19 votes
Accepted

How to understand the time-reversal symmetry in graphene?

Two-band model for graphene. To simplify the discussion of Dirac points, it is sufficient to consider a nearest-neighbor tight-binding 2-band model for graphene. This is a spinless model because spin-...
  • 10.6k
19 votes
Accepted

When do we abandon ab initio methods?

There's three scenarios that come to my mind, for when ab initio methods get abandoned: The cost becomes prohibitive (e.g. too many electrons) The insight is lost It is simply not required for what ...
  • 29.8k
16 votes
Accepted

Difference between potential energy, free energy and Coulomb energy in solid state physics

I can't answer in the context of DFT/VASP, where these terms might have specific uses, but I can offer some general comments. These terms are not necessarily mutually exclusive: Total energy probably ...
15 votes
Accepted

How much does the initial geometry affect the final geometry in optimization calculations?

Alot Optimizations are all about finding the minimum in something. Typically in geometry optimization, it is about finding the minimum in energy. At a minimum the derivative of energy with respect to ...
  • 4,086
12 votes

What significant matter modelling methods are implemented in commercial software, for which there is no freeware alternative?

I think the way this question is asked is a little too simplistic. In order to execute a computational project, there is always more than one calculation required. Even if you are happy with the ...
12 votes

How can we say that the KS equation is describing a noninteracting many-electron system?

First of all, let me emphasize that it is more appropriate to speak of KS equations (plural), which you correctly denoted by an index $i$ in your post. This index goes over all KS orbitals (i.e. ...
11 votes
Accepted

How to start with structural defects in monolayer?

First of all, I assume that you are taking VASP to perform your calculation. Secondly, I assume that your structural defect is taking a Hf atom from your structure. (You can deal with substitutional ...
  • 14.3k
11 votes
Accepted

Is there a list of all universality classes for phase transitions with examples of each?

A locally interacting system displaying a continuous phase transition belongs to a universality class that is determined solely by the system symmetries and dimensionality. Drawing from Wikipedia's ...
  • 786
11 votes

Software for solving quantum systems in 1D and 2D

Note: This answer was written when the question was specifically about exact diagonalization codes. In an ED context, "a large number of spin sites" would maybe mean somewhere in the 24-40 ...
  • 4,453
11 votes

What does “strongly correlated” mean?

Here I assume that we are focus on the condensed matter system which is composed of nuclei and electrons with the fundamental force: Coulomb Force. Furthermore, I ...
  • 14.3k
10 votes

How to convert Unified Pseudopotential Format (UPF) into Ultrasoft Pseudopotential (USPP) format?

USPP is not a format of pseudopotential. It is a type of pseudopotential that enables you to work with lower cutoffs. On the other hand, upf is a popular file extension for pseudopotentials. If your ...
  • 2,212
10 votes
Accepted

What's the past and future of 2D materials since graphene?

Actually, if you allow for quasi-2D systems, graphene has had a recent renaissance starting with the experimental discovery of correlated states in "magic angle" twisted bilayer graphene, ...
  • 4,453
10 votes
Accepted

How to use wavefunctions/density to determine which orbitals lead to edge states?

One way of determining this is using the projected density of states (P-DOS) This resolves the DOS into specific orbitals thereby allowing you to discretize each orbitals weight for a specific energy. ...
  • 2,234
10 votes

Why the band structure of a slab is not smooth?

1. Is it possible that the energy gap of the material is direct in the bulk but indirect in the slab? Yes, this is possible (as is the reverse). The 2D slab does not have the same symmetry as the full ...
  • 4,692
9 votes

What are real examples where fractals were used in Matter Modeling?

The review that @Anyon cited focuses on the use of fractal geometry to classify and model the structure of disordered materials, e.g. structures synthesized by the sol-gel method. The computational ...
  • 801
9 votes

What are some codes available for Dynamical Mean Field Theory (DMFT)?

eDMFT Link to website: eDMFT Developer: Kristjan Haule at Rutgers University Main features from their website: DFT+DMFT derived from the stationary Luttinger-Ward functional. Local correlated ...
  • 10.6k
9 votes

How much does the initial geometry affect the final geometry in optimization calculations?

I won't go over what Charlie Crown already described in his answer, but if you don't have experimental data, you can try starting off with the experimental structure of a similar compound that has the ...
  • 2,931
9 votes
Accepted

What class of materials are closest to realizing the tunable coupling Hamiltonian?

Realizing this Hamiltonian in a natural material I cannot imagine a material in which all nearest-neighbor spin-spin interactions can be adjusted arbitrarily at the same time. Spin-spin couplings ...
  • 29.8k
9 votes

What exactly is a topological insulator and how to recognize one from band structure?

The bulk band structure of a topological insulator would look just like any other insulator, with the Fermi level in the gap between the valence and conduction bands. If your band structure includes ...
9 votes
Accepted

How can we say that the KS equation is describing a noninteracting many-electron system?

As you note, the interacting electrons and the Kohn-Sham non-interacting electrons have the same density. How is this possible when the Hamiltonians for the two systems are so different? The answer is ...
  • 1,853
9 votes

What does “strongly correlated” mean?

Here is my summary of definition of strong correlated systems in different context. For ab initio electronic structure Hamiltonian The eigen value of the system can not be well approximated by single ...
  • 3,599
9 votes
Accepted

What software can I use for gas adsorption calculations?

You can look at RASPA by Snurr et al. (Dubbledam, Calero, Vlugt). It is made for gas adsorption and other simulations. They used to have a Fortran version called Music, but this is a revamped edition ...
  • 1,030
9 votes

How does a beginner condensed matter theorist working on real materials, get up to speed?

Welcome to the club of academic migrants! I didn't take any university-level chemistry courses during my undergrad years (except a special topics course for graduate students called "...
  • 29.8k
9 votes
Accepted

Can deformation potential theory be applied to 2D and 1D systems?

Deformation potential theory is almost always used to study 2D materials. The first line of the abstract of the original Bardeen-Shockley paper mentions "deformations of the crystal lattice"....
  • 29.8k
8 votes

What is the difference between U(1) and O(2) symmetry?

The 1-dimensional unitary group U(1) simply corresponds to all complex numbers with a modulus of 1. This is isomorphic to the special orthogonal group SO(2), which corresponds to all real 2x2 rotation ...
  • 1,411
8 votes

What significant matter modelling methods are implemented in commercial software, for which there is no freeware alternative?

One can also pose the opposite question, which may be more interesting: what significant matter modelling methods are implemented in open source software, for which there is no commercial alternative? ...
  • 15.7k
8 votes

What are some codes available for Dynamical Mean Field Theory (DMFT)?

DCore: integrated DMFT software for Correlated electrons DCore is aimed at model calculations and ab-initio calculations by the dynamical mean-field theory (DMFT). This package consists of programs ...
  • 351
8 votes

What does “strongly correlated” mean?

You got it completely correct when you said: "it seems that the definition of it is ambiguous and sometimes inconsistent in different field of study." As the other answers show, it's a bit ...
  • 29.8k
8 votes

Help with translating Hamiltonian into matrix

For this type of calculation, I find MATLAB/Octave to be easier, at least for demonstrating what to do. You can add the np. everywhere afterwards if you want to use ...
  • 29.8k
8 votes

What is signified by the properties of the electronic band structure?

The band structure determines the dispersion relation for the electrons within the material, the E(k) relation. Its gradient (the inverse of it, to be more precise) is related to the effective mass of ...

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