Topological studies of the charge density of some group 2 metallocenes M(η5-C5H5)2 (M = Mg or Ca)

1996 ◽  
Vol 74 (6) ◽  
pp. 1059-1071 ◽  
Author(s):  
Ian Bytheway ◽  
Paul L.A. Popelier ◽  
Ronald J. Gillespie
Keyword(s):  
Charge Density ◽  
Density Functional ◽  
Search Algorithm ◽  
Functional Theory ◽  
Cyclopentadienyl Ligands ◽  
Hartree Fock ◽  
The Core ◽  
Point Search ◽  
Charge Concentration ◽  
Group 2

Ab initio quantum chemical calculations using both the Hartree–Fock and the B3LYP density functional theory methods have been performed for the group 2 metallocenes M (η5-C5-C5H5)2 (M = Mg or Ca). The topology of the calculated charge density (ρ) and its Laplacian [Formula: see text] have been analysed using a new critical point search algorithm in order to understand why Mg(η5-C5-H5)2 is linear while Ca(η-C5H5)2 is bent. The Laplacian of the core of the Ca atom in Ca(η5-C5H5)2 is perturbed by the polarizing field of the cyclopentadienyl ligands and the bent geometry is a consequence of the interactions between the distorted core and the ligand atoms. In the case of Mg(η5-C5H5)2, charge concentration maxima in the Mg core occur along the vectors connecting the metal to the centroids of the cyclopentadienyl ligands irrespective of whether or not the molecule is linear, and the preferred geometry is linear as expected. The results of these calculations demonstrate that the geometries of the group 2 metallocenes can be understood in terms of the repulsive interactions between the ligands and between the ligands and the distorted core of the metal atom. Key words: atoms in molecules, group 2 metallocenes, VSEPR, charge density, Laplacian of ρ.

Study of NO and CO dissociation on the (100) Cu surface using density functional theory and the topological analysis of the electronic density and its Laplacian

1996 ◽  
Vol 74 (6) ◽  
pp. 1014-1020 ◽  
Author(s):  
Yosslen Aray ◽  
Jesús Rodríguez
Keyword(s):  
Density Functional Theory ◽  
Density Functional ◽  
Good Description ◽  
Topological Analysis ◽  
Molecular Graph ◽  
Electronic Density ◽  
Functional Theory ◽  
Hartree Fock ◽  
Local Charge ◽  
Charge Concentration

Molecular orbital ab initio Hartree–Fock, post-Hartree–Fock at the MP2 and QCISD levels, and density functional theory calculations of the dipole moment, the topology of the electronic density, ρ(r), and its Laplacian, [Formula: see text], for CO and NO molecules are reported. The results obtained confirm that density functional methods provide remarkably good electronic properties and a good description of the topology of ρ(r) and [Formula: see text]. The Becke exchange functional with the correlation functional of Lee, Yang, and Parr was used to calculate the electronic density of the (100) Cu surface. Topological analysis of ρ(r) shows that the crystal graph corresponds to square pyramids between the atoms of the top of the surface and the atoms of the second layer The topological analysis of [Formula: see text] shows that the atomic graph of the Cu surface exhibits one (3,−3) local charge concentration surrounded by four (3,+1) local charge depletion points. Additionally, there is a (3,+3) local depletion in the midpoint between each of four contiguous Cu atoms corresponding to the active site for the adsorption of the (3,−3) local charge concentration on the C atom of the CO or the N atom of the NO molecule. The larger value of the [Formula: see text] at the nonbonded charge concentration on the atoms and the geometrical configuration of these critical points favor the interaction of the NO over the CO molecule with the (100) Cu surface. This result is in accord with the known reaction barriers for these molecules. Key words: density functional theory, Laplacian of the electronic density, (100) Cu surface, carbon monoxide, nitrogen monoxide, molecular graph, atomic graph.

