| 1. |
- Chen, Han, 1986-
(author)
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Characterization and Optimization of Silicon-strip Detectors for Mammography and Computed Tomography
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The goal in medical x-ray imaging is to obtain the image quality requiredfor a given detection task, while ensuring that the patient dose is kept as lowas reasonably achievable. The two most common strategies for dose reductionare: optimizing incident x-ray beams and utilizing energy informationof transmitted beams with new detector techniques (spectral imaging). Inthis thesis, dose optimization schemes were investigated in two x-ray imagingsystems: digital mammography and computed tomography (CT).In digital mammography, the usefulness of anti-scatter grids was investigatedas a function of breast thickness with varying geometries and experimentalconditions. The general conclusion is that keeping the grid is optimalfor breasts thicker than 5 cm, whereas the dose can be reduced without a gridfor thinner breasts.A photon-counting silicon-strip detector developed for spectral mammographywas characterized using synchrotron radiation. Energy resolution, ΔE/Ein, was measured to vary between 0.11-0.23 in the energy range 15-40 keV, which is better than the energy resolution of 0.12-0.35 measured inthe state-of-the-art photon-counting mammography system. Pulse pileup hasshown little effect on energy resolution.In CT, the performance of a segmented silicon-strip detector developedfor spectral CT was evaluated and a theoretical comparison was made withthe state-of-the-art CT detector for some clinically relevant imaging tasks.The results indicate that the proposed photon-counting silicon CT detector issuperior to the state-of-the-art CT detector, especially for high-contrast andhigh-resolution imaging tasks.The beam quality was optimized for the proposed photon-counting spectralCT detector in two head imaging cases: non-enhanced imaging and Kedgeimaging. For non-enhanced imaging, a 120-kVp spectrum filtered by 2half value layer (HVL) copper (Z = 29) provides the best performance. Wheniodine is used in K-edge imaging, the optimal filter is 2 HVL iodine (Z = 53)and the optimal kVps are 60-75 kVp. In the case of gadolinium imaging, theradiation dose can be minimized at 120 kVp filtered by 2 HVL thulium (Z =69).
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| 2. |
- Adamsson, Carl, 1976-
(author)
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Dryout and Power Distribution Effects in Boiling Water Reactors
- 2009
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Doctoral thesis (other academic/artistic)abstract
- Film flow measurements at several axial positions in round pipes with variousaxial power distributions are presented for conditions corresponding to normaloperation of a BWR. It is confirmed that the film flow rate approaches zero atthe onset of dryout. Selected phenomenological models of annular two-phaseflow are shown to reasonably reproduce the measurements. It is concluded thatmodels are in better agreement with measurements if terms corresponding topossible boiling induced entrainment are excluded. A method to perform film flow analysis in subchannels as a post-processto a standard two-field subchannel code is suggested. It is shown that thisapproach may yield accurate prediction of dryout power in rod bundles to alow computational cost and that the influence of the power distribution is wellpredicted by the model.
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| 3. |
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| 5. |
- Akpe, Victor
(author)
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Photophysical and Chemical Approaches to Cellular Biophysics
- 2008
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Licentiate thesis (other academic/artistic)abstract
- The central theme in this thesis is reversibility. Two main attempts has been made to approach reversibility in cellular systems from both chemical and physical points of view. Reversibility of immunolabeling of proteins on the cell surface has been adressed by development of new fluorescent substances optimized for CALI (Chromophore-Assisted Laser Inactivation of protein). Aluminum phthalocyanine (AlPc) is here identified to be a good candidate for a new generation of fluorophores for efficient hydroxyl radical generation. It is shown that cells can be reversibly labeled with antibody-AlPc conjugates. In experiments on living cells the AlPcs were not only active as classic fluorophores but also as photocatalytic substances with destaining properties. Reversibility of cell immobilization is also reported, where cells cultured in microstructures were immobilized and 3D supported using hydrogels. Hydrogel formulation and application was optimized to achieve a system where both viability and ease of use was satisfied. Gel reversibility was actualized with pH and enzyme treatment. The developped method offers the possibility of stop flow culturing cells in controlled and reusable 3D environments.
