, unitary group based coupled cluster method, linear response theory, finite field technique.

Group IIIa Hydrides XH2 and XH2- (X = B, Al, Ga): Electron Affinities and Singlet-Triplet Splittings Revisited

2003 ◽  
Vol 68 (1) ◽  
pp. 75-88 ◽  
Author(s):  
Ivan Černušák ◽  
Alena Zavažanová ◽  
Juraj Raab ◽  
Pavel Neogrády
Keyword(s):  
Ground State ◽  
Basis Set ◽  
Basis Sets ◽  
Coupled Cluster ◽  
Electron Affinities ◽  
Complete Basis ◽  
Complete Basis Set ◽  
Order Of Magnitude ◽  
Complete Basis Set Limit ◽  
Cluster Methods

Geometries, electron affinities (EA) and singlet-triplet (S-T) splittings of XH2/XH2- molecules (X = B, Al, Ga) are calculated by coupled-cluster methods, using the sequence of basis sets. The EA values and S-T splittings for aluminium and gallium dihydrides are an order of magnitude larger (in absolute values) than those for boron. For boron and aluminium dihydrides, two types of extrapolations towards complete basis set limit are applied, leading to EA = 0.24 eV, ST = -0.01 eV (BH2), and EA = 1.10 eV, ST = -0.62 eV. The best calculated values for gallium dihydrides are EA = 1.13 eV and ST = -0.74 eV. All three S-T splittings favour singlet as the ground state, although the S-T splittings of BH2- is exceptionally small. In addition, vertical electron affinities and vertical electron detachments are reported for these molecules.

Evaluating Transition Metal Barrier Heights with the Latest DFT Exchange–Correlation Functionals – the MOBH35 Benchmark Dataset

2019 ◽  
Author(s):  
Mark Iron ◽  
Trevor Janes
Keyword(s):  
Transition Metal ◽  
Density Functional ◽  
Computational Cost ◽  
Basis Set ◽  
Functional Theory ◽  
Exchange Correlation ◽  
Barrier Heights ◽  
Complete Basis Set ◽  
Complete Basis Set Limit ◽  
Hybrid Functionals

A new database of transition metal reaction barrier heights – MOBH35 – is presented. Benchmark energies (forward and reverse barriers and reaction energy) are calculated using DLPNO-CCSD(T) extrapolated to the complete basis set limit using a Weizmann1-like scheme. Using these benchmark energies, the performance of a wide selection of density functional theory (DFT) exchange–correlation functionals, including the latest from the Truhlar and Head-Gordon groups, is evaluated. It was found, using the def2-TZVPP basis set, that the ωB97M-V (MAD 1.8 kcal/mol), ωB97X-V (MAD 2.1 kcal/mol) and SCAN0 (MAD 2.1 kcal/mol) hybrid functionals are recommended. The double-hybrid functionals PWPB95 (MAD 1.6 kcal/mol) and B2K-PLYP (MAD 1.8 kcal/mol) did perform slightly better but this has to be balanced by their increased computational cost.

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