2145-32 spring college on computational...
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2145-32
Spring College on Computational Nanoscience
Risto NIEMINEN
17 - 28 May 2010
COMP/Applied Physics Aalto University School of Sciences & Technology
Espoo Finland
Computational Science as Nanotechnology Pillar
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Spring College in Computational NanoscienceTrieste, May 17-28, 2010
Computational Science asNanotechnology Pillar
Risto Nieminen
COMP/Applied PhysicsAalto University School of Science and Technology
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The three pillars of nanoscience and -technology
Manufacture and manipulation
“top-down”: lithography
“bottom-up”: self-assembly
Characterisation,imaging andprobing
SPMTEMPES…
Theory, modelling andsimulation
• Predictive computation of physical, chemical and biologicalfunctions• Process design• Interpretation of probe data
Computational nanoscience
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Science Paradigms• A thousand years ago….
Empirical, observational
• A few hundred years ago…
Theoretical branch emerges
• A few decades ago …
Computational branch emerges: simulatingcomplex phenomena
• Today …
Data-centric branch emerges: unifiesexperiment, theory and simulation
Data management, mining and curation
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Computational science
• Uses the capabilities of computers through the methodologiesof scientific computing to solve scientific and engineering problems
across all disciplines.
• Draws heavily on the advances in applied mathematics andcomputer science: numerics, graphics, complex optimisation,data structures, statistics etc.
• Contributes to the emergence of data-intensive scientific enquiry.
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Computational Science
Towards increasing complexity
- Nonlinearity- Multi-scale problems- Multi-disciplinary problems- Large-scale problems- Simultaneous deterministic and stochastic phenomena- Inverse problems
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Computational science is technology-driven: the Computational Pyramid
HPC
Mid-range
Servers, desktop
Tip of the iceberg: High Performance Computing
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The ”Fourth Paradigm”:Online Data Pyramid
Literature
Derived andrecombined data
Raw data
Information at your fingertips, everywhere
• Overlapping disciplines share data
• From literature to computationto data – and back
• Universal access through Internet
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2005
2010
Earth SimulatorASCI White
Pacific
EDSAC 1UNIVAC 1
IBM 7090
CDC 6600
IBM 360/195CDC 7600
Cray 1
Cray X-MPCray 2
TMC CM-2
TMC CM-5 Cray T3D
ASCI Red
1950 1960 1970 1980 1990 2000 2010
1 KFlop/s
1 MFlop/s
1 GFlop/s
1 TFlop/s
1 PFlop/s
Scalar
Super Scalar
Vector
Parallel
Super Scalar/Vector/ParallelHPC and Moore’s Law
A computation that in 1980 took 1 full year to complete can today be done in ~ 5 seconds!
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Trends in high-performance computing
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Trends in high-performance computing
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Trends in high-performance computing
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Evolving Computational Pyramid:from ”Metacomputing” to ”Grid Computing” to
”Cloud Computing”
• Larry Smarr (NCSA) 1992 –
• Ian Foster et al. (Argonne) 1998 –
• First implementations (data intensivechallenges – CERN LCG etc.) 2000 –
• NSF Foundational Research in Data-CentricComputing 2010 -
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The Grid
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GRID projects: DEISA, PRACE,…
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The importance of algorithms
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The future facilities for HPC will havehundreds of thousands of processors.
This implies the need for massive re-designof algorithms, compilers and and programming tools.
