Materials Genomics
Unit 5: Monte Carlo Sampling & Continuum Mechanics

Prof. Dr. Philipp Pelz

FAU Erlangen-Nürnberg

FAU Logo IMN Logo CENEM Logo ERC Logo Eclipse Logo

Where We Stand

Recap of Units 1–4

  • Unit 1: postulates of QM, hydrogen atom, Born interpretation
  • Unit 2: multi-electron atoms, Born–Oppenheimer, Slater determinants, LCAO
  • Unit 3: Hartree–Fock, post-HF (MP, CC), density functional theory
  • Unit 4: thermodynamics, statistical mechanics, classical interatomic potentials, molecular dynamics
  • We can now produce finite-temperature trajectories of \(10^4\)\(10^7\) atoms — but only if we trust ergodicity to deliver equilibrium averages

What Unit 5 Adds — The Pivot

  • MD samples the canonical distribution by integrating Newton — slow when barriers \(\gg k_B T\)
  • A complementary sampler: Monte Carlo (MC) moves through configuration space without dynamics
  • For length scales above the atomic, we move to a continuum description — fields, PDEs, balance laws
  • We learn the two workhorses for solving those PDEs numerically: finite differences and a brief look at finite elements / volumes
  • Together with Units 1–4, this completes the simulation toolkit that feeds the ML half of the course

Lecture Roadmap

Part I — Monte Carlo sampling

  • The sampling problem and importance sampling
  • Markov chains, detailed balance, Metropolis
  • Ergodicity, equilibration, move design
  • Ensembles, kinetic MC, MC vs MD

Part II — Continuum & discretization

  • Balance laws and constitutive relations
  • Finite Difference Method (FDM)
  • Weak forms, Finite Element Method (FEM)
  • Finite Volumes (FV) and outlook

A Multiscale View

Length Time Method
0.1–1 Å fs QM / DFT (Units 1–3)
Å–nm fs–ps classical MD (Unit 4)
Å–μm μs–s Monte Carlo (today, Part I)
nm–m ms–years Continuum / FEM (today, Part II)
  • The “right” method tracks the smallest feature you must resolve
  • ML in materials genomics plays at every scale — surrogates for DFT, MLIPs for MD, neural PDE solvers for FEM
  • Today’s content is what those surrogates ultimately reproduce

Part I — Monte Carlo Sampling

The Sampling Problem

Equilibrium thermodynamics computes ensemble averages

\[\langle A\rangle = \int A(\mathbf{r}^N,\mathbf{p}^N)\,p_{\text{Boltz}}(\mathbf{r}^N,\mathbf{p}^N)\,d\mathbf{r}^N d\mathbf{p}^N\]

  • For \(N\sim 10^4\) atoms, the integral is over \(\sim 6\times 10^4\) dimensions
  • Closed-form solutions exist only for trivially decoupled systems
  • We need a sampling strategy instead of a quadrature strategy
  • Goal: draw configurations \(\{(\mathbf{r}^N_m,\mathbf{p}^N_m)\}_{m=1}^M\) such that \(\langle A\rangle\approx \tfrac{1}{M}\sum_m A_m\)

Why Brute Force Fails

For a regular grid with \(n\) points per dimension:

\[\text{cost} = n^{3N}\]

  • \(N=10\) atoms, \(n=10\): \(10^{30}\) evaluations — infeasible
  • This is the curse of dimensionality
  • Even sparse grids and quasi-Monte-Carlo lattices cannot beat the exponential scaling for typical materials problems
  • We need to sample adaptively, putting compute where the integrand is large

Why Uniform Sampling Fails

Uniform random sampling: pick \(\mathbf{r}^N\) at random with probability \(1/V^N\).

  • Almost every uniform draw places atoms on top of each other \(\Rightarrow\) astronomical \(V\), vanishing Boltzmann weight \(e^{-V/k_BT}\approx 0\)
  • The Boltzmann mass concentrates in an exponentially small subset of phase space
  • Estimator variance grows exponentially with \(N\)
  • Effectively no signal — every sample contributes essentially zero

We must bias the sampler toward high-probability regions: importance sampling.

