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Questions tagged [electronic-structure]

For questions concerning electronic structure concepts and calculations.

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Properties that can be deduced from Band structure and DOS

Calculation of band structure and Density of States (DOS) is ubiquitous in matter modelling research publications. What are some properties that can be deduced from a band structure plot and density ...
Thomas's user avatar
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19 votes
3 answers
4k views

Can DFT be considered an ab initio method?

Note added afterwards: This question also has excellent answers on the Chemistry Stack Exchange: Is density functional theory an ab initio method? I have very little experience with DFT, but coming ...
Prateek Goel's user avatar
23 votes
1 answer
279 views

When do relativistic effects need to be explicitly included?

For many applications with heavy metals, pseudopotentials can be used to include some amount of relativistic effects. But for what sort of systems does it become necessary to actually use a ...
Tyberius's user avatar
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15 votes
2 answers
3k views

Total spin and/or multiplicity for transition metal ions?

I am trying to input various charge states of various transition metal ion species into DFT calculations. The program requires an input of multiplicity = 2*(total spin) + 1. I have an undergraduate-...
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15 votes
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Is Heisenberg model or in its simplier form Ising model a good approximation to study magnetic systems?

Heisenberg model $$\hat{H}=-\sum_{\langle i j\rangle}J\hat{S}_i\hat{S}_j$$ And in its simplified version, the Ising model $$\hat{H}=-\sum_{\langle ij\rangle}J\hat{S}_i^z\hat{S}_j^z$$ are widely ...
Paulie Bao's user avatar
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10 votes
1 answer
6k views

What is the physical meaning of occupations='smearing' in Quantum ESPRESSO?

I want to understand, what is the intuition behind using the occupations='smearing' or 'tetrahedra' in Quantum ESPRESSO input file? I mean how we can interpret what is going to happen inside of ...
UJM's user avatar
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10 votes
2 answers
466 views

How can I be sure that I have found the conduction band minimum in DFT?

Let's say I want to find the electronic energy bands from DFT calculations in VASP along a chosen path in the Brillouin zone. Now, I want to make sure I choose a path that captures the conduction band ...
B. Brekke's user avatar
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27 votes
2 answers
650 views

How to determine, a priori, whether a compound has multireference character?

We say a system has multireference character if it is not well described at the SCF level by a single configuration/Slater determinant. Not accounting for this can lead to errors in post-SCF methods, ...
Tyberius's user avatar
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25 votes
2 answers
402 views

How to overcome the exponential wall encountered in full configurational interaction methods?

Similar to how a molecular orbital, also known as a 1-electron wavefunction, can be represented with a linear combination of “basis” functions, e.g., atomic orbitals (LCAO): $$\Phi(\mathbf{r})=\sum_i^...
Cody Aldaz's user avatar
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24 votes
3 answers
617 views

How can quantum computing accelerate materials modeling?

Materials modeling is very computationally intensive and first-principles simulation of a real system of reasonable size typically involves the use of classical supercomputers. How can quantum ...
edwinksl's user avatar
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19 votes
3 answers
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How to incorporate the effect of spin-orbit coupling in electronic structure calculation

Since the effect of spin-orbit coupling plays an important role in many transition metal complexes, what are the common methods to incorporate the effect of spin-orbit coupling?
Paulie Bao's user avatar
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17 votes
1 answer
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Gaussian-type orbitals vs Slater-type orbitals in terms of computational efficiency

When creating molecular orbitals via the linear combination of atomic orbitals method I am told that Slater orbitals provide a more accurate representation whilst Gaussian orbitals are computationally ...
DJA's user avatar
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16 votes
1 answer
352 views

When should geometry relaxation not be performed?

My understanding is that geometry relaxation (i.e. letting the nuclei follow the force gradients of the PES until they reach local minima) should usually be performed in order to get the correct ...
2ndQuantized's user avatar
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13 votes
2 answers
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How to convert Unified Pseudopotential Format (UPF) into Ultrasoft Pseudopotential (USPP) format?

I am working on DFT code having plane wave along xy axis and bspline in z direction. For calculating the properties of TMD materials, spin orbit coupling must be included but i did not find fully ...
Raj Kumar Paudel's user avatar
12 votes
2 answers
160 views

Avoiding charge-migration in electronic structure calculations with solvent-like environments

When performing electronic structure calculations on a molecule, one can surround it with point charges to mimic a solvent environment, and lend polarization effects. There is a bit of a caveat ...
B. Kelly's user avatar
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10 votes
4 answers
2k views

How can I find the parameters in a basis set?

