# Lecture: Computational Many-Body Physics

**summer term 2020**

The lecture will be given as an online course.
The links to the videos can be found on the
ILIAS page.

This lecture gives an introduction to numerical methods
for the investigation of classical and quantum
many-particle
systems. The focus is on models of strongly correlated
electron systems (Hubbard model,
single-impurity Anderson model) and
quantum spin models (Heisenberg model, Kitaev model).
The physical phenomena
(Mott transitions, Kondo physics, spin liquid physics, etc.) these models are
supposed to describe, are quite often out of the reach of
analytical techniques - this triggered the development
of very powerful numerical approaches, see Secs. 4 and 7 in the
table of contents.
The lecture also includes a brief introduction to basic
theoretical concepts, such as Green functions,
continued fraction expansions, reduced density matrices,
and entanglement measures.

Module description
of the primary area of specialization
`Solid State Theory/Computational Physics'

**Contents:**

- classical many-particle systems

1.1 cellular automata

- rule N, Game of Life, ASEP, Nagel-Schreckenberg model

1.2 statistical physics of classical many-particle systems

- Metropolis algorithm for the classical Ising model

This section appears on pages 51-53 in the previous script.

1.3 Newtonian dynamics of classical many-particle systems

- quantum-mechanical spin models - introduction

2.1 quantum spin models

- Heisenberg model, Kitaev model

2.2 diagonalization of small clusters

- Hamilton matrix - quantum-mechanical spin models - calculation of physical properties

3.1 spin correlations

-*T=0*and*T>0*

3.2 entanglement

- reduced density operator, entanglement entropy, Schmidt decomposition - numerical methods

4.1 exact diagonalization (Lanczos)

4.2 iterative diagonalization

- application to the 1d Heisenberg model, truncation, relation to rg methods (see Sec. 7)

4.3 quantum Monte Carlo (QMC)

- application to the 1d Heisenberg model, Suzuki-Trotter decomposition, world lines - fermionic many-particle systems

5.1 second quantization

5.2 fermionic models

- Hubbard model, single-impurity Anderson model

5.3 diagonalization of small clusters - Green functions for fermionic systems

6.1 basic definitions

6.2 equations of motion and continued fractions - renormalization
group methods

7.1 general concepts

7.2 numerical renormalization group (NRG)

7.3 density-matrix renormalization group (DMRG)

The script of the lecture given in the summer term 2017 is available here. This script covers most of the material presented in the current lecture, although in a different order.

**Tutorials:**

The online tutorialis will be held on Mondays, 14:00 - 15:30, via video conference.

Dates: 11.05, 25.05, 15.06, and 13.07

Tutor: Chae-Yeun Park

Solutions can be submitted in groups of up to three students. Please check the submission guidelines.

Exercises:

**Requirements for the admission to the module exam:**

To obtain the signature on the form Anmeldung zur Modulprüfung im Masterstudiengang at least 50% of the points from the exercises are required (for groups of two or three students; for those students who submit all their solutions individually, the limit is 30%). This applies to the overall score, not to the score for the individual sheets.

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