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    Öğe
    A posteriori corrections to the configuration interaction method: a single-reference and multi-reference study
    (Taylor & Francis Ltd, 2015) Erturk, Murat; Meissner, Leszek
    The single-reference (SR) configuration interaction (CI) method is nowadays rather rarely used in quantum-chemical calculations. The reason is that the method is not competitive with the coupled-cluster (CC) approach that uses the same number of parameters but is size-extensive and more accurate. The accuracy of the CI method can be increased by applying size-extensivity a posteriori corrections but even that does not make the SR-CI method much more attractive. The CI scheme has, however, one important advantage over the CC one. Due to its formal simplicity, the SR-CI approach can be easily generalised to the multi-reference (MR) case while such a generalisation for the CC method turned out nontrivial. Two basic MR-CC formulations are formally complicated, numerically demanding, vulnerable to intruder states, and sensitive to the problem of multiple solutions. Contrary to that the MR-type CI schemes are among very few methods that are used in routine calculations for systems requiring a MR description. The problem of improving the MR-type CI results by applying an a-posteriori correction is in this context very appealing. In the paper, we discuss different types of corrections trying to show that the one based on the SR cluster expansion is both well theoretically justified and reliable in numerical applications. That is illustrated on model CI calculations of SR and MR type.
  • [ X ]
    Öğe
    Size-Extensivity Corrections in Single- and Multireference Configuration Interaction Calculations
    (Elsevier Academic Press Inc, 2016) Erturk, Murat; Meissner, Leszek
    It is well known that size-inextensivity is a major drawback of truncated configuration interaction (CI) methods. In order to ease the problem, many attempts have been made to minimize size-extensivity error of the CI methods through a posteriori corrections. In this study, we investigate performance of various size-extensivity corrections proposed in the literature. While the corrections were basically formulated for the single-reference (SR) CI method with singles and doubles, very soon their straightforward generalizations to a multireference (MR) case started to be used without any deeper theoretical justification. The only correction that is universal enough to be applied in case of any kind of CI method is the coupled-cluster (CC) correction. For the purpose of this study, we have used the closed shell (a) over tilde (1)A(1) singlet excited states of the CH2 and NH2+ molecules. The states are of MR character but still can be quite well described by SR approaches. That allows us to investigate the performance of the corrections in both SR and MR cases. For the first time, the second-order configuration interaction method has been used as a kind of the MR-type CI method. The calculations show that the performance of the CC correction is significantly better than other corrections.