Questions tagged [ab-initio-calculations]

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

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9
votes
1answer
84 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
124 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 ...
11
votes
2answers
138 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
86 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
votes
1answer
126 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
58 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
59 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 ...
9
votes
1answer
52 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?
8
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0answers
40 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
votes
1answer
100 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"?
5
votes
0answers
47 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
votes
0answers
47 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
votes
2answers
186 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 ...
12
votes
1answer
157 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 ...
18
votes
5answers
316 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
votes
2answers
171 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
votes
3answers
446 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
votes
2answers
494 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: ...
4
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0answers
74 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
votes
2answers
129 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 ...
32
votes
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,...
10
votes
2answers
151 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 ...
14
votes
1answer
76 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 ...
13
votes
2answers
209 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
votes
1answer
108 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, ...
10
votes
1answer
82 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 ...
15
votes
1answer
59 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 ...
14
votes
1answer
106 views

Can DFT be considered an ab initio method? [closed]

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
101 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
48 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 ...
23
votes
3answers
243 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 ...
24
votes
2answers
199 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, ...
20
votes
1answer
83 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
95 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 ...
15
votes
1answer
105 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 ...
15
votes
1answer
92 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 ...
20
votes
2answers
133 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
90 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 ...
39
votes
4answers
449 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 ...
47
votes
6answers
543 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?