I have a question about the basis functions of Gaussian. When the basis function is set (e.g., 6-31G), I would like to know the specific value of Gaussian's parameters for each atom. In particular, I ...
neco's user avatar
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8 votes
1 answer
491 views

How do the various programs read or write integrals in FCIDUMP format?

The FCIDUMP format was created by Peter Knowles and Nick Handy circa 1989 and now allows many electronic structure software packages to interact with each other. ...
Nike Dattani - No Free Time's user avatar
6 votes
2 answers
727 views

How to find out the multiplicities of molecules containing d and f block species?

I know that the U atom has four unpaired electrons. Therefore it has a total spin S of 2. This means its multiplicity is 2S+1 = 5. Now consider a complicated molecule such as $UF_6$. In theory all I ...
DJA's user avatar
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5 votes
1 answer
83 views

How to obtain imaginary frequency polarizability?

I am trying to find a way to calculate $C_6$ dispersion coefficients for some atoms and molecules. In papers and textbooks, this quantity is introduced using the Casimir-Polder integral from the ...
Szgoger's user avatar
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23 votes
3 answers
810 views

What is the largest system that has been studied by the coupled cluster method?

This is parallel to the analogous question about the largest DFT calculation: What is the largest material that has been studied using density functional theory? I assume we allow high performance ...
Paulie Bao's user avatar
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22 votes
0 answers
244 views

Is there a software that can do derivatives with respect to user defined vibrational modes?

I'm doing some property calculations that depend on a sum of derivatives of some quantity with respect to normal vibrational modes. I was hoping to find some physical intuition relating the type of ...
Tyberius's user avatar
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19 votes
4 answers
737 views

How is group theory used to deduce which of these integrals are equal to 0?

The number of all two-electron integrals: $$ \tag{1} \langle \phi_1 \phi_2|\phi_3\phi_4 \rangle = \int d^3\mathbf r' \int d^3\mathbf r'' \, \phi_1(\mathbf r'') \, \phi_2(\mathbf r') \frac{1}{|\mathbf ...
nougako's user avatar
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17 votes
2 answers
150 views

What approaches are employed to treat amorphous materials?

A lot of materials electronic structure research focuses on periodic structures like polymers, nanotubes/sheets, Metal Organic Frameworks (MOFs) and crystals, which makes them amenable to PBC ...
Tyberius's user avatar
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15 votes
2 answers
162 views

What is the largest system for which vibrationally resolved electronic spectra have been simulated using ML-MCTDH?

Multi-layer Multi-configuration time-dependent Hartree (ML-MCTDH) is a method widely used for simulation of vibrationally resolved electronic spectra that incorporate the coupling between vibrational ...
Paulie Bao's user avatar
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14 votes
2 answers
720 views

How to calculate energy of a molecule in an aligned electric field?

So I am aware that many quantum chemical codes are able to add external fields to molecular calculations (for example to calculate polarizabilities). I also know that in some (eg. Orca) you have some ...
user avatar
12 votes
1 answer
337 views

What are the practical limitations of R12/F12 methods?

Electron correlation methods based on an expansion of the $N$-electron wavefunction in terms of Slater determinants have a slow basis set convergence. Moreover, explicitly correlated methods (such as ...
Verktaj's user avatar
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11 votes
4 answers
2k views

Codes for post-processing Gaussian cube files?

I have generated MOs of a molecular system and I need to recreate its electronic density via $\rho(r)=\sum_{A_i}|\psi_i(\mathbf{r})|^2$ where $A$ is a set of occupied MOs. I am trying to do this via ...
mykd's user avatar
  • 2,352
11 votes
2 answers
1k views

Is there an energy gap correction factor that needs to be used in DFT calculations?

I am a beginner and have performed the DFT calculation in Quantum ESPRESSO for calculation of the direct and indirect bandgap of Silicon. I found the indirect bandgap to be 0.6551 eV. Now, I have ...
UJM's user avatar
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10 votes
2 answers
194 views

Quantifying electronic overlap?

I am trying to understand how to quantify the electronic interaction/conduction/overlap between two metal cores at varying distances. As someone who works primarily with classical MD, and has less ...
anneb101's user avatar
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9 votes
1 answer
787 views

Work Function Problem

2D materials (such as graphene) are mostly exfoliated from their 3D bulk counterpart. I am wondering how we can calculate work function for a 2D material? Normally electrostatic potential in the out-...
Shahid Sattar's user avatar
8 votes
1 answer
318 views

What differences are there when modeling metal-organic-frameworks as isolated clusters or periodic solids?

