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Development of molecular dynamics methodology for simulations of hard materials
Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on molecular dynamics simulations, both classical and ab initio. It is devoted to development of new methods and applications of molecular dynamics based techniques to a series of materials, all of which have the common property of being hard.

I first study grain boundaries in diamond and apply a novel method to better explore the configurational phase space. Using this method several new grain boundary structures are found. The lowest energy grain boundary structure has 20% lower energy then the one obtained with a conventional approach.

Another area is the development of efficient methods for first principles Born-Oppenheimer molecular dynamics. Here a fundamental shortcoming of the method that limits efficiency and introduces drift in the total energy of the system, is addressed and a solution to the problem is presented. Special attention is directed towards methods based on plane waves. The new molecular dynamics simulation method is shown to be more efficient and conserves the total energy orders of magnitude better then previous methods.

The calculation of properties for paramagnetic materials at elevated temperature is a complex task. Here a new method is presented that combines the disordered local moments model and ab initio molecular dynamics. The method is applied to calculate the equation of state for CrN were the connection between magnetic state and atomic structure is very strong. The bulk modulus is found to be very similar for the paramagnetic cubic and the antiferromagnetic orthorhombic phase.

TiN has many applications as a hard material. The effects of temperature on the elastic constants of TiN are studied using ab initio molecular dynamics. A significant dependence on temperature is seen for all elastic constants, which decrease linearly with temperature.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 69 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1454
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-78823ISBN: 978-91-7519-883-5 (print)OAI: oai:DiVA.org:liu-78823DiVA: diva2:536065
Public defence
2012-06-08, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:00 (English)
Opponent
Supervisors
Available from: 2012-06-21 Created: 2012-06-21 Last updated: 2012-06-21Bibliographically approved
List of papers
1. Missing-atom structure of diamond Sigma 5 (001) twist grain boundary
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 14, 144112- p.Article in journal (Refereed) Published
Abstract [en]

We carried out a combined experimental and theoretical study of grain boundaries in polycrystalline diamond, aimed at achieving the conditions in which grain boundaries are equilibrated. Raman spectra of compacted at high-pressure and high-temperature diamond powders allow us to identify signals from sp(2)-bonded atoms, in addition to a strong peak at 1332 cm(-1), corresponding to sp(3)-bonded carbon. To verify our interpretation of the experiment, Sigma 5 (001) twist grain boundaries of polycrystalline diamond were studied by means of molecular dynamics simulations using the technique proposed by von Alfthan et al. [Phys. Rev. Lett. 96, 055505 (2006)]. We find that grain-boundary (GB) configurations, from which one atom is removed, have significantly lower energy compared to those obtained with conventional techniques. These calculated GBs are highly ordered, a few monolayers thick, in agreement with experimental observations, and are primarily sp(2) bonded. This paper underlines the importance of varying the number of atoms within GBs in molecular dynamics simulations to correctly predict the GB ground-state structure.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-72025 (URN)10.1103/PhysRevB.84.144112 (DOI)000296287600002 ()
Note
Funding Agencies|Swedish Foundation for Strategic Research via strategic center MS2E||Goran Gustafsson Foundation for Research in Natural Sciences and Medicine||Swedish Research Council (VR)||DFG|SPP1236|Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2017-12-08
2. Extended Lagrangian Born-Oppenheimer molecular dynamics with dissipation
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2009 (English)In: JOURNAL OF CHEMICAL PHYSICS, ISSN 0021-9606, Vol. 130, no 21, 148075- p.Article in journal (Refereed) Published
Abstract [en]

Stability and dissipation in the propagation of the electronic degrees of freedom in time-reversible extended Lagrangian Born-Oppenheimer molecular dynamics [Niklasson , Phys. Rev. Lett. 97, 123001 (2006); Phys. Rev. Lett. 100, 123004 (2008)] are analyzed. Because of the time-reversible propagation the dynamics of the extended electronic degrees of freedom is lossless with no dissipation of numerical errors. For long simulation times under "noisy" conditions, numerical errors may therefore accumulate to large fluctuations. We solve this problem by including a dissipative external electronic force that removes noise while keeping the energy stable. The approach corresponds to a Langevin-like dynamics for the electronic degrees of freedom with internal numerical error fluctuations and external, approximately energy conserving, dissipative forces. By tuning the dissipation to balance the numerical fluctuations the external perturbation can be kept to a minimum.

Keyword
ab initio calculations, fluctuations, molecular dynamics method
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-19532 (URN)10.1063/1.3148075 (DOI)
Available from: 2009-06-29 Created: 2009-06-26 Last updated: 2012-06-21
3. Wave function extended Lagrangian Born-Oppenheimer molecular dynamics
Open this publication in new window or tab >>Wave function extended Lagrangian Born-Oppenheimer molecular dynamics
2010 (English)In: PHYSICAL REVIEW B, ISSN 1098-0121, Vol. 82, no 7, 075110- p.Article in journal (Refereed) Published
Abstract [en]

Extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] has been generalized to the propagation of the electronic wave functions. The technique allows highly efficient first principles molecular dynamics simulations using plane wave pseudopotential electronic structure methods that are stable and energy conserving also under incomplete and approximate selfconsistency convergence. An implementation of the method within the plane-wave basis set is presented and the accuracy and efficiency is demonstrated both for semiconductor and metallic materials.

