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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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298 views

Is there a reliable way to determine if a chemical system is more suited to be calculated using a multireference method instead of DFT?

Some time ago I recall hearing a professor say that looking at a DFT result (I don't remember exactly what, a TD-DFT, spin contamination etc...), he could tell that the system was multireferenced. My ...
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191 views

Ground state oxidation potential and Ionization potential from density functional theory (DFT)

Can someone please tell me how to calculate the ground state oxidation potential (GSOP)? I understand that according to Koopmans' theorem, the negative of HOMO can be considered as the GSOP. Is there ...
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195 views

Are there other Density Functional Theories other than Kohn Sham DFT?

For some reason I thought that the term DFT and Kohn Sham DFT were interchangeable. Kohn Sham DFT is certainly one of the most popular. But are there other types of DFT that are in use today? I have ...
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360 views

Calculation of rate constant for bimolecular reaction from Gaussian DFT calculations

I am working with a bimolecular reaction for which I have computed the energy profile. I have output files for TS and reactants which I intend to use for rate constant calculations. From TST we have: ...
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219 views

How to define the initial guess for the electron density?

This is a technical question. If I have a one-dimensional box of length L, and an electron density existing in the box and integrating to N electrons. And if I want to determine the electron density ...
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1k views

Warning during geometry optimization via Gaussian

I have some recurrent problem with my DFT calculations [functional: hseh1pbe; basis:6-31+g(d,p), object: porphyrin derivative]. During the MO setting I get: **** Warning!!: The largest alpha MO ...
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4k views

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

Can anyone help me to run a geometry optimization and frequency calculation using B2PLYP-D3 functional including the DFT-D3 dispersion correction? I went through Gaussian manual, however, it was not ...
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210 views

Can I compare DFT calculations with different grids?

When doing DFT calculations, some integrations are commonly done numerically on grids. [In fact, more than a single grid may be used at the same time for different integrals, e.g. approximations such ...
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74 views

Global reactivity parameters from open shell DFT calculations using Koopman's theorem

Koopmans' theorem is a useful approach to calculate the global reactivity parameters from the HOMO-LUMO energies. My question is, does it apply to the open-shell systems where we get two sets of ...
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86 views

Atom-condensed softness matrix

In the ACKS2 polarizable force field paper, I found a thing called the atom-condensed softness matrix. In another paper, I found this expression for it: $$ \chi_{kl} = 2 \sum_{i}^{\text{occ MOs}} \...
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262 views

Electronic density distribution in hydrogen bond

I'm currently studying about the atomic configuration of electrostatic interactions. Concept is there that in case of interactions like in N-H...N-H (amides), the stability depends on the position of ...
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706 views

Predict XPS Spectrum of Organic Monolayers on a Semiconductor

X-ray photoelectron spectroscopy (XPS) is a particularly useful technique for examining the surfaces of all sorts of materials. Organic monolayers are particularly difficult to characterize completely ...
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1k views

Partial Atomic Charge in GAMESS

In GAMESS, a calculation of optimisation gives data about partial atomic charges (like Mulliken or Lowdin charges). Can you please let me know how to calculate the other types of partial atomic ...
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96 views

Coupling constant (Jab) - Why more unpaired electrons means a smaller coupling?

I hope this is not a "homework" question, but I'm having a bad time trying to figure this out. Available literature proposes 3 equations to calculate the coupling constant during a Broken-Symmetry ...
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108 views

Ab initio MD simulations in VASP

I'm not sure if this is an appropriate question for Chemistry.SE, but I'm taking a stab. I'm running ab initio MD calculations in VASP and I'm trying to optimize different input tags. On the VASP ...
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52 views

Paper Data Replication

I read a paper recently - Here. I found the concept of Electride materials very interesting and wanted to mess around with them myself. I tried to replicate the partial electron density plots shown in ...
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855 views

DFT optimization of a molecule in gaussian and find electrophilic sides [closed]

