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22 votes
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There are no wavefunctions in DFT

There are no wavefunctions in DFT. I don't like that "wave function" is used here in the plural form and I feel like it reflects OP's misconception. The right way to say this is There is no wave ...
Wildcat's user avatar
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20 votes
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PBE vs. PBEPBE functional

The designation PBEPBE is an actual terrible artefact from researchers using Gaussian. The actual publication[1, 2] only refers to it as PBE, and most other program packages (I know) implement it as ...
Martin - マーチン's user avatar
16 votes
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Why are correlation consistent basis sets used with DFT calculations?

It is generally recommended not to use a cc basis set with a DFT method (and I guess conversely, a basis set aimed at DFT should not be used with a coupled cluster method). This statement glosses ...
pentavalentcarbon's user avatar
14 votes

PBE vs. PBEPBE functional

If you want to use a LDA or GGA functional in Gaussian you always need specify the desired Exchange and Correlation functional. In case of PBE you need to specify it twice, only using ...
Feodoran's user avatar
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14 votes

Is there any software that can do geometry optimization using machine learning?

There is one Python package that I know: AMP (Atomistic Machine-learning Package). It is based on ASE (Atomic Simulation Environment), a homogeneous interface to a lot of computational chemistry ...
schneiderfelipe's user avatar
12 votes
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B3LYP Explained

'The level of theory' is a fancy word for scheme used to calculate energy of a molecule. There are quite many ways to do it, most very computationally expensive. However, quite recently it was proven ...
permeakra's user avatar
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12 votes
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Determining Kohn-Sham and Hartree Fock virtual orbitals: The underlying field

That is true: in the Kohn–Sham model electrons both in occupied and in virtual orbitals are "moving" in the field $n-1$ electrons, while in the Hartee-Fock model electrons in occupied orbitals are "...
Wildcat's user avatar
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11 votes

Do modern dispersion-corrected DFT methods give more accurate molecular geometries?

I am afraid I do not know of a paper specifically discussing the effects of DFT dispersion corrections on intramolecular properties. However, it should be noted that for short-range descriptors (such ...
F'x's user avatar
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11 votes
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Why do we integrate square of wave function over N-1 electron coordinates?

$\newcommand{\el}{_\mathrm{e}} \newcommand{\dif}{\mathrm{d}} \newcommand{\braket}[2]{\langle{#1}\vert{#2}\rangle}$Consider an $n$-electron system in an arbitrary electronic state represented by the ...
Wildcat's user avatar
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11 votes
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How DFT-D3 incorporates coordination number (CN) into C6

I will quote the paper quite a bit, but I'll try and summarize a bit after the quotes and equations. You might want to start at the bottom and work backward, a lot of this is just for later reference. ...
pentavalentcarbon's user avatar
11 votes
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Calculating dipole moments from atomic partial charges

From a theoretical standpoint, that approach is correct: $$ \mu_i = \sum_{a}^{N_{\text{atoms}}} \sum_{i\in\{x,y,z\}} r_{ia}q_{a}, $$ where the set of atomic partial charges $\{q\}$ could come from ...
pentavalentcarbon's user avatar
10 votes

Density Functional Definition

Electron density, is defined as the probability density of finding any of the $n$ electrons with arbitrary spin at some point $\vec{r}_{1}$ in space, $$ \newcommand{\el}{_\mathrm{el}} \...
Wildcat's user avatar
  • 19.1k
10 votes

How to run a B2PLYP-D3 calculation in Gaussian 09?

Search for B2PLYP on the Moller-Plesset page: B2PLYPD3 requests the same but with Grimme's D3BJ dispersion [Grimme11, Goerigk11]. The correct D3 parameters will be set for you automatically. <...
pentavalentcarbon's user avatar
10 votes

Why are correlation consistent basis sets used with DFT calculations?

Because in fact it is appropriate. In most cases there is not a huge difference (quality/efficiency) among basis set families. For example Dunning (cc) basis sets work reasonable well for DFT, and ...
user1420303's user avatar
  • 2,027
10 votes
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How to calculate the energy of a single proton?

