Questions tagged [density-functional-theory]
Density functional theory (DFT) is a quantum mechanical model used to estimate the electronic structure of molecules and condensed matter. In broad terms, DFT works by treating all the electrons in the system as a single electron density, and computing physical quantities of interest as functionals of that electron density. (from Physics.SE)
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Striking examples where Kohn-Sham orbitals clearly have no physical meaning
In Density Functional Theory courses, one is often reminded that Kohn-Sham orbitals are often said to bear no any physical meaning. They only represent a noninteracting reference system which has the ...
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DFT Functional Selection Criteria
I have a very very general question:
In DFT functional selection , mostly people speak about the most recent ones. For example my professor always asks: " which DFT Functional did you select ? " ...
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Is density functional theory an ab initio method?
The following comment by Wildcat made me think about whether density functional theory (DFT) can be considered an ab initio method.
@Martin-マーチン, this is sort of nitpicking, but DFT (where the last ...
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d-orbital splittings in WS2 monolayer
The formerly degenerate $d$ orbitals of the tungsten atoms in the $\ce{WS2}$ monolayer are split into three groups: (1) $\mathrm{d}_{z^2}$, (2) $\mathrm{d}_{x^2-y^2}, \mathrm{d}_{xy}$ and (3) $\mathrm{...
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How do quantum software packages Work?
If one wants to calculate a moderate size Alkane (with say 10-15 carbons, assuming 100 electrons, with Restricted Hartree Fock based methods) we can simply say that the electron-electron part will be $...
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Suggest methods and basis sets for a variety of systems
Please help me with any/all of the cases below. In the following cases, the named method and basis set are not suitable for the chemical systems. Why aren't they? Could you suggest a suitable method/...
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What's the difference between PBE and B3LYP methods?
I can't find an answer to that question. I was told that in B3LYP, more variables implemented in the method are empirical, but I can't find anywhere if it's true, and I'm sure it's not the only ...
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How should I go about picking a functional for DFT calculations?
I've been looking to use time-dependent density functional theory, but reading the literature I keep getting overwhelmed by how many different functionals there are. How should I go about selecting ...
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Equivalent of Szabo and Ostlund book for DFT
Szabo and Ostlund book Modern quantum chemistry [1] is extremely useful to understand Hartree-Fock and post-Hartree-Fock methods. Not only it explains the theory behind such methods, but it is also ...
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DFT Code for Atoms : Sources
Some time ago I implemented the restricted and unrestricted Hartree-Fock methods. I had a lot of fun and I learned a lot. It goes without saying that Szabo and Ostlund's book "Modern Quantum Chemistry"...
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Do modern dispersion-corrected DFT methods give more accurate molecular geometries?
I was discussing with a colleague the use of modern dispersion-corrected density functionals. I take it almost as a given that the methods generally produce "better" (for some definition) geometries, ...
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Interpretation of TD-DFT results
For the first time I'm doing TD-DFT calculations (wB97XD functional) in Gaussian 09 for an open-shell system and the results look like the hell of a mess for me. The molecule is rather big, so I ...
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PBE vs. PBEPBE functional
I've found good explanation what the PBE functional is. However, sometimes I see PBEPBE and I cannot find any good information about what this means and how it's different from PBE.
The Gaussian ...
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What do short-range and long-range corrections mean in DFT methods?
Currently I am looking for the most accurate calculation method for a simple non-conjugate molecule consist of C, N, O and H. Normally I would try each method for a similar and known molecule then ...
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Why is Density Functional Theory notoriously bad at describing oxygen molecules?
I read in several papers that DFT is notoriously bad at describing free oxygen molecules but in none of them an explanation or reference concerning the problem has been provided. I guess this has ...
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Are there any full worked examples of DFT calculations?
I just started learning DFT and now I am totally confused.
Assuming I want to use B3LYP:
\begin{align}
v_s\left(\textbf{r}\right) &= v_\text{ext}\left(\textbf{r}\right) + \int d^3r\frac{n\left( \...