Importance Sampling — The Idea

If we can draw \(\mathbf{x}_m \sim p(\mathbf{x})\) directly, then

\[\langle A\rangle = \int A(\mathbf{x})\,p(\mathbf{x})\,d\mathbf{x}\;\approx\;\frac{1}{M}\sum_{m=1}^{M} A(\mathbf{x}_m)\]

  • The estimator collapses to a simple average when samples follow \(p\)
  • Variance is now \(\mathrm{Var}(A)/M\) instead of an exponentially bad worst case
  • The target distribution here is the Boltzmann distribution \[p_{\text{Boltz}}(\mathbf{x}) = \frac{1}{Z}\,e^{-\mathcal{H}(\mathbf{x})/k_B T}\]
  • The catch: we cannot draw from \(p_{\text{Boltz}}\) directly because \(Z\) is intractable

The Partition Function Problem

  • The partition function is itself an integral over all of phase space \[Z = \int e^{-\mathcal{H}(\mathbf{x})/k_B T}\,d\mathbf{x}\]
  • Computing \(Z\) has exactly the same dimensionality problem as the original average
  • Key observation: ratios of probabilities do not depend on \(Z\) \[\frac{p_{\text{Boltz}}(i)}{p_{\text{Boltz}}(j)} = \exp\!\left[-\frac{E_i - E_j}{k_BT}\right]\]
  • A sampler that uses only ratios can target \(p_{\text{Boltz}}\) without ever evaluating \(Z\)
  • Markov-chain Monte Carlo (MCMC) is exactly such a sampler

Markov Chains — Setup

A Markov chain is a sequence of random states \(X_0,X_1,X_2,\dots\) with

\[P(X_{t+1}=x'\mid X_0,\dots,X_t) = P(X_{t+1}=x'\mid X_t) \equiv T(x_t\to x')\]

  • Memoryless: the next state depends only on the current state
  • \(T\) is the transition kernel — a probability over \(x'\) for each \(x\)
  • A distribution \(\pi\) is stationary if pushing it through \(T\) leaves it unchanged
  • If the chain is ergodic (irreducible + aperiodic), it converges to \(\pi\) from any starting state

Stationary Distributions

\(\pi\) is stationary if and only if

\[\pi(x') = \sum_x \pi(x)\,T(x\to x')\qquad\text{(global balance)}\]

A stronger, easier-to-design condition is detailed balance:

\[\pi(x)\,T(x\to x') = \pi(x')\,T(x'\to x)\]

Sum over \(x\) on both sides: detailed balance \(\Rightarrow\) global balance. The converse is not true — detailed balance is sufficient, not necessary.

Detailed Balance

Detailed balance demands that probability flux between every pair of states cancels:

\[\pi(i)\,P(i\to j) = \pi(j)\,P(j\to i)\]

  • Imposes a local condition rather than a global sum
  • Almost trivial to verify for a proposed acceptance rule
  • Sufficient (with ergodicity) to guarantee convergence to \(\pi\)
  • The standard design recipe for equilibrium MCMC algorithms

Proposal × Acceptance

Decompose the transition kernel:

\[P(i\to j) = P_q(i\to j)\,P_a(i\to j)\]

  • \(P_q\): proposal — how we suggest a trial move (random displacement, swap, …)
  • \(P_a\): acceptance — whether we keep the trial move
  • Inserting into detailed balance and dividing yields \[\frac{P_a(i\to j)}{P_a(j\to i)} = \frac{\pi(j)\,P_q(j\to i)}{\pi(i)\,P_q(i\to j)}\]
  • For symmetric proposals, \(P_q(i\to j) = P_q(j\to i)\), the proposal cancels

Deriving the Metropolis Criterion

For symmetric proposals, the acceptance ratio must satisfy

\[\frac{P_a(i\to j)}{P_a(j\to i)} = \exp\!\left[-\frac{E_j - E_i}{k_B T}\right]\]

The simplest rule satisfying this is the Metropolis acceptance:

\[\boxed{\;P_a(i\to j) = \min\!\left[1,\;\exp\!\left(-\frac{\Delta E}{k_B T}\right)\right]\;}\]

  • Down-hill moves (\(\Delta E\leq 0\)): always accepted
  • Up-hill moves (\(\Delta E>0\)): accepted with Boltzmann probability

The Metropolis Algorithm

Metropolis, Rosenbluth, Teller (1953).