I am not a pro, but I have some experience with gaussian. For example, I can optimize a structure, calculate the frequency and rotational energies. In the lab I made a nucleophilic substitution ...
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3answers
126 views

Reference states for molecular orbital energies in DFT calculations

When doing a population analysis using density functional theory (specifically as implemented in Gaussian), what are the orbital energies (i.e. alpha and beta eigenvalues) with respect to? If I have ...
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188 views

Geometry optimization: dependence on initial geometry

One would think that initial configuration should not significantly impact the final optimized geometry. Starting with two moieties, the final "optimized" geometry is not unique and depends highly on ...
4
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1answer
980 views

Is there a relation between transition density and density differences?

When I have an excited state, is there a relation between the transition density associated with this electronic transition and the density difference between the excited state density and the ground ...
4
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82 views

Finding mathematically the ground state density in DFT

To find the ground state density in DFT, you set the following Lagrangian: $$L = E[\rho(\vec r)] - \mu\left(\int \rho(\vec r) \mathrm{d}\vec r - N\right)$$ While minimising with respect to the ...
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1k views

How to use dispersion correction with TPSSh in Gaussian 16?

A colleague of mine suggested to use TPSSh as a functional. Since I am applying Grimme's D3 dispersion correction by default, I was surprised that it wasn't implemented in the standard density ...
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266 views

Computing reaction mechanisms using solvation effects

I am about to compute reaction steps for a series of compounds using DFT. A key point in our discussion is the dependency of the experimental rate of reaction w.r.t. solvent used, so it is very ...
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96 views

Correlation energy in quantum chemistry

In density functional theory (DFT) there is classic Coulomb energy and non-classic correlation energy. But what is difference between them? In books these energies are explained very similarly. Both ...
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469 views

How to optimize graphene geometry with Gaussian?

I am new to Gaussian and I want to optimize graphene using DFT. After the the geometry optimization it has been severely changed, and some bonds were set as double and triple. Why am I unable to get ...
4
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367 views

Defining the Turbomole named BP86 functional in Gaussian [closed]

According to Turbomole, the BP86 density functional has the following definition: $$ \mathrm{Exchange = LDA + B88\\ Correlation = LDA(VWN(V)) + P86} $$ What is the equivalent definition in Gaussian ...
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171 views

What is the “curse of dimensionality” in molecular dynamics?

The 'curse of dimensionality' or the 'bottleneck problem' in molecular dynamics is explained in page 5 and 6 of Ab Initio Molecular Dynamics: Basic Theory and Advanced Methods by Dominic Marx and Jurg ...
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110 views

Do dipole dipole interactions need to be added explicitly to vdW forces while doing DFT simulations for organic molecules?

I am modelling the adsorption of organic molecules on metal surfaces using plane-wave dft. While there is literature on how important van der Waals forces are for organic systems, and there are also ...
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234 views

Transition state and intermediate very close to each other

I've been studying a particular reaction with DFT, where a transition state and the following intermediate are very close to each other. The change in geometry is rather small and therefore the ...
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77 views

Signs of Exchange and Correlation Potentials

The exchange and correlation potentials refer to those defined in density functional theory. (See also http://en.wikipedia.org/wiki/Local-density_approximation) Define the exchange potential as $V_{x}...
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857 views

How to calculate the energy of a single proton? [closed]

Using e.g. the VWN5 functional, what is a simple way to calculate the energy of a single proton? Of course, it is not possible to perform a calculation of a proton in isolation, as it bears no ...
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90 views

What's the most complete basis set for calculating chemical shifts with DFT?

I'm trying to benchmark some NMR chemical shifts. The literature suggests that the pcS-4 basis set will be the most accurate. I was wondering if there were any basis sets that are more complete than ...
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340 views

Hybrid functional calculations using different approach

In hybrid functional such as B3LYP, PBE0, HSE etc, some part of HF exchange and/or correlation is added to some part of DFT X/C part in order to get better results. My question is if I am adding 1/4 ...
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234 views

DFT Calculations, Atomic Ionization Potentials — Which Exchange-Correlation Functional to Use, to Preserve Koopmans Theorem?

