scholarly journals Independent amplitude approximations in coupled cluster valence bond theory: Incorporation of 3-electron-pair correlation and application to spin frustration in the low-lying excited states of a ferredoxin-type tetrametallic iron-sulfur cluster

2018 ◽  
Vol 149 (14) ◽  
pp. 144103 ◽  
Author(s):  
David W. Small ◽  
Martin Head-Gordon
Keyword(s):  
Excited States ◽  
Electron Pair ◽  
Pair Correlation ◽  
Valence Bond ◽  
Coupled Cluster ◽  
Spin Frustration ◽  
Iron Sulfur Cluster ◽  
Iron Sulfur ◽  
Independent Amplitude ◽  
Bond Theory

The Atomic Exciton Model for the Excited States of p-Electron Systems

1985 ◽  
Vol 38 (10) ◽  
pp. 1529
Author(s):  
PE Schipper
Keyword(s):  
Excited States ◽  
Ground State ◽  
Valence Bond ◽  
Essential Difference ◽  
Electron Systems ◽  
Exciton Model ◽  
Valence Bond Theory ◽  
Bond Theory ◽  
Atomic Excitations ◽  
Insight Into

A new model is presented to describe the π-π* excitations of π-electron systems in terms of intra-atomic excitations. The atomic exciton model combines features of conventional exciton and valence bond theory, reducing to the former in the non-exchanging limit, and the latter in the ground-state limit with covalent structures. The model is ideally suited to the approximate or exact incorporation of exchange, and highlights the opposition of excitation and electron interchange in determining the energetics of the excitation manifold, eliciting thereby the essential difference between ground and excited states. Applications to some simple π-systems are considered, providing new insight into their excited states.

The molecular orbital theory of chemical valency XIII. Orbital wave functions for excited states of a homonuclear diatomic molecule

1953 ◽  
Vol 216 (1126) ◽  
pp. 424-433 ◽  
Keyword(s):  
Molecular Orbital ◽  
Excited States ◽  
Valence Bond ◽  
Present Theory ◽  
Wave Functions ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
Bond Theory ◽  
Homonuclear Diatomic Molecules ◽  
Approximate Wave

The expansions for the exact wave functions for excited states of homonuclear diatomic molecules derived in part XII are used as the basis for discussing various approximate wave functions of the orbital type. The states considered in detail are the lowest states of symmetries 1 Σ u + , 3 Σ u + . The calculus of variations is used to determine the optimum forms for the component orbital functions. A transformation to equivalent orbitals is used to bring out the physical significance of the various wave functions, and to relate the present theory to earlier theories, in particular the molecular orbital theory, the valence-bond theory and their generalizations.

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