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| {{Unreferenced|date=December 2009}}
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| In [[quantum chemistry]], the '''multireference configuration interaction''' method consists in a [[configuration interaction]] expansion of the [[eigenstate]]s of the [[electronic molecular Hamiltonian]] in a set of [[Slater determinant]]s which correspond to excitations of the [[ground state]] [[electronic configuration]] but also of some [[excited state]]s. The Slater determinants from which the excitations are performed are called '''reference determinants'''. The higher excited derterminants (also called [[configuration state function]]s (CSFs) or shortly configurations) are then chosen either by the program according to some [[perturbation theory|perturbation theoretical]] [[ansatz]] according to a threshold provided by the user or simply by truncating excitations from these references to singly, doubly, ... excitations resulting in MRCIS, MRCISD, etc.
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| For the ground state using more than one reference configuration means a better [[electronic correlation|correlation]] and so a lower energy. The problem of [[size consistency|size inconsistency]] of truncated CI-methods is not solved by taking more references.
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| As a result of a MRCI calculation one gets a more balanced [[electronic correlation|correlation]] of the ground and [[excited state]]s. For quantitative good energy differences (excitation energies) one has to be careful in selecting the references. Taking only the dominant configuration of an excited state into the reference space leads to a correlated (lower) energy of the excited state. The generally too high excitation energies of CIS or CISD are lowered. But usually excited states have more than one dominant configuration and so the ground state is more correlated due to: a) now including some configurations with higher excitations (triply and quadruply in MRCISD); b) the neglect of other dominant configurations of the excited states which are still uncorrelated.
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| Selecting the references can be done manually (<math> \Phi_1, \Phi_2, \Phi_5, ...</math>), automatically (all possible configurations within an active space of some orbitals) or semiautomatically (taking all configurations as references that have been shown to be important in a previous CI or MRCI calculation)
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| This method has been implemented first by [[Robert Buenker]] and [[Sigrid D. Peyerimhoff]] in the seventies under the name '''Multi-Reference single and Double Configuration Interaction (MRDCI)'''.
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| The MRCI method can also be implemented in [[Semi-empirical quantum chemistry method|semi-empirical]] methods. An example for this is the OM2/MRCI method developed by [[Walter Thiel (Chemist)|Walter Thiel's]] group.
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| ==See also==
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| * [[Configuration interaction]]
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| {{DEFAULTSORT:Multireference Configuration Interaction}}
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| [[Category:Quantum chemistry]]
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| <!-- [[Category:Computational chemistry]] redundant -->
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Revision as of 15:55, 11 February 2014
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