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CI: Configuration Interaction (Read 3837 times)
Gerrit-Jan Linker
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CI: Configuration Interaction
21.08.08 at 12:19:13
 
CI: Configuration Interaction
 
Using CI electron correlation is brought into account by making a wave function for the system as a linear combination of Configuration State Functions (CSF's) based on a reference CSF, usually the HF wave function. The other CSF's are excitations from this reference wave function. When only single and double excitations are considered the technique is called CI Singles and Doubles: CISD.
 
An important feature of CI is that it leaves the reference one electron functions (orbitals) unchanged. This is why the reference wave function (the HF wave function usually) needs to be of sufficient quality to make CI successful. Optimisation is only done of the coefficients used in the linear expansion of CSF's and the one electron functions that constitute the reference wavefunction are not optimised. Note that this is different in e.g. MCSCF calculations where the one electron orbitals are optimised in a SCF step thus improving the quality of the reference wave function.
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« Last Edit: 21.08.08 at 12:27:48 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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Gerrit-Jan Linker
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Posts: 75
Re: CI: Configuration Interaction
Reply #1 - 28.03.09 at 18:46:59
 
On the following website you find a good set of notes on configuration interaction. I have copied the outline below.
http://vergil.chemistry.gatech.edu/notes/ci/node1.html
 
    * Contents
    * Introduction and Notation
    * Fundamental Concepts
         o Scope of the Method
         o Why Configuration Interaction?
         o The Correlation Energy
         o Slater's Rules  
    * The Variational Theorem
         o The Method of Linear Variations
         o Variational Theorem for the Ground State
         o Why are Coupled-Cluster and MBPT Energies not Variational?
         o Application of the Variational Theorem to Other States
         o Convergence of the Wavefunction
         o Variational Theorem Bounds on Excited States  
    * Reducing the Size of the CI Space
         o Symmetry Restrictions on the CI Space
         o Classification of Basis Functions by Excitation Level
         o Energy Contributions of the Various Excitation Levels
         o Size of the CI Space as a Function of Excitation Level
         o The Frozen Core Approximation
         o Truncated CI is not Size Extensive  
    * Second Quantization
    * Determinant-Based CI
         o Introduction to Determinant CI
         o Alpha and Beta Strings
         o Restricted Active Space CI
         o Full CI Algorithm  
    * Bibliography
    * Index
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Gerrit-Jan Linker
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