An isolated proton will have energy given by the Schrodinger equation (just like any molecular system): $$ -\frac{\hbar^2}{2\mu}\nabla^2\psi + V\psi = E\psi $$ ($\mu$ is reduced mass; $\hbar$ is $h/...
khaverim's user avatar
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10 votes
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Use of basis set in DFT (Density Functional Theory)

This answer only deals with the most common variety of Density Functional Theory, namely, Kohn-Sham DFT. This is what most people mean by "DFT", but, as noted in the comments, things such as orbital-...
TAR86's user avatar
  • 6,881
9 votes
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Excited-states properties with traditional density functional theory

Conventional density functional theory (DFT) is strictly limited to describing the electronic density of ground electronic states. This is because the Hohenberg–Kohn theorems, on which it is based, ...
F'x's user avatar
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9 votes
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When is it necessary to check wavefunction stability in density functional theory?

The very short answer is: whenever you are in doubt. The stability check is probably unnecessary when you are dealing with closed shell molecules, where you can be pretty sure that a restricted ...
Martin - マーチン's user avatar
9 votes
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Can I compare DFT calculations with different grids?

Yes, your error analysis is valid for energy differences. However, I believe it is also valid for the absolute error $\epsilon_i$ of any calculated quantity, not just the error $\Delta\epsilon$ of an ...
hBy2Py's user avatar
  • 17.4k
9 votes
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Is it rigorous enough to change a DFT functional after geometry optimization to calculate excited state properties?

What is sometimes done is use the geometry and thermochemical corrections (i.e. the results of the frequency calculation) of method X, but use the electronic energy (SCF energy) of another method Y. ...
Raphaël's user avatar
  • 1,618
9 votes
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Oscillator Strength and Absorption Intensity in TDDFT

I'll address the last part of your question. I think the first part What formulae relates the transition dipole moments, the oscillator strength and the observed absorption relative intensity? of ...
Hans Wurst's user avatar
  • 1,245
8 votes
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Is there a relation between transition density and density differences?

By definition, the transition density is not the same as the difference density. In the following derivation, the transition density is $\mathbf{T}^{\mathrm{CIS}}$ and the difference density is $\...
pentavalentcarbon's user avatar
8 votes
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Why wasn't Lu Jeu Sham awarded the 1998 Nobel Prize with Walter Kohn?

Unfortunately, there is very rarely going to be an official statement as to why someone did not receive the Nobel Prize. A number of factors probably played into the decision. One of the big ones is ...
Tyberius's user avatar
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8 votes

Why do DFT calculations output molecular orbitals?

The short answer is they do not. We can identify two distinct ways of doing DFT calculations, OF-DFT (orbital-free DFT) and KS-DFT (Kohn-Sham DFT). Let us start with the OF-DFT formalism, as it is the ...
Erik Kjellgren's user avatar
7 votes
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Do modern dispersion-corrected DFT methods give more accurate molecular geometries?

Disclaimer: These articles are from my old group and myself. But they may serve as a starting point for someone's own investigation, even if one disagrees with our conclusions. The Grimme group, me ...
TAR86's user avatar
  • 6,881
7 votes

Calculating singlet-triplet gap and open-shell singlet character

There is a related discussion on which method to use for a singlet triplet guess, which I encourage you to also read: U- or RO-method for Singlet-Triplet Gap? This post is going to be a little longer,...
Martin - マーチン's user avatar
7 votes

What to do with (large) imaginary frequencies for constrained minimum structures?

The first I noticed is you said: The two gradient tolerances have been loosened slightly in order to converge the calculations. By using the default gradient tolerances, the calculations simply do ...
user1420303's user avatar
  • 2,027
7 votes
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Hartree product and the Slater determinant

The Hartree product can definitely be used for approximate calculations; as you might know Hartree developed his method for atoms in 1927 (for reference, the Schrödinger equation was discovered in ...
Lorents's user avatar
  • 258
7 votes
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DFT vs. MP2 for stacked dimer

Both papers you refer to used methods which were basically state of the art at their time (Gaussian 98/03 compared to Gaussian 16 today). But that is almost twenty (or fifteen) years ago. Nowadays we ...
Martin - マーチン's user avatar

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