Yazar "Erturk, Murat" seçeneğine göre listele

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    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.
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    An Improvement on Noninteger n-Generalized Exponential Type Orbitals with Hyperbolic Cosine in Atomic Calculations
    (Chemical Soc Japan, 2012) Erturk, Murat
    An improvement on the total energy results reported in Ref. 9 in atomic calculations based on a modified type hyperbolic cosine function cosh(beta(mu) + gamma) is presented. It is shown that the noninteger n-generalized exponential type orbitals r(n*-1)e(-zeta r mu) with modified type hyperbolic cosine as radial basis functions are a much better approximation to the Hartree-Fock orbitals than a double-zeta basis set of Slater type functions. The efficiency of the new basis function is tested by application to some closed and open shell neutral atoms and their ions. A substantial improvement in both the total and orbital energies is obtained within the minimal basis sets framework. The total energy values obtained in this work are significantly close to the numerical Hartree-Fock results. These results supersede all previous minimal basis function total energies achieved in the literature.
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    Exponential type orbitals with hyperbolic cosine function basis sets for isoelectronic series of the atoms Be to Ne
    (Walter De Gruyter Gmbh, 2023) Sahin, Ercan; Erturk, Murat; Ozdogan, Telhat; Orbay, Metin
    Exponential type orbital with hyperbolic cosine basis functions, proposed recently for Hartree-Fock-Roothaan calculations of neutral atoms, are studied in detail for the calculations of isoelectronic series of atoms from Be to Ne. Calculations are performed for the neutral and the first 20 cationic members of the isoelectronic series of each atom in its ground state. Three of the most popular exponential type orbitals (Slater type functions, B functions and psi((alpha) )functions with alpha = 2) are combined with modified hyperbolic cosine function cosh(beta r + gamma) to improve the basis function quality within the minimal basis sets framework. Performances of the basis functions are compared with each other by using the same number of variational parameters in them. The obtained results are also compared with numerical Hartree-Fock and extended Slater type basis set results. The presented accuracy of the minimal basis descriptions of atomic systems supports the usage of these unconventional basis functions in electronic structure and property calculations.
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    Generalized B functions applied to atomic calculations
    (Elsevier, 2020) Erturk, Murat; Sahin, E.
    During the past years, there has been an increasing interest in the construction of alternative basis sets for electronic systems. .In this work, by the use of generalized exponential functions approach, new efficient exponential type basis sets which include the B functions in the radial part have been constructed. Additional variational flexibility for the radial part of B functions is obtained by replacing the radial variable by r(mu), with mu optimisable. Thus, the new variational parameter mu in generalized B functions greatly improves the quality of atomic orbitals in the LCAO calculations. Results obtained using generalized B function basis sets within the minimal basis framework are compared to the results of the standard B functions basis sets and numerical Hartree-Fock values for atomic systems.
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    Is contralateral carotis artery occlusion a risk factor for carotid andarterectomy?
    (Baycinar Medical Publ-Baycinar Tibbi Yayincilik, 2016) Reyhanoglu, Hasan; Asgun, Halil Fatih; Ozcan, Kaan; Erturk, Murat; Durmaz, Isa
    Background: In this study, we aimed to investigate whether contralateral carotis artery occlusion poses an additional risk for morbidity and mortality in carotid endarterectomy. Methods: Between September 2010 and March 2015, a total of 135 patients (103 males, 32 females; mean age 68.3 years; range 46 to 92 years) who were operated with the diagnosis of carotid stenosis in our clinic were divided into two groups depending on the presence of an occluded contralateral artery. Thirty-two patients undergoing carotid endarterectomy with contralateral carotid artery occlusion were compared to 103 patients without contralateral occlusion. Baseline demographic characteristics, risk factors, type of surgical technique, postoperative early morbidity and mortality results of both groups were compared. Results: Both groups were similar according to their demographic and perioperative characteristics. There was no statistically significant difference in postoperative neurological [carotid occlusion group (CO) 6.3%, control group 6.8%, p= 0.638] or nonneurological (group CO 15.6%, control group 12.6%, p= 0.430) complications between the two groups. The length of hospital stay did not differ between both groups statistically while length of intensive care unit stay was significantly longer in the group with occlusion (p= 0.029). There was no significant difference in the postoperative mortality between the groups (group CO 0%, control group 1%, p= 0.763). Conclusion: Although morbidity and mortality are expected to be high in patients with contralateral carotid artery occlusion due to the limited cerebral blood supply, there was no significant increase in postoperative neurological complications, particularly. Therefore, we believe that contralateral occlusion does not pose an additional risk for surgery in patients undergoing carotid endarterectomy.
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    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.