Questions tagged [ab-initio-calculations]

For questions about ab-initio (first principle) calculations.

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12
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1answer
218 views

Comparing GAMESS, OpenMOLCAS and Psi4

Over the years many open-source ab initio packages have been developed and the diversity in subroutines, methods and portability (ability to interface with other software) is quite varied for ...
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2answers
66 views

How to calculate formation energy using DFT+U?

I noticed that a lot of articles use DFT+U methods to calculate different formation energies when using regular DFT and DFT+U methods. For example, in the attached paper[1] on $\ce{In2O3}$, the ...
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0answers
50 views

Orbital space clustering

I am interested in how the orbital space of a system can be partitioned into clusters. I read reading 'Cluster many-body expansion: A many-body expansion of the electron correlation energy about a ...
5
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0answers
54 views

How to calculate energy gradient and Hessian with respect to only a few internal coordinates using Gaussian?

I would like to form the (numerical) gradient and the Hessian of the total electronic energy with respect only to a few internal coordinates (namely, the six intermolecular coordinates of a weakly ...
9
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0answers
40 views

Difference of high-frequency and static dielectric constants?

I found in an article [1] the following definition of FNV (Freysoldt, Neugebauer and Van de Walle) energy correction term for a charged defect in a cubic supercell: $$E_{\text{corr}}=\frac{\alpha_Mq^2}...
12
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2answers
305 views

For DFT and Hartree-Fock, how can we know that we have a true minimum? Is there an equivalent to the "frequency analysis" for geometry optimization?

In a geometry optimisation, one can check if they have reached a minimum by checking the phonons. And to a certain extent, the global minimum can be confirmed by relaxing from different starting ...
12
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2answers
204 views

What are the references in multi-reference methods?

After reading What exactly is meant by 'multi-configurational' and 'multireference'? I still don't understand what the multiple "references" are. I understand that you can for example use a ...
12
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2answers
84 views

What are properties of interest for energy storage materials?

In the field of simulation for energy storage materials, there is always a gap between theoretical calculations and demand for experimentalists. What could be directly calculated from ab initio ...
11
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1answer
88 views

Translating fractional coordinates resulting from conventional cell and primitive cell

I have calculated the band structure of some material where the unit cell was the conventional one (i.e. the lattice vectors are orthogonal). I'm trying now to calculate the same band structure only ...
7
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2answers
97 views

What quantity to perform CBS extrapolation on for adsorption/binding energies?

When calculating complete basis set extrapolations for quantities, such as adsorption energies or binding energies, which depend on the difference between three quantities (i.e. $E_\textrm{prop} = E_\...
8
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1answer
128 views

Can I run an nscf calculation in Quantum ESPRESSO with disk_io='none'?

I had run an SCF calculation on a crystal system in Quantum ESPRESSO and had collected the converged charge density. However, during the NSCF calculation, there is the generation of ...
12
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6answers
905 views

How may I calculate the bond length between two atoms?

I have been looking into Lennard-Jones Potential and Morse potential. And what the formulas do is to calculate a potential and the 'well depth' and 'distance at which the intermolecular potential ...
15
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6answers
2k views

How do I simulate the interaction between two atoms?

I want to build a rough atom interaction simulator, where pushes two (or more) atoms toward each other and they should behave physically correctly: attract, bond or repel with some force. So that is ...
4
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1answer
62 views

Has there been any work done on fluid dynamic or acoustic modeling of the human oral (or nasal) cavity?

The purpose is for me to develop a deeper understanding of speech production and how distinct features that identify a phenome are produced. ab initio models based on classical & statistical ...
19
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3answers
3k views

Solving the Schrödinger equation as a service?

Does anyone know of a company with a cloud-based service which takes some representation of a molecule as an input, and then somehow solves the Schrödinger equation to some level of accuracy and ...
8
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2answers
522 views

Pressure applied DFT calculations in Quantum ESPRESSO

I am trying to perform the pressure-induced DFT calculations in Quantum ESPRESSO. For this, I have calculated the lattice constant of the cubic structure by vc-relax...
9
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1answer
94 views

What do the parameters at the top of an FCIDUMP file mean?