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SCALABILITY
Let N measure the problem size (e.g. ”number of electrons” in first-principles molecular dynamics)
Strong scalability
- Fixed problem size, increase NCPU- Ideally, time ~ O(1/NCPU)- Hard to achieve
Weak scalability
- Fixed problem size, increase NCPU- Not well defined if cost is not O(N)- Cost is often polynomial in N or NlogN
(F. Gygi)
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LIMITS FOR COMPUTATIONAL EFFICIENCY
•Time scales
- one floating point operation (add or multiply): 125 ps- fetch a double-precision number from memory: 4 ns- fetch a double–precision number from another node: 8 ns
•The only viable strategy is to overlap communication and computation
- reuse data in processor cache- use asynchronous communication
•Try to achieve tcompute > tcommunicate
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LIMITS FOR COMPUTATIONAL EFFICIENCY
• Fundamental characteristics
F = flop rate (e.g. AMD Opteron: 8 Gflop/s)B = bandwidth (e.g. Infiniband 1 GB/s)S = local data size (e.g. matrix block size 8 MB)N(S) = number of operations (e.g. matrix multiply N(S) = S3/2)
tcompute = N(S)/Ftcommunicate = S/B
N(S)/S > F/B
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LIMITS FOR COMPUTATIONAL EFFICIENCY
• Trends of new CPU architectures
Multicore processors: flop rate (F) increasesBandwidth (B) ~ constant
N(S)/S > F/B
Only possible for very large problems (S)
N(S) must be superlinear
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ON THE ROAD TO PETASCALE COMPUTING
• Future HPC platforms will consist of O(106) CPUs
• CPUs do not get much faster
• We can use the extra computational power to- simulate moderate-sized systems faster (requires strong scalablity)- use more accurate and sophisticated theory/models
• Petascale computing will require new algorithms- main issues: data compression, simplified communication patterns
• New bottlenecks appear:- CPU architectures change fast – software must adapt quickly- I/O performance will become critical- Fault-tolerant algorithms may become a necessity
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“The general theory of quantum mechanics is now almost complete. The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble.
It therefore becomes desirable that approximate practicalmethods of applying quantum mechanics should bedeveloped, which can lead to an explanation of the main features of complex atomic systems without too muchcomputation.”
P.A.M. Dirac, 19291902-1984Physics Nobel Prize 1933 (with E. Schrödinger)
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Five Grand Challenges for Science and the Imagination (courtesy of Mark Ratner)Imagination (courtesy of Mark Ratner)
• How do we control materials and processes at the level of electrons?
• How do we design and perfect atom-and energy-efficient synthesis of new forms of matter with tailored properties?
• How do remarkable properties of matter emerge from complex correlations of atomic and electronic constituents and how can we control these properties?
• Can we master energy and information on the nanoscale to create new technologies with capabilities rivaling those of living systems?
• How do we characterize and control matter away—especially very far away—from equilibrium?
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Handshaking of…..• quantum and classical phenomena• equilibrium and non-equilibrium phenomena• top-down and bottom-up approaches• reductionist and emergent viewpoints (“more is different”)
QUANTUM WORLD:
• “weird” = counterintuitive : wave-particle dualism• inherently “parallel” : quantum computing• phase information• many-particle symmetry/antisymmetry :mathematical description of electrons very complex(“fermion sign problem”)
• coupling to (classical) environments: decoherence, dissipation
NANOWORLD:
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Multi-scale materials modelling and simulation:bridging length and time scales
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Bridging the different length scales:three-dimensional etching Si MEMS
Interrelation between microscopic, mesoscopic and macroscopic featuresof the etching process
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Corner Compensation Simulation using Monte Carlo Atomistic Method
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� Results from ETCH3D Anisotropic Etch
Simulator
� Results from ETCH3D Anisotropic Etch
Simulator
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Scientific software used at COMP
QUANTUM SCALEQMC
ABINIT VASPCASTEP WIEN2kFHI-AIMS OCTOPUSSIESTA GPAW
Quantum Monte Carlo
Density-functionaltheory
ATOMISTIC SCALE CPMD GROMACS MOLDYCASHEW
Molecular dynamicsKMC CASINOTAPAS
MESOSCOPIC SCALE Kinetic Monte CarloCellular automata
ELMERCONTINUUM SCALE Finite-element methods
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Anatomy of a package: GPAW
https://wiki.fysik.dtu.dk/gpawPsi-k Highlight April 2010J. Phys. CM: Condensed Matter (to be published)
• Real-space grid: finite-differences
• Projector-augmented-wave for the core electrons
• Static and time-dependent DFT
• LDA, GGA, hybrid, van der Waals
• TDDFT: linear-response and time propagation
• Many utilities
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PAW: Projector-augmented wave approach (Peter Blöchl)
Total energy
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Generalised eigenvalue problem
Hamiltonian
Overlap matrix
Potential
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Grid-based approach: finite differences
Iterative solution (RMM-DIIS, CG, Davidson,…)
Residuals, preconditioning
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Exchange and correlation: GGA, meta-GGA, EEX, hybrids, …
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The role of the van der Waals interaction in the adsorption of organic
molecules on surfaces
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Is van der Waals interaction weak?
