Questions tagged [ab-initio-calculations]

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

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11
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1answer
110 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, ...
13
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1answer
209 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. This doesn't make much sense to me since how can I write a research ...
10
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3answers
364 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
votes
1answer
109 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 ...
6
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0answers
40 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 ...
7
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1answer
32 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 ...
8
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1answer
70 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 ...
8
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0answers
55 views

ab-initio 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 ...
4
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0answers
19 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 from an ab-initio perspective?
6
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2answers
97 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 ...
10
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1answer
125 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
129 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
149 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
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1answer
91 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
164 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
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1answer
73 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
votes
1answer
62 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 ...
11
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1answer
62 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?
9
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0answers
46 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 ...
8
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1answer
110 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"?
6
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0answers
52 views

What is the difference between Ehrenfest dynamics and Born-Oppenheimer Molecular Dynamics? [duplicate]

What is the fundamental difference(s) between Ehrenfest dynamics and Born-Oppenheimer MD? Also, in what situation(s) or case(s) one is preferred than the other one?
10
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0answers
51 views

Why are single excitations ignored in the MP2 component of double hybrid functional calculations?

In the original paper by Grimme introducing double hybrid functionals (also summarized in just 3 paragraphs here), it says: "[As] opposed to nonempirical versions of KS-PT2 [19], the single ...
12
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2answers
188 views

What ab initio methods are better suited to modelling disorder in materials?

In contrast to the perfect periodic bulk materials, now computational approaches are moving towards modelling ‘real’ materials or solid solutions with dopants, dislocations, grain boundaries and ...
14
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1answer
180 views

Great computer graphics for matter modelling

I am trying to write some code to automatically generate a judicious set of graphics from DFT/fn-DMC calculations so that users can sanity check their results quickly. My literature review has found ...
19
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5answers
356 views

What are the types of SCF?

Many of us know the most common types of SCF (though we can do better than Wikipedia at explaining them): RHF (Restricted Hartree-Fock), RKS (Restricted Kohn-Sham) [link to answer] UHF (Unrestricted ...
10
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2answers
179 views

What are some methods for modeling bulk phase infrared spectra?

I am interested in predicting the infrared spectra of bulk phase molecules, it seems that AIMD (ab initio molecular dynamics) is the current best approach. I have found a tutorial on using CP2K to do ...
18
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3answers
483 views

What are the types of ab initio Molecular Dynamics?

I am new to the world of Molecular Dynamics, and am curious to know exactly what is considered to be an ab initio Molecular Dynamics (AIMD) method, and how do they work? The seminal work by Roberto ...
12
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2answers
535 views

What do the rows and columns of a Fock matrix represent?

I am using the pyscf code, where the Fock matrix can be obtained by: ...
3
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0answers
114 views

How ductile is C60? [closed]

According to a web page I can no longer find, some brittle materials such as glass have a very high theoretical strength but they tend to have surface cracks which magnify tension at the tip greatly ...
13
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2answers
139 views

A CCSD(T) geometry optimization in Gaussian09 is deviating too far from the minimum. How do I deal with it?

I am doing a CCSD(T) geometry optimization on a series of molecules. One of the geometry optimizations took about 2 weeks even though an analogous molecule with the same number of basis functions took ...
33
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1answer
1k views

Was Walter Kohn wrong about this?

In Kohn's Nobel Lecture, he claimed that: "In general the many-electron wave function $\Psi(\mathbf{r}_1,\ldots,\mathbf{r}_N)$ for a system of $N$ electrons is not a legitimate scientific concept,...
11
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2answers
163 views

Is there an example where TD-DFT fails to provide sufficient accuracy for a small system, where RI-CC2 is sufficient?

Time-dependent density functional theory (TD-DFT) is considered to be more efficient (in terms of its accuracy/speed ratio) than alternatives such as the approximate coupled-cluster singles and ...
15
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1answer
83 views

How accurately are magnetic effects treated in *ab initio* methods?