A while ago I wrote a simple program that takes in an FCIDUMP file (generated by MOLRPO) and can determine matrix elements (or ...
10
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3answers
310 views

Electronic structure calculations without SCF?

Is there an ab-initio calculation of electronic structure that does not involve self-consistent iteration? What I mean by self-consistent iteration is the iteration to solve the equation $$ \mathbf{y} ...
9
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1answer
177 views

How to calculate FCI matrix elements from an FCIDUMP file

I'm interested in writing a simple program (as an exercise) that takes an FCIDUMP file (e.g. from a Molpro calculation) and can determine FCI matrix elements (e.g. to construct the entire FCI matrix ...
7
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0answers
55 views

How to reproduce 2D small-angle X-ray scattering (SAXS) interferogram of water via ab-initio MD methods?

I am trying to reproduce the 2D SAXS interference pattern of bulk water via ab initio MD methods as shown in the figure below: The main aim is to recreate figure (a) via averaging MD configurations ...
13
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1answer
102 views

What is a good practical way to determine whether or not I need to use a multiconfigurational method?

I’m planning to run a calculation of single electron transfer where my system goes from singlet to triplet biradical (presumably). So I’m wondering how to decide if I need to use milticonfigurational ...
7
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1answer
329 views

Molecular Dynamics Simulations in VASP

I want to run molecular dynamics (MD) simulations with VASP for Mg2Si to investigate the simulation time vs ENCUT and KPOINTS, ...
7
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0answers
89 views

How to fix gauge in Quantum ESPRESSO?

I am trying to use Quantum ESPRESSO to manually calculate the Berry phase using the Berry connection, due to evolution along a closed loop in k-space. I want to do this manually instead of using ...
10
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1answer
94 views

Can I do a "constrained" geometry optimization in Quantum ESPRESSO, such that certain dopants appear in certain positions even after relaxation?

I am studying the magnetic properties of bulk ZnO, so I am doping it with transition metal elements in order to observe the magnetic spectrum. I replace a Zn atom with a transition metal element at ...
11
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1answer
448 views

How to understand the spin of exciton?

In very recent years, exciton physics has been intensively explored as the emergence of 2D materials. Exciton is a bound electron-hole pair and is bosonic, namely hosting an integer spin. My first ...
12
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1answer
556 views

When do we abandon ab initio methods?

This question is related to (and was originally asked in) another post about "quantum protectorates" I made here. Ab initio methods are nice because they directly solve a sort of "...
10
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1answer
228 views

"Quantum protectorates" and the limits of grand unified theories

I've recently made myself more familiar with Laughlin and Pines' "quantum protectorate" arguments (1, 2), which seems built on Anderson's famous More is Different article (3). This does seem ...
7
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1answer
203 views

How to understand the wavefunction outputed by QE with HDF5 format?

Following the answer to my question: What is the best program to manipulate numerical DFT wavefunctions to calculate custom matrix elements?, I compiled Quantum ESPRESSO with the hdf5 flag on, and now ...
10
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1answer
273 views

How to make sure that my SCF calculation is correct?

hope you are doing well. I am trying to study the electronic properties of ZnO doped with transition metals (Co, Ni, Fe) using DFT calculation, I am a beginner on DFT calculation using quantum ...
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0answers
59 views

Calculating single electron transfer

I'm thinking about doing some computational research about pyrazoline oxidation in chlorinated solvents under light. I've found plausible mechanism (article) that involves single electron transfer ...
13
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1answer
135 views

Are nuclear ab-initio methods related to materials ab-initio methods?

I just read this synopsis in Physics where scientists used first-principles nuclear physics calculations to predict the stability of 700 isotopes up to iron. I didn't know that this was possible, ...
14
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1answer
286 views

DFT Knowledge Check for Posed Problem

Currently I am trying to apply to a graduate program overseas. They require me to include a research proposal in my application. My topic of interest is computational materials science. So, one ...
9
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3answers
391 views

What is the procedure for constructing an ab initio potential energy surface for CH3Cl + Ar?

I would like to construct a model analytical potential function for collision between CH3Cl and Ar. This would be the sum of CH3Cl intramolecular + CH3Cl---Ar intermolecular potential. First I would ...
12
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1answer
141 views

What's the physics contained in the exchange-correlation functional in the framework of KS-DFT?