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| 6. |
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| 7. |
- Almlöf, Jonas
(author)
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Quantum error correction
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Quantum error correction is the art of protecting quantum states from the detrimental influence from the environment. To master this art, one must understand how the system interacts with the environment and gives rise to a full set of quantum phenomena, many of which have no correspondence in classical information theory. Such phenomena include decoherence, an effect that in general destroys superpositions of pure states as a consequence of entanglement with the environment. But decoherence can also be understood as “information leakage”, i.e., when knowledge of an encoded code block is transferred to the environment. In this event, the block’s information or entanglement content is typically lost.In a typical scenario, however, not all types of destructive events are likely to occur, but only those allowed by the information carrier, the type of interaction with the environment, and how the environment “picks up” information of the error events. These characteristics can be incorporated into a code, i.e., a channel-adapted quantum error-correcting code.Often, it is assumed that the environment’s ability to distinguish between error events is small, and I will denote such environments “memory-less”. But this assumption is not always valid, since the ability to distinguish error events is related to the temperature of the environment, and in the particular case of information coded onto photons, kBTR «ℏω typically holds, and one must then assume that the environment has a “memory”. In the thesis I describe a short quantum error-correction code adapted for photons interacting with a “cold” reservoir, i.e., a reservoir which continuously probes what error occurred in the coded state.I also study other types of environments, and show how to distill meaningful figures of merit from codes adapted for these channels, as it turns out that resource-based figures reflecting both information and entanglement can be calculated exactly for a well-studied class of channels: the Pauli channels. Starting from these resource-based figures, I establish the notion of efficiency and quality and show that there will be a trade-off between efficiency and quality for short codes. Finally I show how to incorporate, into these calculations, the choices one has to make when handling quantum states that have been detected as incorrect, but where no prospect of correcting them exists, i.e., so-called detection errors.
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| 8. |
- Almlöf, Jonas, 1973-
(author)
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Quantum error correction
- 2012
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Licentiate thesis (other academic/artistic)abstract
- This thesis intends to familiarise the reader with quantum error correction, and also show some relations to the well known concept of information - and the lesser known quantum information. Quantum information describes how information can be carried by quantum states, and how interaction with other systems give rise to a full set of quantum phenomena, many of which have no correspondence in classical information theory. These phenomena include decoherence, as a consequence of entanglement. Decoherence can also be understood as "information leakage", i.e., knowledge of an event is transferred to the reservoir - an effect that in general destroys superpositions of pure states.It is possible to protect quantum states (e.g., qubits) from interaction with the environment - but not by amplification or duplication, due to the "no-cloning" theorem. Instead, this is done using coding, non-demolition measurements, and recovery operations. In a typical scenario, however, not all types of destructive events are likely to occur, but only those allowed by the information carrier, the type of interaction with the environment, and how the environment "picks up" information of the error events. These characteristics can be incorporated into a code, i.e., a channel-adapted quantum error-correcting code. Often, it is assumed that the environment's ability to distinguish between error events is small, and I will denote such environments "memory-less". This assumption is not always valid, since the ability to distinguish error events is related to the \emph{temperature} of the environment, and in the particular case of information coded onto photons, typically holds, and one must then assume that the environment has a "memory". In this thesis, I describe a short quantum error-correcting code (QECC), adapted for photons interacting with a cold environment, i.e., this code protects from an environment that continuously records which error occurred in the coded quantum state.Also, it is of interest to compare the performance of different QECCs - But which yardstick should one use? We compare two such figures of merit, namely the quantum mutual information and the quantum fidelity, and show that they can not, in general, be simultaneously maximised in an error correcting procedure. To show this, we have used a five-qubit perfect code, but assumed a channel that only cause bit-flip errors. It appears that quantum mutual information is the better suited yardstick of the two, however more tedious to calculate than quantum fidelity - which is more commonly used.