  1. Initialise the system in some state (random, lattice, snapshot, …)
  2. Propose a symmetric trial move \(i\to j\) (e.g. displace one atom by \(\delta\in[-\Delta,\Delta]^3\))
  3. Compute the energy difference \(\Delta E = E_j - E_i\)
  4. Draw \(u\sim\mathcal{U}(0,1)\). Accept if \(u<\min[1,e^{-\Delta E/k_BT}]\)
  5. If rejected, stay in \(i\) and count it again
  6. After equilibration, average observables along the chain

Why Metropolis Works

With symmetric proposals, the Metropolis rule satisfies detailed balance for \(\pi=p_{\text{Boltz}}\):

\[\frac{P(i\to j)}{P(j\to i)} = \frac{P_a(i\to j)}{P_a(j\to i)} = e^{-\Delta E/k_BT} = \frac{\pi(j)}{\pi(i)}\]

  • The chain therefore converges to the canonical distribution
  • Accepting unfavourable moves lets it escape local minima
  • The rule never needs \(Z\) — only differences of energies
  • “Reject = stay = count again” preserves the proper invariant measure

Ergodicity

Detailed balance fixes the invariant distribution, but ergodicity is needed for convergence:

  • The chain must be able to reach every state with nonzero \(\pi\) in finite time (irreducibility)
  • It must not get stuck in a cyclic pattern (aperiodicity)
  • Proposal moves must be rich enough to cross all relevant barriers
  • Conservative moves \(\Rightarrow\) the chain may explore only a basin and give wrong averages
  • In practice: combine multiple move types (local + global) and check independence of the answer to the starting state

MC Workflow — Equilibration & Production

  • Equilibration / burn-in: discard the first \(N_{\text{eq}}\) samples — they reflect the initial state, not \(\pi\)
  • Production: record observables for \(M\) samples
  • Block averaging or autocorrelation analysis: estimator variance is set by independent samples, not total samples
  • Effective sample size \(M_\text{eff} = M / (1 + 2\tau)\) with autocorrelation time \(\tau\)
  • Run multiple independent chains from different seeds
  • Compare within-chain and between-chain variance (Gelman–Rubin \(\hat R\))
  • Diagnose poor mixing before trusting the average
  • Hint for the exercise: \(\tau\) is often much larger than you think

Tuning Step Size

Acceptance rate \(\alpha\) as a function of step size \(\Delta\):

  • Tiny \(\Delta\): \(\alpha\to 1\) but each step barely moves — slow exploration
  • Huge \(\Delta\): almost every proposal rejected, \(\alpha\to 0\)
  • Sweet spot: 20–50% acceptance for atomistic systems (theoretical optimum 23% for Gaussian targets)
  • Adapt \(\Delta\) during equilibration; freeze it for production
  • Auto-tuning during production breaks detailed balance — separate the phases

Move Catalogue for Materials

  • Atom displacement: random \(\delta\) on one atom — workhorse for fluids and amorphous systems
  • Species swap: exchange the labels of two atoms — fast equilibration of order/disorder
  • Volume / cell move: rescale box and coordinates — required for NPT
  • Particle insertion / removal: required for grand-canonical (\(\mu\)VT) sampling
  • Configurational-bias moves: regrow polymer chains segment by segment
  • Cluster moves: flip whole connected clusters (Swendsen–Wang) — Ising-type problems
  • Hybrid MC: short MD trajectory used as a proposal, Metropolis accept/reject

Ensembles in MC

Ensemble Fixed Extra move type
NVT (canonical) \(N,V,T\) displacement
NPT \(N,P,T\) + volume move
\(\mu\)VT (grand canonical) \(\mu,V,T\) + insertion / removal
Gibbs (multi-box) \(N_\text{tot},V_\text{tot},T\) + particle transfer
  • Same Metropolis acceptance, generalised by the appropriate Boltzmann weight
  • \(\mu\)VT and Gibbs are unique to MC — MD cannot create/destroy atoms
  • Phase coexistence (e.g. liquid–vapour) is naturally obtained in Gibbs MC
  • Mirror the experimental boundary conditions

MC vs MD — Side by Side

MD strengths

  • True dynamics: diffusion, viscosity, spectra
  • Non-equilibrium relaxation
  • Forces already available from MLIPs
  • Familiar Newtonian intuition

MC strengths

  • No forces required, only energies
  • “Unphysical” moves: species swap, volume change, particle exchange
  • Grand-canonical sampling
  • Often faster equilibrium sampling for dense / stiff systems
  • Trivially parallel over independent chains

In practice they are complementary: MD for kinetics, MC for equilibrium and rare events.