This question is inspired by this post. In the Kohn-Sham framework of density functional theory, the total energy is expressed as: $$E=E_{kin}^{non}+E_{ext}+E_{H}+E_{xc}$$ in which The first term is ...
8
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1answer
80 views

Floquet's Theorem and ab-initio Calculations

Are there any ab initio packages that take advantage of Floquet's theorem (which one can think of as a generalization of Bloch's theorem from space to time) in order to study the impact of an ...
6
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1answer
35 views

Rotational magnetic moment and ab-initio calculations

Rotating molecules have an associated magnetic moment that can couple with both nuclear and electronic spin magnetic moments, thereby impacting the hyperfine energies of rotating molecules. Can one ...
7
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1answer
72 views

Signatures of Competing Orders in Ab Initio Calculations

Some materials have various competing and/or synergistic orders, such as charge density waves and superconducting pairing. Are there signatures of such an interplay between orders that can be seen in ...
10
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0answers
93 views

First principles calculations beyond s-wave BCS superconductors

Conventional superconductors are often modelled using Bardeen-Cooper-Schrieffer (BCS) theory with an s-wave order parameter, but not all superconductors describable with BCS theory have an s-wave ...
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0answers
26 views

How are pseudogap phases typically modelled?

Cuprate superconductors have a pseudogap phase that is not well-understood. What attempts have been made to understand the pseudogap phase?
5
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2answers
186 views

Calculating the band structure with different unit cell

I have to do a electronic band structure calculation on a heterostructure using DFT calculation. First I do the calculation on each phase separately. I use a primitive unit cell to do the calculation ...
9
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1answer
318 views

AIMD and velocity scaling in VASP

Typically one does velocity scaling to "heat" their sample with MD. How do we do this in VASP? Is it by setting the SMASS=-1 and ...
11
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2answers
159 views

Which properties of a material calculated using density functional theory are affected by smearing?

Smearing (smearing width $\sigma$, to be precise) has always been confusing. I understand what it is but can't pin it down when it comes to DFT calculations. Should it be converged like k-points and ...
12
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2answers
155 views

Permissible amount of forces on ions in a relaxed structure

An equilibrium structure would be at a local minimum but that doesn't happen with all relaxation calculations. Relaxed structures tend to have some resultant forces on ions. What amount of these ...
5
votes
1answer
110 views

What are the other matrix elements in singlet symmetry-adapted CISD?

Szabo/Ostlund list the CI matrix elements between singlet symmetry-adapted configurations (SAC) in Table 4.1 of their book: $$ \langle ^1\Psi^r_ a \lvert \mathcal{H} - E_ 0 \rvert ^1\Psi^r_ a \rangle, ...
9
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1answer
327 views

How to derive the k-path for monoclinic lattice structure?

Good afternoon all, My post is inquiring about the mathematical derivation behind the Monoclinic crystal structure (c axis unique). The Brillouin zone (BZ) I really would like to derive is shown below:...
5
votes
1answer
111 views

Best vdW correction functional for transition metal dichalcogenides

For relaxation of transition metal dichalcogenides (e.g.: WS2) using the VASP code, what is the best choice for vdW correction method (e.g.: DFT-D2, DFT-D3 or optB88-vdW) that can give you good ...
6
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1answer
75 views

Charge density wave and band inversion

Is there any connection between charge density wave and band inversion? Or is there any system in which band inversion follows the same mechanism as the CDW? I am just trying to find a connection ...
10
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1answer
79 views

Edge states in Topological Semimetals

Does it make any sense to calculate edge states for topological semimetals while they don't have any global gap?
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82 views

What are the ways to add dynamic correlation to an MCSCF wavefunction?

We recently learned about the many types of MCSCF, which treat "static" electron correlation. There are then many ways to treat further "dynamic" electron correlation to try and ...
10
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1answer
139 views

What is the "overcoherence problem" in Ehrenfest dynamics?

I am a novice in Ehrenfest dynamics and would like to know why Ehrenfest dynamics suffers from the "overcoherence problem"?