An Efficient Saddle Point Search Method Using Kriging Metamodels

2015 ◽  
Author(s):  
Lijuan He ◽  
Yan Wang
Keyword(s):  
Potential Energy ◽  
Saddle Point ◽  
Density Functional ◽  
Search Algorithm ◽  
Saddle Points ◽  
Surrogate Models ◽  
Functional Theory ◽  
Search Results ◽  
Point Search ◽  
Large Systems

Simulating phase transformation of materials at the atomistic scale requires the knowledge of saddle points on the potential energy surface (PES). In the existing first-principles saddle point search methods, the requirement of a large number of expensive evaluations of potential energy, e.g. using density functional theory (DFT), limits the application of such algorithms to large systems. Thus, it is meaningful to minimize the number of functional evaluations as DFT simulations during the search process. Furthermore, model-form uncertainty and numerical errors are inherent in DFT and search algorithms. Robustness of the search results should be considered. In this paper, a new search algorithm based on Kriging is presented to search local minima and saddle points on a PES efficiently and robustly. Different from existing searching methods, the algorithm keeps a memory of searching history by constructing surrogate models and uses the search results on the surrogate models to provide the guidance of future search on the PES. The surrogate model is also updated with more DFT simulation results. The algorithm is demonstrated by the examples of Rastrigin and Schwefel functions with a multitude of minima and saddle points.

scholarly journals Unraveling the electrochemical and spectroscopic properties of neutral and negatively charged perylene tetraethylesters

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Christian Wiebeler ◽  
Joachim Vollbrecht ◽  
Adam Neuba ◽  
Heinz-Siegfried Kitzerow ◽  
Stefan Schumacher
Keyword(s):  
Absorption Spectra ◽  
Density Functional ◽  
Energy Levels ◽  
Spectroscopic Properties ◽  
Absorption Bands ◽  
Functional Theory ◽  
Vertical Excitation ◽  
Hartree Fock ◽  
Tauc Plot ◽  
Electrochemical Approach

AbstractA detailed investigation of the energy levels of perylene-3,4,9,10-tetracarboxylic tetraethylester as a representative compound for the whole family of perylene esters was performed. It was revealed via electrochemical measurements that one oxidation and two reductions take place. The bandgaps determined via the electrochemical approach are in good agreement with the optical bandgap obtained from the absorption spectra via a Tauc plot. In addition, absorption spectra in dependence of the electrochemical potential were the basis for extensive quantum-chemical calculations of the neutral, monoanionic, and dianionic molecules. For this purpose, calculations based on density functional theory were compared with post-Hartree–Fock methods and the CAM-B3LYP functional proved to be the most reliable choice for the calculation of absorption spectra. Furthermore, spectral features found experimentally could be reproduced with vibronic calculations and allowed to understand their origins. In particular, the two lowest energy absorption bands of the anion are not caused by absorption of two distinct electronic states, which might have been expected from vertical excitation calculations, but both states exhibit a strong vibronic progression resulting in contributions to both bands.

Polaronic and Bipolaronic Enhancement of Second Hyperpolarizabilities in Dithienyl Polyenes from Ab Initio Quantum Methods

1999 ◽  
Vol 597 ◽  
Author(s):  
Steven Trohalaki ◽  
Robert J. Zellmer ◽  
Ruth Pachter
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Valence Bond ◽  
Closed Shell ◽  
Functional Theory ◽  
Molecular Conformations ◽  
Hartree Fock ◽  
Molecular Charge ◽  
Field Methodology ◽  
Moller Plesset

AbstractSpangler and He [1,2] have shown that dithienyl polyenes form extremely stable bipolaronic dications when oxidatively doped in solution. Previous theoretical studies applied empirical methods to predict bipolaronic enhancement of hyperpolarizabilities for simple polyenes [3,4]. Here, we employ density functional theory to optimize the gas-phase molecular conformations of neutral, cationic, and dicationic forms of a series of dithienyl polyenes, where the number of ethene units, N, is varied from 1–5. Ab initio Hartree-Fock, generalized valence bond, configuration interaction, and Møller-Plesset calculations demonstrate that the dications are farily well described with a closed shell and therefore have little biradicaloid character. Second hyperpolarizabilities, γ, are subsequently calculated using ab initio Hartree-Fock theory and a finite field methodology. As expected, γ increases with the number of ethene units for a given molecular charge. The cations also show the largest increase in γ with N. For a given value of N, the cations display the largest γ values. However, if we treat the dication as a triplet, which might be present in solution, then it displays the largest γ.

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