41 votes
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DFT Functional Selection Criteria

What's up with all that magic? (A chapter formerly known as Introduction) The hunt for for the holy grail of density functional theory (DFT) has come a long way.[1] Becke states in the introduction of ...
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31 votes
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What's the difference between PBE and B3LYP methods?

PBE The PBE functional${}^{[1]}$ belongs to the class of generalized gradient approximation (GGA) functionals for the exchange-correlation energy $E_{\mathrm{xc}}$. Considering that the dependence $...
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29 votes
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Is density functional theory an ab initio method?

First note that the acronym DFA I used in my comment originates from Axel D. Becke paper on 50 year anniversary of DFT in chemistry: Let us introduce the acronym DFA at this point for “density-...
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21 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 ...
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20 votes
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How do Quantum Software Packages Work?

There are many tricks. Since I'm not a developer, I'm not privy to all of them, but there are some simple things to start. Some integrals are predictably close to zero: Even in fully first-principals ...
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18 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 ...
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15 votes
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Exchange-correlation hole

What the exchange-correlation hole means in common words? Assuming that the probability is the common word, one could say that the exchange-correlation hole, is a region of space around an electron ...
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15 votes
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What do short-range and long-range corrections mean in DFT methods?

There are many good review articles, but I really like the freely available "The ABC of DFT" by Kieron Burke (and friends) and a recent review article that tries to keep to understandable language. ...
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15 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 ...
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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 ...
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13 votes

DFT Functional Selection Criteria

There are many criteria that you can follow to choose a functional, in no special order: Availability: Do you have access to a software package in which the functional is implemented? If yes, are ...
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12 votes
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How to simulate chemical reactions in GAMESS?

As in geometry optimization, you are searching for stationary point on the potential energy surface (PES). Not for local minimum, but for saddle point, therefore in GAMESS, you specify ...
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  • 4,417
12 votes
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Interpretation of TD-DFT results

Time-dependent DFT can be used to predict excitation energies through a linear-response formulation. In this Gaussian result, beyond the first line, you are looking at the largest coefficients in the ...
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12 votes

Is density functional theory an ab initio method?

The convention used by many is that ab initio refers solely to wave-function based methods of various sorts and that first principles refers to either wave-function or DFT methods with little ...
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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 ...
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11 votes

DFT Functional Selection Criteria

In addition to the good answers that have already been given, it is important to note that for DFT to be properly implemented, you must calibrate the functional(s) to whatever it is you are examining. ...
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  • 4,549
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 ...
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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 ...
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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. ...
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11 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 ...
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11 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 "...
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  • 18.4k
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 ...
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10 votes

How do Quantum Software Packages Work?

First things first, I would check your code: there is no reason 400 unique integrals should take an hour, even with the most naive implementation. You are computing at a rate of an integral every ten ...
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10 votes

How do Quantum Software Packages Work?

To further reduce storage requirement, integrals may be calculated on the fly, emplpoying different caching strategies. For some methods storing all the integrals is plainly impossible.
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  • 20.6k
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}} \...
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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. <...
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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 ...
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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/...
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  • 3,514
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-...
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