Metropolis–Hastings & Beyond

Asymmetric proposal \(\Rightarrow\) acceptance ratio carries the proposal correction:

\[P_a(i\to j) = \min\!\left[1,\;\frac{\pi(j)\,P_q(j\to i)}{\pi(i)\,P_q(i\to j)}\right]\]

  • Foundation for biased moves, parallel tempering, replica exchange
  • Kinetic Monte Carlo (kMC): choose events with rates \(r_{i\to j}\), advance the clock by \(\Delta t = -\ln u / \sum r\)
  • kMC samples kinetics of rare events (diffusion, defect migration) when a rate table is available
  • Modern variants: Hamiltonian / hybrid MC, NUTS, normalising-flow proposals — all share the same backbone

Part II — Continuum Mechanics

Why Continuum?

  • Below ~1 nm: atomistic discreteness matters \(\Rightarrow\) MD / MC / DFT
  • Above ~1 μm: \(10^{15}\) atoms is hopeless. Treat matter as a continuous medium
  • Fields \(\rho(\mathbf{x},t)\), \(T(\mathbf{x},t)\), \(\mathbf{u}(\mathbf{x},t)\), \(\sigma(\mathbf{x},t)\) live at every point
  • Atomic information enters only through material laws (constitutive relations)
  • This is the regime of structural mechanics, heat transport, fluid flow, electromagnetism
  • Same mathematical machinery serves every conserved quantity: mass, momentum, energy, charge, …

1D Balance Setup

Take a small interval \([x, x+dx]\), with flux \(q(x)\) flowing through the boundary and a source/sink density \(s(x)\):

  • Influx from source: \(s(x)\,dx\)
  • Net flux through boundaries: \(q(x+dx)-q(x)\)
  • If the two balance, total inside is constant

\[s\,dx = q(x+dx) - q(x)\] \[\Rightarrow s = \frac{q(x+dx)-q(x)}{dx}\]

Taking \(dx\to 0\) gives the differential form.

Stationary Continuity Equation

Limit of the balance:

\[\boxed{\;s = \frac{\partial q}{\partial x}\;}\]

  • Net divergence of flux equals the source
  • “What flows out must be produced inside”
  • Valid for any conserved quantity at steady state
  • Independent of the physical meaning of \(q\) — particles, heat, momentum, money

Transient Continuity Equation

If influx and out-flux don’t balance, the amount inside changes in time. Defining number density \(\rho_{\text{nmbr}} = N/dx^{\text{dim}}\) and taking \(dx,dt\to 0\):

\[\frac{\partial \rho_{\text{nmbr}}}{\partial t} = -\frac{\partial q}{\partial x} + s\]

  • The first term moves material in/out
  • The second adds material from sources
  • This is the transient continuity equation in 1D
  • Generalises directly to higher dimensions

Multi-Dimensional Form

In \(d\) dimensions the flux becomes a vector \(\mathbf{q}\in\mathbb{R}^d\) and the divergence replaces \(\partial/\partial x\):

\[\boxed{\;\frac{\partial \rho}{\partial t} = -\nabla\cdot\mathbf{q} + s\;}\]

where

\[\nabla\cdot\mathbf{q} = \sum_{i=1}^{d}\frac{\partial q_i}{\partial x_i}\]

This is the universal form of conservation. To close the system we need constitutive laws that specify \(\mathbf{q}\) in terms of the field.

Constitutive Laws

Constitutive laws connect flux to gradient of the field:

  • Fick’s law (diffusion): \[\mathbf{q} = -D\,\nabla c\]
  • Fourier’s law (heat): \[\mathbf{q} = -k\,\nabla T\]
  • Darcy’s law (porous flow): \[\mathbf{q} = -k_{\text{hyd}}\,\nabla p\]
  • All have the same form: flux opposes gradient
  • Coefficients (\(D\), \(k\), \(k_{\text{hyd}}\)) carry the material identity
  • Combining with continuity yields the diffusion / heat / Darcy PDEs
  • e.g. \(\partial_t c = D\nabla^2 c + s\)

Isotropy vs Anisotropy

  • Scalar coefficient \(\Rightarrow\) isotropic material — same response in all directions
  • Many real materials are not: layered composites, fibre-reinforced polymers, single-crystal metals
  • General form uses a material tensor \(\mathbf{A}\): \[\mathbf{q} = -\mathbf{A}\,\nabla\varphi\]
  • \(\mathbf{A}\) has up to \(d^2\) independent entries (symmetry usually reduces this)
  • The principal axes of \(\mathbf{A}\) are the directions of fastest / slowest transport
  • ML potentials and ML constitutive models routinely predict \(\mathbf{A}\) from microstructure

Part III — Discretization

Why Discretize?