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| 9. |
- Álvarez Ruiz, Jesús
(author)
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Photon induced fluorescence studies of molecules using synchrotron radiation
- 2003
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Licentiate thesis (other academic/artistic)abstract
- This Licentiate thesis presents research accomplished at theSection of Atomic and Molecular Physics at the Royal Instituteof Technology in Stockholm using photon induced fluorescencespectroscopy (PIFS) during the last two years.The main results presented are summarized:- Neutral photodissociation in CO was observed aftersynchrotron photon excitation in the range 19-26 eV bycollecting dispersed fluorescence from excited neutral C atoms.Follow-up ab initio calculations point out CO Rydberg seriesconverging to the CO+ C and D states as precursors.- The branching ratio between N2 + (B-X)(v’=1,v’’=2) and (v’=0,v’’=1)transitions in the 20-46 eV energy range reveals strongnon-Franck-Condon effects. Ab initio calculations indicate thatthe autoionization of certain superexcited states areresponsible for some of the structures present in the branchingratio curve, confirming the important role of non-Rydbergdoubly excited resonant states (NRDERS) in de-excitationprocesses above the ionization potential.- Photon induced neutral dissociation processes in NO arereported. Neither Rydberg series nor other molecular states inNO known so far can account for the collected data. From abinitio calculations more information regarding the NO precursorstates and the mechanism behind the observed neutraldissociation were obtained.- The details of a new experimental set-up for gas phasefluorescence measurements using synchrotron radiation aredescribed. It is able to perform simultaneous measurements ofdispersed and total fluorescence in the visible range. Thefirst results obtained with this set-up are presented,concerning fluorescence after excitation of the N2 molecules inthe N 1s edge.These four studies conform the set of papers enclosed in theLicentiate thesis.Finally a pre-study to further apply PIFS to speciespreviously excited by microwave discharge is included as futureplans.
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| 10. |
- Álvarez Ruiz, Jesús, 1975-
(author)
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Synchrotron radiation induced fluorescence spectroscopy of gas phase molecules
- 2004
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Doctoral thesis (other academic/artistic)abstract
- A new experimental set-up for gas phase fluorescence studies using synchrotron radiation has been designed and constructed to perform simultaneously total and dispersed fluorescence measurements. Neutral photodissociation of CO has been investigated after excitation with 19-26 eV photons. Fluorescence from 3p 3P, 3p 3S and 3p 1D excited states in carbon was recorded and interpreted by ab initio calculations. The population and dissociation of states belonging to the C and D Rydberg series in CO seem to explain the production of the observed triplet states but not the 3p 1D state. Neutral photodissociation of NO is reported in the 17-26 eV energy range. No known molecular states can account for the collected data. New information regarding the precursor states of the observed neutral dissociation is provided by ab initio calculations. Autoionization of superexcited states in molecular nitrogen is evidenced by strong deviations of the Franck-Condon ratio in the fluorescence of the N2+ B state. Ab initio calculations predict the existence of autoionizing-excited states that may account for some of the observed structures in the 20-46 eV energy range. Selective molecular fluorescence from the npó1Óu+ and npð 1Ðu (n=3-7) Rydberg levels to the E,F 1Óg+ state in H2 was recorded and rotationally analyzed. Vibrational levels of the E,F 1Óg+ state (vEF =0,1,3,6-10) are determined. The predissociation of npð 1Ð+ levels is observed in agreement with the literature. Fragmentation of SF6 was investigated after excitation with 25–80 eV photons. Dispersed fluorescence measurements reveal the emission of S, S+, F and F+ excited atoms. These fragments are produced after single, double and triple excitations as well as direct ionizations and shake-ups in SF6. Photoabsorption and fluorescence yield have been measured in SF5CF3 using 10-30eV photons. The photoabsorption spectrum can be explained in terms of its similarities to those of the SF6 and CF4 molecules. The dispersed and un-dispersed fluorescence resemble those of the CF3X family. Several features suggest the migration of an F atom across the S-C bond that fragments the molecule producing excited CF4. Doubly excited states of H2 have been investigated in the range of 26-60 eV by monitoring Balmer á emission. The experimental data show the already known emission correlated with the fragmentation of the Q1 and Q2 states, and new features which could be attributed to dissociative photoionization and higher lying doubly excited states Qn (n>2) of the hydrogen molecule
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| 11. |