Place, publisher, year, edition, pages
American Physical Society, 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-58775 (URN)10.1103/PhysRevB.82.075110 (DOI)000280758500002 ()
Note
Original Publication: Peter Steneteg, Igor Abrikosov, Valery Weber and Anders M N Niklasson, Wave function extended Lagrangian Born-Oppenheimer molecular dynamics, 2010, PHYSICAL REVIEW B, (82), 7, 075110. http://dx.doi.org/10.1103/PhysRevB.82.075110 Copyright: American Physical Society http://www.aps.org/ Available from: 2010-08-27 Created: 2010-08-27 Last updated: 2012-06-21
4. Extended Lagrangian free energy molecular dynamics
Open this publication in new window or tab >>Extended Lagrangian free energy molecular dynamics
2011 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 135, no 16, 164111- p.Article in journal (Refereed) Published
Abstract [en]

Extended free energy Lagrangians are proposed for first principles molecular dynamics simulations at finite electronic temperatures for plane-wave pseudopotential and local orbital density matrix-based calculations. Thanks to the extended Lagrangian description, the electronic degrees of freedom can be integrated by stable geometric schemes that conserve the free energy. For the local orbital representations both the nuclear and electronic forces have simple and numerically efficient expressions that are well suited for reduced complexity calculations. A rapidly converging recursive Fermi operator expansion method that does not require the calculation of eigenvalues and eigen-functions for the construction of the fractionally occupied density matrix is discussed. An efficient expression for the Pulay force that is valid also for density matrices with fractional occupation occurring at finite electronic temperatures is also demonstrated.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-72659 (URN)10.1063/1.3656977 (DOI)000296521200012 ()
Note
Funding Agencies|US-DoE|DE-AC52-06NA25396|(U.S.) Department of Energy through the LANL LDRD/ER||Swedish Foundation for Strategic Research (SSF) via Strategic Materials Research Center on Materials Science for Nanoscale Surface Engineering|MS2E|Gran Gustafsson Foundation for Research in Natural Sciences and Medicine||T-Division Ten-Bar Java Group||Available from: 2011-12-02 Created: 2011-12-02 Last updated: 2017-12-08
5. Equation of state of paramagnetic CrN from ab initio molecular dynamics
Open this publication in new window or tab >>Equation of state of paramagnetic CrN from ab initio molecular dynamics
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 14, 144404- p.Article in journal (Refereed) Published
Abstract [en]

The equation of state for chromium nitride has been debated in the literature in connection with a proposed collapse of its bulk modulus following the pressure-induced transition from the paramagnetic cubic phase to the antiferromagnetic orthorhombic phase [F. Rivadulla et al., Nature Mater. 8, 947 (2009); B. Alling et al., ibid. 9, 283 (2010)]. Experimentally the measurements are complicated due to the low transition pressure, while theoretically the simulation of magnetic disorder represents a major challenge. Here a first-principles method is suggested for the calculation of thermodynamic properties of magnetic materials in their high-temperature paramagnetic phase. It is based on ab initio molecular dynamics and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moments molecular dynamics method to the case of CrN and simulate its equation of state. In particular the debated bulk modulus is calculated in the paramagnetic cubic phase and is shown to be very similar to that of the antiferromagnetic orthorhombic CrN phase for all considered temperatures.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-76936 (URN)10.1103/PhysRevB.85.144404 (DOI)000302401900004 ()
Note
Funding Agencies|Swedish Research Council (VR)|621-2011-4426621-2011-4417|Swedish Foundation for Strategic Research (SSF)|10-0026|Available from: 2012-05-03 Created: 2012-04-27 Last updated: 2017-12-07
6. Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
Open this publication in new window or tab >>Temperature dependence of TiN elastic constants from ab initio molecular dynamics simulations
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 9Article in journal (Refereed) Published
Abstract [en]

Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C11, C12, and C44 are calculated. A strong dependence on the temperature is predicted, with C11 decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

Place, publisher, year, edition, pages
American Physical Society, 2013
National Category
Natural Sciences Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-78754 (URN)10.1103/PhysRevB.87.094114 (DOI)000316791600001 ()
Note

Funding Agencies|Swedish Research Council|621-2008-5535621-2011-4426|Swedish Foundation for Strategic Research (SSF) programs|10-0026|project Designed Multicomponent Coatings (MultiFilms)||Erasmus Mundus doctoral program DocMase||Ministry of Education and Science of the Russian Federation within the framework of Program Research and Pedagogical Personnel for Innovative Russia|14.B37.21.089010.09.2012|

Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2017-12-07Bibliographically approved
7. Configurational disorder effects on adatom mobilities on Ti1-xAlxN(001) surfaces from first principles
Open this publication in new window or tab >>Configurational disorder effects on adatom mobilities on Ti1-xAlxN(001) surfaces from first principles
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2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 24, 245422- p.Article in journal (Refereed) Published
Abstract [en]

We use metastable NaCl-structure Ti0.5Al0.5N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution during film growth. First-principles calculations were employed to obtain energy potential maps of Ti and Al adsorption on an ordered TiN(001) reference surface and a disordered Ti0.5Al0.5N(001) solid-solution surface. The energetics of adatom migration on these surfaces are determined and compared to isolate effects of configurational disorder. The results show that alloy surface disorder dramatically reduces Ti adatom mobilities. Al adatoms, in sharp contrast, experience only small disorder-induced differences in migration dynamics.

Place, publisher, year, edition, pages
american physical society, 2012
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-78822 (URN)10.1103/PhysRevB.85.245422 (DOI)000305089700007 ()
Available from: 2012-06-21 Created: 2012-06-21 Last updated: 2017-12-07Bibliographically approved

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