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Determining Kohn-Sham and Hartree Fock virtual orbitals: The underlying field
In Frank Jensen Introduction to Computational Chemistry from 2007, I stumbled upon this paragraph about whether to assign meaning to the KS orbitals or not
Another difference is that the unoccupied ...
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What to do with (large) imaginary frequencies for constrained minimum structures?
I am performing DFT calculations using ORCA 4.0.1 on an enzyme active site model. The model contains 89 atoms including the substrate (see Animation 1), five of which are fixed in space (the spherical ...
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How to define custom force in quantum chemistry packages?
I don't know if this is really applicable but I want to add a custom force (say, $F(x,y,z)$) to some atoms in my geometry optimization. Is there any way to add such forces to the DFT or molecular ...
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Strange "far out" orbitals in HF and DFT calculations. Why do they occur?
I calculated some orbitals and I get quite strange results with orbital geometries I wouldn't expect from MO theory. A more experienced colleague just said: "That's always the case with big basis sets,...
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Use of basis set in DFT (Density Functional Theory)
Basis sets are used to guess the electronic wave functions for Hartree Fock or similar methods, which are quite legitimate since these methods deal with the wave function of each and every electron.
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Exchange-correlation hole
I did a google and chemistry.exchange search and found several technical definitions of the exchange correlation hole.
The questions that I want to proposes are:
What the exchange-correlation hole ...
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There are no wavefunctions in DFT
I have seen this phrase several times across DFT textbooks. However, I am not sure if it still holds. Was there a change or a theorem that proved it otherwise? Several programs display wavefunctions ...
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How to simulate chemical reactions in GAMESS?
I am rather a newb in the GAMESS field but finally, learned to make input files for most molecular simulations and could even use TD-DFT for excited states (special thanks to Geoff Hutchison for his ...
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Better functionals to work with transition metals
My Masters in inorganic chemistry is focused in magnetic properties of transition metals; great part of the work is done with DFT methods using softwares like ORCA and NWChem, but since my University ...
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When is it necessary to check wavefunction stability in density functional theory?
In Gaussian, there is a stable keyword that checks the stability of the wavefunction. Using stable=opt reoptimizes the ...
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Can PBE (and LDA) actually be a better choice sometimes?
Part of my work as an inorganic chemist is to investigate the magnetic coupling between metallic centers in coordination compounds. After some time, I've noticed that the classic PBE functional is the ...
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Why are correlation consistent basis sets used with DFT calculations?
Reading some papers, there is a bit of a curious observation that does not exactly make a lot of sense to me.
In Quantum Chemistry, different methods exist to carry out geometry optimisation and ...
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Defining and testing custom made DFT functionals?
What is a good and free software for this endeavor? Any recommended articles on the subject?
I want to design dft functionals, trying out different parametrizations and optimizing mixing constants for ...
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How does one actually get the energy from the Kohn–Sham equations?
This might be a silly question, but how does one acquire the energy of the system from the Kohn–Sham equations?
$\left[ -\frac{1}{2}\nabla^2 + V_{eN}(\vec{r}) + V_{ee}(\vec{r}) + V_{xc}(\vec{r}) \...
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Spin in meta-GGAs
In density functional theory (DFT) we know that the local density approximation (LDA) leads to an exchange-correlation energy kernel of the form:
$$E_\mathrm{xc}^\mathrm{LDA} =\int e_\mathrm{xc}\left(...
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Estimating stability and mean life of anomalous tautomers
A pair of bases in the DNA, say, A-T, have a tautomerized form A*-T* (resulting from switching the sides of both protons along the hydrogen bridges).
I have studied how, by means of DFT computations, ...
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Density Functional Theory with Generalized Gradient Approxmations (GGA) - What Happens if the Electron Density Shows a "Kink" at the Ion Position?
My question may be stupid, so please correct me if you find anything which is obviously erroneous.
In the following I will place a question mark (?) besides points/steps I consider doubtful.
My ...