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| 12. |
- Andersson, Andreas, 1982-
(author)
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Simulations of thermoelectric transport in granularsuperconductors
- 2010
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Licentiate thesis (other academic/artistic)abstract
- This thesis presents results from numerical simulations of the Nernst effect dueto phase fluctuations in models of two-dimensional granular superconductors. Inaddition other transport properties, such as thermal conductivity and electrical re-sistivity are calculated. The models are based on a phase only description withLangevin or resistively and capacitively shunted Josephson junction (RCSJ) dy-namics, generalized to be valid for any type of two-dimensional lattice structure.All transport coefficients are evaluated from equilibrium correlation functions usingKubo formulas. In Paper I, anomalous sign reversals of the Nernst signal eN , corresponding tovortex motion from colder to hotter regions, are observed. These are attributedto geometric frustration effects close to magnetic fields commensurate with theunderlying lattice structure. The effect is seen also in systems with moderategeometric disorder, and should thus be possible to observe in real two-dimensionalgranular superconductors or Josephson junction arrays. Paper II presents two different derivations of an expression for the heat current inLangevin and RCSJ dynamics. The resulting expression is through our simulationsseen to obey the required Onsager relation, as well as giving consistent resultswhen calculating κ and eN via Kubo formulas and through the responses to anapplied temperature gradient. In zero magnetic field and at low-temperatures, thecontribution to the thermal conductivity κ in RCSJ dynamics is calculated usinga spin-wave approximation, and is shown to be independent of temperature anddiverge logarithmically with system size. At higher temperatures, κ shows a non-monotonic temperature dependence. In zero magnetic field κ has a anomalouslogarithmic size dependence also in this regime. The off-diagonal component ofthe thermoelectric tensor αxy is calculated and displays the very same ∼1/T dependence at low temperatures predicted from calculations based on Gaussiansuperconducting fluctuations.
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| 13. |
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| 16. |
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| 18. |
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| 19. |
- Andersson, Sebastian, 1981-
(author)
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Spin-diode effect and thermally controlled switching in magnetic spin-valves
- 2012
-
Doctoral thesis (other academic/artistic)abstract
- This thesis demonstrates two new device concepts that are based on the tunneling and giant magnetoresistance effects. The first is a semiconductor-free asymmetric magnetic double tunnel junction that is shown to work as a diode, while at the same time exhibiting a record high magnetoresistance. It is experimentally verified that a diode effect, with a rectification ratio of at least 100, can be obtained in this type of system, and that a negative magnetoresistance of nearly 4000% can be measured at low temperature. The large magnetoresistance is attributed to spin resonant tunneling, where the parallel and antiparallel orientation of the magnetic moments shifts the energy levels in the middle electrode, thereby changing their alignment with the conduction band in the outer electrodes. This resonant tunneling can be useful when scaling down magnetic random access memory; eliminating the need to use external diodes or transistors in series with each bit. The second device concept is a thermally controlled spin-switch; a novel way to control the free-layer switching and magnetoresistance in spin-valves. By exchange coupling two ferromagnetic films through a weakly ferromagnetic Ni-Cu alloy, the coupling is controlled by changes in temperature. At room temperature, the alloy is weakly ferromagnetic and the two films are exchange coupled through the alloy. At a temperature higher than the Curie point, the alloy is paramagnetic and the two strongly ferromagnetic films decouple. Using this technique, the read out signal from a giant magnetoresistance element is controlled using both external heating and internal Joule heating. No degradation of device performance upon thermal cycling is observed. The change in temperature for a full free-layer reversal is shown to be 35 degrees Celsius for the present Ni-Cu alloy. It is predicted that this type of switching theoretically can lead to high frequency oscillations in current, voltage, and temperature, where the frequency is controlled by an external inductor or capacitor. This can prove to be useful for applications such as voltage controlled oscillators in, for example, frequency synthesizers and function generators. Several ways to optimize the thermally controlled spin switch are discussed and conceptually demonstrated with experiments.