Should I take MOF (metal-organic framework) as a periodic structure or an isolated cluster? What's the difference between them? If it's periodic, can I do an analysis of the electronic structure, such ...
Jack's user avatar
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8 votes
1 answer
634 views

Do the U values affect structure optimization with the DFT+U simulation to optimize crystal structure?

I am trying to do structure optimization of TiO2, since titanium is a transition metal, I need to use DFT+U to simulate the right band structure, but the electronic band structure would also affect ...
Jack's user avatar
  • 2,067
7 votes
1 answer
282 views

Computing analytic derivatives of molecular Hamiltonians obtained from solving Hartree-Fock equations

I'm currently working on a quantum chemistry project, I am dealing with a second-quantized electronic Hamiltonian (corresponding to some molecule), written in terms of fermionic creation and ...
Jack Ceroni's user avatar
7 votes
1 answer
438 views

How is the zero energy defined for molecular orbitals?

In DFT and Hartree-Fock bonding orbitals have negative energies while antibonding orbitals have positive energies. Why is this the case? It seems this is due to the zero-point energy being defined for ...
lex2763's user avatar
  • 390
7 votes
2 answers
438 views

Optical Band gap determination from absorption coeffecient using DFT

I am currently working with a perovskite material which shows forbidden transitions. Older studies used bigger values of $\alpha$ in HSE06 calcualtions(to match with experimental Band Gap) but a ...
Chan's user avatar
  • 659
7 votes
2 answers
680 views

What are the limitations of FCI?

I am aware of the fact that Full Configuration Interaction scales poorly. Taking into account that the number of $n$-times excited Slater determinants of an $N$-particle System with $M$ basis ...
Jannis Erhard's user avatar
7 votes
1 answer
430 views

Why use PySCF/OpenMolcas instead of VASP/QuantumESPRESSO

While I am aware that VASP and QE are widely used for solid-state materials and PySCF/OpenMolcas are primarily designed for molecular systems. I want to know why should I use PySCF/OpenMolcas for ...
Jaafar Mehrez's user avatar
7 votes
2 answers
336 views

How to rotate polarizability tensor depending upon the molecular coordinates?

My question is somewhat related to molecular rotation. I have calculated the polarizability tensor of $\ce{HCHO}$ molecule in PSI4. The output tensor is this, ...
Pro's user avatar
  • 1,056
6 votes
2 answers
488 views

How can I get started on DFT?

This question is simply: How can we use DFT in the field of electrochemistry? What are the best resources (Books, reviews, papers, software ...) to learn about using DFT in electrochemistry? ...
Elie H's user avatar
  • 1,411
6 votes
1 answer
151 views

Conductivity Computations of Molecular Wires [closed]

I am interested to compute the conductivity properties of some molecular organic wires in SIESTA. Since I am new in SIESTA, I would appreciate a guide for beginners to this field.
Aggelos Avramopoulos's user avatar
5 votes
1 answer
212 views

Calculating first and second derivatives of a molecular Hamiltonian?

I'm interested in computing first and second derivatives of molecular Hamiltonians with respect to nuclear coordinates. I've been using Psi4 and PySCF to perform Hartree-Fock calculations, and I was ...
Jack Ceroni's user avatar
4 votes
3 answers
603 views

Does the RI method actually accelerate basis convergence of MP2, "by one more zeta"?

It is often said that the resolution of the identity (RI) method accelerates the basis set convergence rate of MP2 "by one more zeta", e.g. that RI-MP2/cc-pVTZ/cc-pVTZ results are close to ...
Kanghun Kim's user avatar
3 votes
0 answers
840 views

What input setting in Quantum ESPRESSO will allow me to calculate the free energy of a solid? [closed]

I understand that it's possible to calculate the Gibbs free energy of a system ab initio, see for example: How can we calculate the Gibbs free energy from first principles? I'm wondering how I can do ...
Paulie Bao's user avatar
  • 3,973
3 votes
0 answers
185 views

Understanding derivation of geminal based methods [closed]

Cross-posted on Reddit. I have been reading through [1] to get a better understanding of geminal based methods. This is one of two questions that I'm asking about that paper on this site, the other ...
kevin's user avatar
  • 61
3 votes
1 answer
218 views

Need help about how I can get/generate semicore pseudopotentials to be used in Quantum Espresso DFT?

I would like to know if I can get/generate semicore pseudopotentials (some core electrons are also considered along with valence electrons) to be used in Quantum Espresso DFT. Actually, it would be ...
Sak's user avatar
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