Consider six electrons allocated in three orbitals in a closed shell restrictred HF ground state determinant. Now, consider three excited states: How many CI configurations can result from this system when only two electrons are excited per configuration? Is it true that there are 9 * 9 + 36 = 117 configurations, which would be as follows:(adsbygoogle = window.adsbygoogle || []).push({});

Same orbitals --> Same orbtials: 9, Same orbitals --> Different orbitals: 9, Different orbitals --> Same orbitals: 9, --> Different orbitals --> Different orbitals: 9. This is 36 possibilites. I assume that the 81 possibilities arise from spin combinations. If this is true (if not, where do they then origin from), which of the four cases would have different spin combinations?

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# Combinatorics within Configuration Interaction

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