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| 20. |
- Andgren, Karin, 1979-
(author)
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Lifetime measurements of excited states in 165Lu and 107Cd
- 2006
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Licentiate thesis (other academic/artistic)abstract
- Measuring lifetimes of excited nuclear states can give important information on the internal structure of the nucleus. This thesis is based on two experiments performed in Italy and in the USA in order to deduce the lifetimes of excited states in 165Lu and 107Cd. The lifetimes were measured using the Recoil Distance Method and the reduced transition probabilities between states have been calculated from the lifetimes in a model independent way. In the analysis of the data from the experiment on 165Lu, the shape of the nucleus is investigated using a collective rotational model to describe the observed excited states. A possibility of a triaxial shape of this nucleus is discussed. In the second experiment the obtained reduced transition probabilities for the nucleus 107Cd are compared to theoretical predictions for vibrational and rotational excitation modes
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| 21. |
- Andgren, Karin, 1979-
(author)
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Studies of collective phenomena in neutron deficient nuclei : by means of lifetime measurements, angular correlation measurements and the recoil-decay tagging technique
- 2008
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Doctoral thesis (other academic/artistic)abstract
- The nucleus is a mesoscopic system that retains features from both the quantum and macroscopic worlds. A basic property of a macroscopic body is its shape. Nuclear shapes can be deduced from experimental data as they influence the excitation mode of the nucleus and hence the energies and lifetimes of its excited levels. Various short-lived nuclei were created in fusion-evaporation experiments performed at international heavy-ion accelerator facilities. The emitted γ rays and, in some experiments, also the charged particles and neutrons emitted in the reactions were detected. The studied neutron-deficient isotopes were either selected by the type and number of particles emitted in the reactions, or by using their characteristic decays. The excited states of the different isotopes were extracted from the γ-ray analyses. Spectroscopic properties, such as the lifetimes of the excited states or the angular distribution of the emitted γ rays were measured when possible. The experimentally obtained level schemes together with the other spectroscopic information were used to deduce the excitation modes and the shapes of the studied nuclei. The detector systems are described in the first chapter and in the second chapter some techniques used to extract information from the experimental data are explained. Finally, a brief theoretical overview on the nuclear models which were used to interpret the experimental results is given.
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| 22. |
- Anghel, Ionut, 1971-
(author)
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Experimental and theoretical study of post-dryout heat transfer in annuli with flow obstacle
- 2013
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Doctoral thesis (other academic/artistic)abstract
- An experimental study on post dryout heat transfer regime in annuli with flow obstacles wasconducted in the High-pressure Water Test (HWAT) loop at the Royal Institute of Technologyin Stockholm, Sweden. An annulus with flow obstacles, consisting of two concentric heatedpipes (12.7x24.3) mm, with total heated length equal to 3650 mm was employed as a testsection. The experimental investigations were performed in a wide range of the operationalconditions: mass flux (500-1750) kg/(m2s), inlet subcooling (10-40) K and system pressure(5-7) MPa. The wall superheat was measured at 88 different axial positions. A significanteffect of the flow obstacles on the wall temperature has been observed. A new correlation toevaluate the wall superheat in the post-dryout developing region and downstream of the flowobstacles was suggested. The new approach is taking into account in a combined manner theonset of the dryout point and the flow obstacle location. The coefficients and constants of thecorrelation have been optimized based on 1211 points obtained experimentally. Thecorrelation is applicable starting with the point of the onset of the dryout towards fullydeveloped post-dryout heat transfer regime and shows a correct asymptotical trend. Toaccount for the flow obstacle effect on the critical quality, an expression similar to theLevitan-Lanstman dryout correlation is suggested. The newly developed methodology can beused to predict the wall temperature in the post-dryout heat transfer regime over a wide rangeof mass fluxes and pressures typical for boiling water reactors.
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| 23. |
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| 24. |
- Armiento, Rickard, 1976-
(author)
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The many-electron energy in density functional theory : from exchange-correlation functional design to applied electronic structure calculations
- 2005
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Doctoral thesis (other academic/artistic)abstract
- Att förutsäga egenskaper hos material och kemiska system är en viktig komponent för teoretisk och teknisk utveckling i fysik, kemi och biologi. Ett systems egenskaper styrs till stor del av dess elektrontillstånd. Datorprogram som baseras på täthetsfunktionalsteori kan beskriva elektronkonfigurationer mycket noggrant. Täthetsfunktionalsteorin hanterar all kvantmekanisk energi exakt, förutom ett mindre bidrag, utbytes-korrelationsenergin. Avhandlingen diskuterar existerande approximationer av utbytes-korrelationsenergin och presenterar en ny metod för konstruktion av funktionaler som hanterar detta bidrag---delsystems-funktionalmetoden. Flera teoretiska resultat relaterade till funktionalutveckling ges. En utbytes-korrelations-funktional har konstruerats helt utan empiriska antaganden (dvs, från första-princip). Funktionalen har använts för att beräkna gitterkonstant, bulkmodul och vakansenergi för aluminium, platina och kisel. Arbetet förväntas vara generellt tillämpbart inom området för täthetsfunktionalsteoriberäkningar
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| 25. |
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