lecture37_umn - Lecture 37 More about Electron Correlation...

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Lecture 37 December 14, 2009 More about Electron Correlation Configuration Interaction with a Single Determinant Reference
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Full CI wave functions A full CI treatment can be applied only to the smallest molecules. CH 3 OH: basis set that has roughly two functions for each atomic orbital of the atoms involved and also puts some d functions on the heavy atoms C and O. Total number of basis functions: 38. Full CI every possible way to occupy the 38 MOs with 18 electrons, we would have to optimize 2.4 x 10 13 expansion coefficients
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Configuration Interaction In most cases, it is impractical to include all possible excited configurations and do a full CI . Reduce the CI problem by allowing only a limited number of other determinants i and j occupied MOs in Ψ HF r and s virtual MOs in Ψ HF additional CSFs are generated by exciting an electron from the occupied orbital(s) into the virtual orbital(s). First summation all possible single electronic excitations Second all possible double excitations, etc. " = a 0 " HF + a i r " i r r vir.
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