  • Analytic solutions exist only for highly symmetric, often academic geometries
  • Real engineering problems involve complex shapes, multiphase media, nonlinearities
  • We replace the continuous PDE by an algebraic system on a finite grid
  • Two big families: finite differences (point values on a regular grid) and finite elements (basis functions on a mesh)
  • Both share a foundation: Taylor expansion of the field

Taylor Expansion

For a smooth scalar field \(\varphi\):

\[\varphi(x_0+\delta) = \sum_{j=0}^{\infty}\frac{1}{j!}\frac{\partial^j\varphi(x_0)}{\partial x^j}\,\delta^j\]

  • Truncating at order \(J\) leaves an error \(\mathcal{O}(\delta^{J+1})\)
  • We will obtain derivative approximations by rearranging truncated expansions
  • The choice of which neighbours to use determines accuracy and stability
  • All FDM stencils, including the gold-standard Crank–Nicolson and Runge–Kutta schemes, derive from this template

Forward & Backward Differences

Truncating at first order:

Forward:

\[\frac{\partial\varphi(x_0)}{\partial x} \approx \frac{\varphi(x_0+\delta) - \varphi(x_0)}{\delta}\]

Backward:

\[\frac{\partial\varphi(x_0)}{\partial x} \approx \frac{\varphi(x_0) - \varphi(x_0-\delta)}{\delta}\]

  • Both have leading error \(\mathcal{O}(\delta)\) — only first-order accurate
  • One-sided; useful at boundaries
  • Cheap and stable for advection-dominated problems with upwinding

Central Difference

Subtract the backward from the forward expansion: the second-order terms cancel.

\[\boxed{\;\frac{\partial\varphi(x_0)}{\partial x} \approx \frac{\varphi(x_0+\delta) - \varphi(x_0-\delta)}{2\delta}\;}\]

  • Leading error \(\mathcal{O}(\delta^2)\)one order better than one-sided
  • Symmetric: no preferred direction
  • Requires neighbour information on both sides \(\Rightarrow\) needs special treatment at boundaries
  • Standard choice for diffusive PDEs

Second-Derivative Stencil

Add forward and backward expansions to order 2: first-order terms cancel.

\[\frac{\partial^2\varphi(x_0)}{\partial x^2} \approx \frac{\varphi(x_0+\delta) - 2\varphi(x_0) + \varphi(x_0-\delta)}{\delta^2}\]

  • Error \(\mathcal{O}(\delta^2)\) (odd-order terms cancel)
  • Three-point symmetric stencil
  • Generalises to higher derivatives and higher orders
  • This is the discrete Laplacian — appears in heat, Poisson, Schrödinger, Helmholtz, …

Worked Example: 1D Laplace

Solve \(\partial^2\varphi/\partial x^2 = 0\) on \(x\in[0,4]\) with \(\varphi(0)=0\), \(\varphi(4)=1\).

Three grid points, \(\delta=2\):

\[x_1=0,\; x_2=2,\; x_3=4\]

Boundary values: \(\varphi_1=0\), \(\varphi_3=1\).

Central stencil at interior node:

\[\frac{\varphi_3 - 2\varphi_2 + \varphi_1}{\delta^2} = 0\;\;\Rightarrow\;\;\varphi_2 = \tfrac{1}{2}\]

  • The discrete problem became a single algebraic equation
  • More interior nodes \(\Rightarrow\) a tridiagonal linear system
  • Larger meshes: sparse linear solvers (e.g. conjugate gradient)
  • Same structure carries to 2D / 3D Poisson — the workhorse of FDM

Boundary Conditions

Three families dominate practice:

  • Dirichlet: value of \(\varphi\) is prescribed on \(\partial\Omega\) — fixed temperature, fixed concentration
  • Neumann: normal derivative \(\partial\varphi/\partial n\) is prescribed — insulated wall, fixed flux
  • Robin (mixed): linear combination \(\alpha\varphi + \beta\,\partial\varphi/\partial n = \gamma\) — convective heat transfer
  • One-sided differences are used at the edge of the domain
  • Bad boundary handling is the single most common bug in undergraduate FDM code

Time Stepping & CFL

For transient problems, march in time with finite differences as well:

