Intermediate Effective Operators in Valence-Bond Model Spaces

1991 ◽  
Vol 02 (01) ◽  
pp. 549-551
Author(s):  
A.V. ZAITSEVSKII ◽  
A.I. DEMENT’EV
Keyword(s):  
Ab Initio ◽  
Valence Bond ◽  
Effective Operator ◽  
Atomic Orbitals ◽  
Bond Model ◽  
Model Spaces ◽  
Operator Construction ◽  
State Functions ◽  
Effective Operators

We developed and implemented an algorithm of ab initio intermediate effective operator construction for model spaces spanned by valence-bond-like configuration state functions built from orthogonalized atomic orbitals.

Ab initio band theory approach to electron energy loss near edge structures

1988 ◽  
Vol 46 ◽  
pp. 506-507
Author(s):  
Xudong Weng ◽  
O.F. Sankey ◽  
Peter Rez
Keyword(s):  
Ab Initio ◽  
Local Density ◽  
Interpolation Method ◽  
Atomic Orbital ◽  
Plane Waves ◽  
Large Set ◽  
Theory Approach ◽  
Band Theory ◽  
Atomic Orbitals ◽  
Pseudopotential Calculations

Single electron band structure techniques have been applied successfully to the interpretation of the near edge structures of metals and other materials. Among various band theories, the linear combination of atomic orbital (LCAO) method is especially simple and interpretable. The commonly used empirical LCAO method is mainly an interpolation method, where the energies and wave functions of atomic orbitals are adjusted in order to fit experimental or more accurately determined electron states. To achieve better accuracy, the size of calculation has to be expanded, for example, to include excited states and more-distant-neighboring atoms. This tends to sacrifice the simplicity and interpretability of the method.In this paper. we adopt an ab initio scheme which incorporates the conceptual advantage of the LCAO method with the accuracy of ab initio pseudopotential calculations. The so called pscudo-atomic-orbitals (PAO's), computed from a free atom within the local-density approximation and the pseudopotential approximation, are used as the basis of expansion, replacing the usually very large set of plane waves in the conventional pseudopotential method. These PAO's however, do not consist of a rigorously complete set of orthonormal states.

scholarly journals The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

2021 ◽  
Author(s):  
Christopher Antony Ramsden ◽  
Wojciech Piotr Oziminski
Keyword(s):  
Ab Initio ◽  
Type A ◽  
Nbo Analysis ◽  
Ring Systems ◽  
X Ray ◽  
X Ray Crystallography ◽  
Bond Model ◽  
Lone Pairs ◽  
Mp2 Calculations ◽  
Good Agreement

AbstractBased on structures determined by X-ray crystallography, ab initio MP2 calculations on type A mesoionic rings give geometries in good agreement with observed values. A study of four mesoionic ring systems, each with exocyclic oxygen, nitrogen or carbon groups, shows that the presence and configuration of exocyclic lone pairs significantly influences the geometry and configurational preference. Using a localised bond model and NBO analysis, these effects are rationalised in terms of an anomeric interaction of lone pairs with the antibonding orbitals of adjacent σ bonds. In agreement with experiment, similar effects are calculated for pyran-2-imines.

The orthogonal valence bond interpretation of ab initio chemisorption cluster model wavefunctions

1993 ◽  
Vol 177 (1) ◽  
pp. 61-67 ◽  
Author(s):  
A. Clotet ◽  
J.M. Ricart ◽  
J. Rubio ◽  
F. Illas
Keyword(s):  
Ab Initio ◽  
Cluster Model ◽  
Valence Bond

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.

The valence-bond theory of molecular structure II. Reformulation of the theory

1954 ◽  
Vol 223 (1154) ◽  
pp. 306-323 ◽  
Keyword(s):  
Valence Bond ◽  
Closed Shell ◽  
Numerical Illustration ◽  
Atomic Orbitals ◽  
Spin Eigenfunctions ◽  
Spin Pairing ◽  
Vb Structures ◽  
Bond Theory ◽  
Strong Repulsion ◽  
A Charge

The construction of spin eigenfunctions and the evaluation of matrix elements between ,them are discussed generally in preparation for a development of the valence bond (VB) theory along the lines indicated in I. The customary approximation of considering explicitly only the electrons outside a ‘closed shell’ is shown to be defensible. The reformulation of the VB theory is now straightforward, but its final description of bonding is quite new. Atomic orbitals (AO’s) are replaced, whenever they appear, by orthogonalized atomic orbitals (AO’s); but when the assumptions of the conventional theory are rigorously validated in this way the ‘covalent’ structures (now ‘VB’ structures) are found, quite generally, to indicate only strong repulsion between the ‘bonded’ atoms, and formal descriptions of bonding and of bond orders, in terms of ‘spin-pairing’, become nonsensical. Bonding can be described only by admitting into the wave functions polar VB structures; a bond between two atoms demands the appearance (with considerable weight) of pairs of structures differing by a ‘charge hop’ between the atoms concerned. The conventional VB structures are found to be equivalent to certain groupings of VB structures (non-polar and polar) and do, indeed, predict bonds between spin-paired atoms and repulsion between the atoms of different pairs. It is then possible to make full use of chemical intuition, using a plausible combination of conventional structures as a starting approximation in the more rigorous theory. A numerical illustration is provided by a discussion of the Kekulé structures of benzene. Some important characteristics of energy calculations in the VB theory are pointed out. Quantities of intra - and inter -atomic origin are well separated, and the method is apparently well suited to development along either ab initio or empirical lines.

Ab Initio valence bond treatment of simple molecules

2009 ◽  
Vol 8 (S8) ◽  
pp. 285-291 ◽  
Author(s):  
M. Raimondi ◽  
G. F. Tantardini ◽  
M. Simonetta
Keyword(s):  
Ab Initio ◽  
Valence Bond ◽  
Simple Molecules

Covalent Excited States of Polyenes C2nH2n+2(n= 2−8) and Polyenyl Radicals C2n-1H2n+1(n= 2−8): An Ab Initio Valence Bond Study

2008 ◽  
Vol 4 (12) ◽  
pp. 2101-2107 ◽  
Author(s):  
Junjing Gu ◽  
Yonghui Lin ◽  
Ben Ma ◽  
Wei Wu ◽  
Sason Shaik
Keyword(s):  
Ab Initio ◽  
Excited States ◽  
Valence Bond
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