I am a condensed matter theorist and I mostly use quantum Monte Carlo methods. I use models like the Heisenberg model, an represent an extreme simplification of real materials to just localized spin ...
14
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2answers
242 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 ...
21
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1answer
114 views

Which basis set families were optimized to give the lowest possible variational energy for a given number of orbitals?

Dunning's basis set families (such as cc-pVXZ and aug-cc-pCVXZ) were not optimized to give the lowest variational energy for a given number of orbitals. For example the S-type orbitals are optimized, ...
11
votes
1answer
120 views

When running DFT on Q-Chem with an HPC cluster, why is parallelization with OpenMP faster than with MPI?

How to maximize the efficiency/time-to-solution of the calculations in Q-Chem on HPC clusters? MPI parallelization is supposed to improve time-to-solution as compared to OpenMP. The system I tested ...
16
votes
1answer
65 views

Are there any ab intio/DFT methods to simulate solubility?

When designing molecules or materials (as drug carriers) in drug design, one important properties is the system solubility. Are there any software that using ab intio/DFT methods can be used to ...
18
votes
3answers
207 views

Can DFT be considered an ab initio method?

I have very little experience with DFT, but coming from more of a coupled-cluster background, where the improvements are "systematic", to me it seems that the choice of functional in DFT is somewhat ...
17
votes
2answers
108 views

Can ab initio crystal structure methods predict the structure of cuprates from their stoichiometry and quantify the brittleness of those materials?

Current ab initio methods may not be able to predict the electronic transport properties of cuprate superconductors but can they be used to predict their crystal structure? Furthermore those materials ...
11
votes
1answer
52 views

What limits the accuracy of ab initio methods in predicting the transition temperature of conventional superconductors?

Here I assume that conventional superconductors refer to materials that undergo a superconducting transition at low temperature as described by the Bardeen-Cooper-Schrieffer theory (BCS). For those ...
25
votes
3answers
278 views

What is the largest material that has been studied using density functional theory?

Quantum-chemical calculations are quite expensive with a cost that scales greatly with the number of atoms in the system. What is the largest material that has been modeled using density functional ...
29
votes
3answers
294 views

What are the positives and negatives of periodic DFT codes that don't use plane-wave basis sets?

Most periodic density functional theory (DFT) codes use plane-wave basis sets in conjunction with three-dimensional periodic boundary conditions. In contrast, for molecular systems of finite size, ...
22
votes
1answer
127 views

What are some materials discovered from first-principles calculations that ended up being incorporated in a commercial product?

High-throughput computational screening, inverse materials design, and advances in machine learning have really accelerated the pace in which we can identify novel materials for a wide range of ...
14
votes
1answer
106 views

What methods can be used to calculate excited-states in molecular crystals?

In periodic systems plane-wave basis functions are often used. In a previous question on this site it was questioned if plane-wave basis-sets are reliable for modeling adsorption-processes The ...
18
votes
3answers
158 views

Generating special quasirandom structures (SQS) for surface calculations?

Given that most of the available codes for performing DFT calculations work with periodic boundary conditions, there are tools like mcsqs as a part of the Alloy ...
16
votes
1answer
99 views

k-points convergence for special quasirandom structures (SQS) of the same material containing different # of atoms

For an ordered crystal, we generally converge the k-mesh resolution for a primitive cell or other smaller supercells. We then use this resolution for any other size of the supercell for the same ...
21
votes
2answers
160 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^...
18
votes
2answers
100 views

When using the “even tempered method” for augmenting basis sets, what justification is there for the numerical factor which is used?

Diffuse functions are often added to basis sets using "even-tempered" exponents, or sometimes I have heard the phrase "even tempering". In this paper by Jacek Koput, the author says "the customary ...
41
votes
4answers
499 views

What is the closest thing we have to “the” universal density functional?

It is understood that finding "the" universal functional is an NP-complete problem. However, progressively better functionals have been constructed decade after decade and fitted to more and more ...
51
votes
7answers
644 views

Is there a database where we can find previously determined geometries of materials?

For small molecules, NIST has a database of experimentally determined geometries here, and ab initio geometries here. I wonder if there is something similar for materials?