I have a program which can perform density-functional calculations for atoms, given a density functional. Of course the simplest form of exchange potential to use is one relevant for a uniform ...
3
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1answer
902 views

Most efficient level of theory for sulfur-containing proline analogues (one that will work)

I'm modelling a small set of proline analogues - extra functional groups, sulfur in the ring. Using Gaussian / DMACRYS. ...
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769 views

What is the difference between Dielectric and Capacitive Polarizable Continuum Model?

I have good background knowledge on Quantum Chemistry calculations but my knowledge on Solvation Models is not that amazing. Many times I read a paper that uses TD-DFT, they also assign some solvent ...
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1answer
235 views

Possible to Calculate Electronic Charge Distributions/Densities for Organic Molecules?

I was wondering if there are any programs that can calculate the electronic charge distributions around an organic molecule, possibly in the form of a volumetric charge volume density? I am reading up ...
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1answer
231 views

Convergence of spectra calculated from molecular dynamics trajectories

How can we check whether or not the power spectra (vibrational density of states) calculated by taking the Fourier transform of the velocity auto-correlation function (VACF) from a molecular dynamics ...
3
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1answer
197 views

What is density in density functional theory?

This may be dumb question. Please bear with me. Density functional theory (DFT) is a successful theory for electronic structure calculations of materials. In DFT, electron density is the fundamental ...
3
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1answer
189 views

How are imaginary frequencies and force constant calculated by DFT?

Computational calculations allow us to simulate the frequencies of molecules. They can even tell us if the optimized structure is a minimum, a saddle point or a maximum according to the number of ...
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20 views

How to do RPA (Random Phase Approximation) in VASP?

I'm new to VASP and as such don't know much of its intricacies yet. I want to find out the interlayer binding energy of a given molecule and to do that I already opted for the use of different ...
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55 views

Why can we know a materials properties by knowing its ground state at 0K - DFT

I have had this doubt ever since I was introduced to DFT. For what I gathered whatever results you obtained with DFT (by using VASP, quantum expresso or any other software) you can say those ...
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131 views

calculate SPE using dummy/ghost atoms in NWChem

I have been making some failed attempts to calculate interaction energy of 2 monomers using a dummy/ghost atom in NWChem6.5. The code I was using to calculate the interaction energy is: ...
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359 views

Semi-numeric or analytic Hessian for DFT (GAMESS)

I'm trying to understand what parameter in the $FORCE group is better to use in computing vibrational frequencies (NVIB=2) of ...
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2answers
210 views

Density Functional Definition

Is it wrong to say that Density Functional means that Electron Density is a function of the orbitals (wave function) of all electrons in 3 dimensions, if so, why ?
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85 views

High errors in frontier orbital energy calculation using DFT

In a recent paper by Faber et . al., Mean Absolute Error and DFT (B3LYP) error (relative to the experiments) for 9 properties of QM9 dataset molecules has been reported. \begin{equation} \begin{array}...
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1answer
77 views

On mathematics behind molecular orbital energies

When explaining the mathematics behind calculating molecular orbital energy , namely HOMO and LUMO energies, using quantum chemistry, Chainer Chemistry documentation states that From mathematical ...
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551 views

Periodic boundary conditions in DFT for cylindrical system

As I understand it, a DFT calculation uses periodic boundary conditions, whenever you want to simulate a bulk material. This is convenient because all information can then be extracted from the ...
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54 views

Magnetic couplings using DFT in a spin frustrated system

I’m working with a trimeric copper system that have the classic triangular shape and experiences spin frustration. This is my first time dealing with such system and, so far, from what I’m reading, I ...
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495 views

Search materials by HOMO, LUMO and work function

I need to search organic semiconductors for organic photovoltaics (OPV) by their HOMO, LUMO, work function at 300K (as I understand, WF cannot be derived from HOMO and LUMO because semiconductor can ...