  • Forward (explicit) Euler: \(\varphi^{n+1} = \varphi^n + \Delta t\,F(\varphi^n)\) — cheap, but conditionally stable
  • Backward (implicit) Euler: \(\varphi^{n+1} = \varphi^n + \Delta t\,F(\varphi^{n+1})\) — needs a linear solve, unconditionally stable
  • Crank–Nicolson: average of the two — second-order accurate
  • For diffusion with explicit Euler: stable only if \(\Delta t \leq \delta^2/(2D)\)
  • For advection: CFL condition \(\Delta t\leq \delta/|v|\) — information cannot move faster than the grid

Toward FEM: Weak Form

Multiply the PDE by an arbitrary test function \(w\) and integrate over the domain:

\[\int_\Omega \frac{\partial}{\partial x}\!\left(A(x)\,\frac{\partial\varphi}{\partial x}\right)\,w\,dx = \int_\Omega q\,w\,dx\]

Integrate the left-hand side by parts to lower the order of differentiation on \(\varphi\):

\[-\int_\Omega A\,\frac{\partial\varphi}{\partial x}\,\frac{\partial w}{\partial x}\,dx + \text{boundary terms} = \int_\Omega q\,w\,dx\]

The PDE is now stated as an integral identity for all admissible \(w\). Discretise both \(\varphi\) and \(w\) on a finite basis \(\Rightarrow\) a linear system.

Finite Element Method

Approximate \(\varphi(x)\approx\sum_i \varphi_i\,N_i(x)\) with local shape functions \(N_i\) (typically piecewise polynomials on a mesh).

  • Mesh handles complex 2D/3D geometry
  • Local support \(\Rightarrow\) sparse stiffness matrix
  • Higher-order \(N_i\) trade memory for accuracy (h- vs p-refinement)
  • Natural treatment of Neumann boundaries through the boundary term in the weak form
  • Industry standard: ANSYS, Abaqus, COMSOL, Code_Aster, FEniCS, deal.II
  • Strength: complex geometries, mixed BCs, multiphysics couplings
  • Weakness: mesh generation can dominate the workflow
  • Cousin: spectral methods (global shape functions, exponential accuracy on smooth domains)

Finite Volumes Variant

Set the test function \(w\equiv 1\) on each control volume and apply the divergence theorem:

\[\int_{V_k}\nabla\cdot\mathbf{q}\,dV = \oint_{\partial V_k}\mathbf{q}\cdot\hat{\mathbf{n}}\,dA\]

  • The PDE becomes a local flux balance between neighbouring cells
  • Conservative by construction — total mass / energy is preserved to round-off
  • Method of choice for CFD (OpenFOAM, ANSYS Fluent), reservoir engineering, shock-capturing schemes
  • Conceptually closest to the original “balance law” derivation we started with

Closing

Where ML Plugs In

  • Surrogate force fields (MACE, M3GNet, CHGNet, …) deliver \(E,\,\mathbf{F}\) for MD/MC at ~DFT accuracy
  • Neural samplers (Boltzmann generators, normalising flows) propose MC moves that vault over barriers — Noé et al. 2019
  • Neural PDE solvers (PINNs, Fourier neural operators, graph neural PDE solvers) learn the discretised continuum operator
  • Operator learning (FNO, DeepONet) generalises across geometries — promising for FEM-scale problems
  • Inverse design treats the continuum solver as a differentiable layer in a generative loop
  • Every technique in the rest of this lecture series sits on top of the simulation tools we just introduced

Key Takeaways

  • Importance sampling: integrate by drawing from a target distribution, not on a grid
  • Markov-chain MC + detailed balance: design a chain whose invariant distribution is \(p_{\text{Boltz}}\) — Metropolis is the canonical example
  • Ergodicity is non-negotiable: move design and diagnostics determine whether averages are physical
  • Continuum mechanics replaces atoms with fields. Conservation gives a continuity equation; constitutive laws close the system
  • FDM turns derivatives into Taylor stencils; FEM/FV start from a weak form / flux balance to handle real geometries
  • These three sampling paradigms (DFT, MD, MC, FEM) span the length-scale ladder. ML in materials genomics learns surrogates for each rung

Outlook — Unit 6

  • Unit 6: how do we encode a local atomic environment so an ML model can read it?
  • Classical descriptors (Magpie, matminer, RDF) and the local-environment ladder (ACSF, SOAP, ACE)
  • Those descriptors power universal ML force fields (MACE-MP-0, M3GNet, CHGNet) that feed straight back into the MD/MC machinery built today
  • Unit 7 then takes the conceptual leap to graph-based representations (CGCNN, MEGNet, SchNet, ALIGNN) and message passing on periodic crystal graphs

Continue

References