21 votes
Accepted

How to deduce phase transitions from a phonon calculation?

Background theory. In the harmonic approximation, the potential energy surface (PES) is expanded about an equilibrium point to second order, to obtain the Hamiltonian: $$ \hat{H}=\sum_{p,\alpha}-\frac{...
ProfM's user avatar
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15 votes

What does it mean to say a material is 'anharmonic'?

This is a difficult question without a straight-forward answer. In general you have to perform a test to decide whether the harmonic approximation is sufficient or whether you need to include higher ...
ProfM's user avatar
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12 votes
Accepted

What do phonon dispersion (or lattice dynamics) studies include?

TLDR: When you calculate phonons, you can describe electrons at different levels of theory, typically semilocal DFT, but also hybrids or dynamical mean-field theory. Phonons do include zero-point ...
ProfM's user avatar
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11 votes
Accepted

Pressure applied DFT calculations in Quantum ESPRESSO

I would not recommend applying external hydrostatic pressure by changing the lattice parameter by hand. This is because fixing the lattice parameters by hand may lead to the incorrect structure. For ...
ProfM's user avatar
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8 votes
Accepted

Why volume of unit cell is not the same as cubing the lattice constant?

Please read the pw.x documentation on the Bravais lattice index (ibrav). The cell vectors for the primitive cell corresponding ...
leopold.talirz's user avatar
8 votes
Accepted

What's the Difference between Lattice Statics and Lattice Dynamics?

Lattice statics refer more to the minimum energy lattice structure such as spacegroup, lattice constant, bond distances, or any other property you can derive from a geometric standpoint on that ...
Tristan Maxson's user avatar
7 votes

Can we do anything to reduce the number of displacements in a frozen phonon lattice dynamics study?

ProfM's answer gets the core idea perfectly right: Symmetry really is your best friend here. However, symmetry analysis is often quite involved, especially for larger unit cells. I recently ...
Anubhab Haldar's user avatar
6 votes

Can we do anything to reduce the number of displacements in a frozen phonon lattice dynamics study?

You can (should) use symmetry to reduce the number of displacements needed to construct the matrix of force constants. A nice pratical description of how to do this can be found in the description of ...
ProfM's user avatar
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5 votes

Phonon density of states in statistical thermodynamics

There are two possible things tripping you up here: Phonons are collective oscillations: they involve the motion of all the atoms together. Therefore it only makes sense to talk about the phonons of ...
taciteloquence's user avatar
5 votes

Pressure applied DFT calculations in Quantum ESPRESSO

In your case, the crystallographic axes look to be equivalent, so just use 'relax' to start with. This will fix the lattice constants (equivalent to applying pressure) but will move the atoms around - ...
Xivi76's user avatar
  • 2,392
5 votes

Is there an upper limit of temperature after which the quasi-harmonic approximation (QHA) fails?

The starting point of most calculations is the Born-Oppenheimer approximation, which separates the electronic and nuclear degrees of freedom. The electronic structure problem is then solved using a ...
ProfM's user avatar
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4 votes

What does it mean to say a material is 'anharmonic'?

A "pure harmonic system" does not allow opportunity for evolution. It is the equivalent of a fixed point. In initial consideration, its stability seems appealing, as it appears to be a goal (...
MAAT's user avatar
  • 49
4 votes
Accepted

What is the meaning of Coulomb Subtraction?

Got a bit too long for a comment, so I'll leave it as a semi-complete answer for now. In case the paper is inaccessible to other readers, much of the same information is in the gulp5.2 manual. In ...
Tyberius's user avatar
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3 votes

How to quantify the lattice distortion from XRD?

First of all it must be clarified that a lattice is a collection of geometrical point all characterized by a same surrounding (that is, each lattice point has the same number of nearest neighbour ...
gryphys's user avatar
  • 651
1 vote

How to quantify the lattice distortion from XRD?

In my group we develop continuous symmetry and chirality measures that can be used to quantify the distortion of a structure, given the 3D coordinates. Please see: https://csm.ouproj.org.il/ See also ...
Inbal's user avatar
  • 11

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