K. Autumn et al., PNAS 99, 11993 (2002)
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vdW-DF correlation energy: Langreth, Lundqvist et al.
Tabulated conveniently
Contains also and
The integral is evaluated on adaptive grid
Phys. Rev. Lett. 95, 109902(E) (2005)
Phys. Rev. B 79, 201105 (2009)
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Adsorption of planar molecules on Au(111)
PBE vdW-DFMelamine -0.25 -0.88NTCDA -0.10 -1.31PTCDA -0.17 -1.88Melamine dimer – -2.08
Adsorption energies (eV)
The vdW interaction contributes to the adsorption considerably
Phys. Chem. Chem. Phys. 12, 4759 (2010)
Andris Gulans et al.
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Interaction of molecular dimers
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PTCDA molecule
• PTCDA (3,4,9,10-perylene-…) is an organic semiconductor
• derivatives are suitable for manufacturing solar cells
• organizes in well-ordered films on substrates
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Computational details
• Surface is modelled by slabs with 3–4 atomic
layers• DFT calculations done
with the LCAO code SIESTA
• PBE and vdW-DF xc-functionals
• Double-zeta with polarisation basis set
• BSSE estimated using counterpoise correction
scheme
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Diffusion over a step on KBr (100) surface (GGA results)
-0.50 eV -0.24 eV -0.07 eV
The molecules are likely to detach during the diffusion over the step
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Diffusion over a step (revised)
-0.50 eV -0.24 eV -0.07 eV
-2.36 eV -1.63 eV -0.51 eV
PBE
vdW-DF
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Diffusion diagram
Phys. Rev. B 80, 085401 (2009)
*The PBE curve is displaced in the energy scale
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• GGA fails in cases where the adsorption is dominated by the vdW interaction. This may have
also implications on the accuracy of diffusion calculations.
• The interaction energies of the molecular dimersare influenced not only by hydrogen bonds
(electrostatic interaction), but also by dispersion.
Non-covalent interactions are particularly tricky for common DFT methods. Yet there are approaches
to overcome these difficulties.
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Time-dependent DFT: time-propagation
Assume: overlap matrix independent of time
Time propagation: Crank-Nicolson with predictor-corrector steps
Initial (time-dependent) perturbation
External (electromagnetic) vector potential
Nonlinear effects included
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Time-dependent DFT: linear reponse
Transition energies between initial and final states
Coupling matrix
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Time-dependent DFT: kernel for linear response
Coulomb part (Random Phase Approximation)
Exchange-correlation part
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Excitation energies from linear response (ALDA)
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TDDFT: Optical absorption
Delta pulse excitation
Time propagation: record time-dependent dipole moment �(t)Fourier transform � dipole strength tensor, oscillator strengths
Linear response: eigenvectors needed Kohn-Shamtransition dipoles
Oscillator strengths
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Nonlinear effects: higher harmonics in emission spectra
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Photoelectron cross sections
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Transport: open boundaries, finite-bias Landauer - Büttiker
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Also: localised basis sets
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Other utilities
• �SCF
• Wave function overlaps
• X-ray absorption spectra
• Wannier orbitals
• Density analysis and partitioning
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Parallel calculations
Key issue: real-space domain decomposition
TDDFT
STATIC DFT
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Overall Challenge: Making the Leap from Observation Science to Control Science
The things we want to do (i.e. designing materials to have the properties we want & directing synthesis to achieve them) require the ability to see functionality at the relevant time, length & energy scales.
We will need to develop & disseminate new tools capable of viewing the inner workings of matter—transport, fields reactivity, excitations & motion
This new generation of instruments will naturally lead to devices capable of directing matter at the level of electrons, atoms, or molecules.
(Mark Ratner)