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Theoretical Concepts for
Chemical Energy Conversion
processes and functional materials
Luca M. GhiringhelliFritz Haber Institute, Theory group
held during summer semester 2014
at Technische Universität Berlin
Course material (slides and list of suggested textbooks):
http://www.fhiberlin.mpg.de/~luca/TCCEC.html
Technicalities on the course
Chemical energy conversion: catalysis
Fre
e en
ergy
Non-catalytic free-energy barrier
Adsorption
Reaction
Desorption
Reaction coordinate
Product(s)
Reactant(s)
Issues:● Reaction rate: proportional to exp ( F / kT) Δ● Selectivity: eliminate or at least reduce the undesired products
ΔFnon-cat
ΔFcat
The example of heterogeneous catalysis:
A catalyst usually gets active after a macroscopic “induction time”.Thus: We introduce a material, but the material that exists at the steady state of catalysis may be different from the one that was introduced.
Which material is formed and active at reactive conditions?
Veracity and reliability
Veracity: Some Words about Theory
We need errorcontrolled links between the different simulation techniques
We need a reliable base!If the electronicstructure theory base is not reliable, everything that follows may be wrong, and we may miss the key issue.
We don’t (necessarily) want to identify THE key novel material, but just the most promising ones. And we want to identify anomalies in materials properties and functions that fall outside the established trends.How good is our theory? We don’t want to miss the best candidate.
Calculation of the function of materials typically demands multiscale modeling.
Semiempirical methods (AM1, PM6, CNDO, tightbinding)
Densityfunctional theory with (semi)local and hybrid functionals
Empirical potentials (“force fields”)(no explicit electrons)
Beyond independent electrons
(MP2, RPA, CCSD(T),...)
FullCI
Accuracy,Reliability,
and Predictive
Power
Computational Cost
Current state of the art in atomistic modeling
Realistic predictions: bulk oxide vs surface oxide, Pd (100)
E. Lundgren et al., Phys. Rev. Lett. 92, 046101 (2004)
(√5 × √5)R27 °
(√5
× √5
)R27
°p(
2 ×
2)
p(2 × 2)
bulk oxide
bulk
oxi
de
metalmetal
in-situ SXRD Theory
Obr / COcus
CObr / COcus
Obr / Ocus
Obr / -
ΔμO (eV)
Δμ C
O (
eV)
Realistic predictions: CO oxidation on RuO2 (110)
K. Reuter and M. Scheffler, Phys. Rev. Lett. 90, 046103 (2003); Phys. Rev. B 68, 045407 (2003)
Diamond nucleation on … white dwarfs
Ghiringhelli et al., PRL (2007).Discovery of V886 Centauri (BPM 37093), later PSR J17191438 b, 55 Cancri e
1. Recapitulation of key concepts in thermodynamics and statistical mechanics.2. Introduction to importance sampling Metropolis Monte Carlo, canonical ensemble and more.3. Introduction to (classical) molecular dynamics, microcanonical ensemble and more.4. Ab initio molecular dynamics: BornOppenheimer molecular dynamics and beyond (stateoftheart)5. Ab initio atomistic thermodynamics: phase diagrams. Application to surface/cluster corrosion and reactivity of realistic materials. Neutral and charged defects in semiconductors.6. Stochastic sampling of the Schrödinger equation: quantum Monte Carlo. Theory and application to realistic materials.
Topics
7. Sampling free energy I: (ab initio) phase diagrams. Thermodynamics integration, smart advanced techniques, and applications to realistic materials.8. Sampling free energy II: enhanced sampling (biased sampling, metadynamics, and more). Theory and application to realistic materials.9. Sampling free energy III: replica exchange, the problem of the choice of order parameters and reaction coordinates (from many to few “relevant” dimensions). Theory and application to realistic materials.10. Chemical reactions as rare events: transition state theory and beyond. Methods (transition path sampling, transition interface sampling, and more) and application to realistic materials.11. Stochastic sampling beyond equilibrium: ab initio kinetic Monte Carlo. Theory and application to realistic materials.12. Multiscale approaches. QM/MM, adaptive schemes and beyond. Theory and application to realistic materials.13. Materials discovery. The quest for descriptors.
Topics
Another equivalent formulation of the 2nd principle tells us that Any spontaneous change in a closed system (i.e. a system exchanging neither heat nor particles with its environment) can never lead to a decrease of entropy.
Thermodynamics, reversible engine, entropy
No work done on the system:
Entropy is extensive (weakly coupled systems)
1st principle, rewritten:
Everything we do not know: lack of information
Thermodynamics, entropy, generalized 1st principle
Thermodynamics, free energy
Let's define:
Combining 1st and 2nd principles:
It means that, at constant N and T , the maximum amount of work that the system can do (−δW ) equals (the negative of) the change in free energy F . Hence the name free energy: the part of internal energy that is actually available to produce work.
Extending the scale
Essentials of computational chemistry: theories and models. 2nd edition.C. J. Cramer, JohnWiley and Sons Ltd (West Sussex, 2004).Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functionsK. Reuter, C. Stampfl, and M. Scheffler, in: Handbook of Materials Modeling Vol. 1, (Ed.) S. Yip, Springer (Berlin, 2005). http://www.fhiberlin.mpg.de/th/paper.html
{Ri}
E
Potential Energy Surface: {Ri}
(3N+1)dimensional
10-9
10-6
10-3
1
10-15 10-9 10-3 1
Length(m)
Time (s)
Microscopicregime
Mesoscopicregime
Macroscopicregime
few processes
few atoms
many atoms
many processes
continuum
average overall processesmore
details
more proc
esses
Thermodynamics:p, T, V, N
Statistical mechanics, microcanonical ensemble
System at constant energy U, consisting of N particles in a box of volume V. If known, we can solve the equations of motion. This is useless: we are more interested in average properties of the system than detailed properties.
Hypothesis: U, assuming a particular value E, is all we need to know about the system (together with N and V ) to describe the equilibrium state. We call the set of all state at energy E, the microcanonical ensemble. Defining Ω(E): the number of states between E and E + Eδ :
Ensemble average:
All state with a fixed energy E are equally probable
Statistical mechanics, properties of highdimensional spaces
Properties of Ω(E):
Number of states between 0 and E
Energy per degree of freedom
Number of states between 0 and ε α of order 1, e.g. free particle: harmonic oscillator:
E.g., Ddimensional hypersphere
Statistical mechanics, definition of temperature
Two systems, E1 + E2 = EProbability that 1 is in state i :
At equilibrium:
Statistical mechanics, definition of temperature
Definition of entropy in statistical mechanics:S is maximum at equilibrium and extensive
Invoking thermodynamics:Systems 1 and 2 at equilibrium have the same temperature
Statistical mechanics: free energy, alternative derivation
Important derivative of F:
Thermal length:
Configurational partition function: factorizing momenta
Partition function of ideal gas:
Statistical mechanics: free energy as a probabilistic concept
Energy: mapping from 3N coordinates into one scalar
so that:
Formally:
(in this page, Helmholtz free energy, F(N,V,T))
if we can calculate E and write analytically on approximation for S for our system, we use this expression. Example: ab initio atomistic thermodynamics.
Ab initio
or similar derivatives that yield measurable quantities (in a computer simulation): one can estimate the free energy by integrating such relations. This is the class of the so called thermodynamicintegration methods.
Thermodynamics
Thermodynamic Integration
Ab initio
Free energy, one quantity, many definitions
Classical statistics (for nuclei):
● Fundamental statistical mechanics thermodynamics link↔
Ab initioAb initio
● Probabilistic interpretation of free energy
Free energy, one quantity, many definitions