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B3LYP Explained
In my inorganic chemistry class, we were introduced to computational chemistry. We are told to use B3LYP theory in the program, but we never actually learned any of the computation yet. Could anyone ...
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How DFT-D3 incorporates coordination number (CN) into C6
I'm reading Grimme's DFT-D3 paper
but really puzzled by how the $\mathrm{CN}$ is considered in the calculation of $C_\mathrm{6}$ coefficients. (Stefan Grimme, Jens Antony, Stephan Ehrlich, and Helge ...
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Excited-states properties with traditional density functional theory
Recently I was shown the book "Density Functional Theory II, Relativistic and Time Dependent Extensions" by Nalewajski in which a chapter from E. K. U. Gross et al. is printed. [1] This chapter ...
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How to calculate the amount charge transfer from a natural population analysis?
I am trying to reproduce the quantity referred to as the degree of charge transfer reported in this publication by Zhu et al. [Ref. 1] for the F4-TCNQ molecule. I am interested in section 3.2 in the ...
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Can you explain DFT and TDDFT functioning (math aside)? [closed]
I have been recently reading a lot on the quantum mechanical theory regarding Density Functional Theory, DFT, and Time Dependent Density Functional Theory, TDDFT (Oscillatory and Rotatory Strengths in ...
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Is there any software that can do geometry optimization using machine learning?
DFT is a computational tool that is used in optimizing and calculating the electronic structure properties of molecules.
Are there any machine learning codes that can do something similar in a ...
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Ab initio effective potentials
I have been reading a journal article on ab-initio pseudo-potentials, and I need some help understanding it. The article is C. F. Melius and W. A. Goddard, III. Phys. Rev. A 1974, 10, 1528.
A summary ...
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DFT vs. MP2 for stacked dimer
Recently, I looked at these two papers analyzing the excited-state properties of modified DNA bases (2-aminopurine and 8-vinyl-A) and how they are influenced by stacking with natural nucleobases:
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Can density functional approximations including dispersion correction perform worse than without it?
I am trying correlate some experimental properties of a molecule to its calculated equilibrium geometry. To do so, I calculated the geometry and energy of some conformers.
Now, I obtain mixed results ...
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Why are the calculated NMR values on the DFT & GIAO level of cyclopentane-1,3-dione so far off?
I'm trying to simulate the 13C NMR spectrum of cyclopentane-1,3-dione (PubChem CID: 77466; CAS 3859-41-4; ChemSpider ID: 69875; SDBS No: 15258; SMILES: C1CC(=O)CC1=O...
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Difference between single point energy and dimerization energy
I am investigating potential energy surface characteristics of some electrostatic monomeric interactions e.g. $\ce{F-H}$...$\ce{F-H}$.
I'm using DFT and SCS-MP2 to perform the necessary calculations ...
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Hartree Fock and DFT - Exchange term
this might be a stupid question but I am really confused at the moment. I'm currently preparing for an exam in computational chemistry. In the lecture I noted always down that the exchange term (Pauli-...
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Is there proof that the exact exchange correlation functional is an analytic function?
Mathematics show that some problems cannot be solved analytically. Could this be true for the exact exchange correlation functional?
Most textbooks state that the form of the exact exchange ...
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Calculating dipole moments from atomic partial charges
How can I calculate dipole moment of a complex molecule where the partial charges are available from density functional theory? I have the coordinates of each atom from atomistic simulations of an ...
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Why do we integrate square of wave function over N-1 electron coordinates?
Quote form F.Jensen's Introduction to computational chemistry:
"The electron density is the square of the wavefunction, integrated over N-1 electron coordinates..."
Why do we integrate over N-1 ...
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Oscillator Strength and Absorption Intensity in TDDFT
When I run a linear-response time-dependent DFT calculation in my favourite electronic structure program, for each excitation I'm given an oscillator strength, $f_i$, and transition dipole moment $$\...
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High-temperature DFT
Why is it so difficult to perform DFT calculations that consider temperature?
I have seen that time-dependent DFT is needed to model systems at high temperature.
Why is this the case? What about ...
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