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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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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A thorough benchmark of DFT functionals without dispersion corrections

So, I'm looking for a benchmark of DFT functionals not including dispersion corrections. Something similar to Grimme's and coworkers GMTKN30 database, but for benchmark calculations done without ...
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220 views

Computing of strengthening of π-interaction

I need some practical advise. What would be the easiest approach to compute strengthening or weakening of π-interaction in a bond with multipole-bond character. I'm not really fond of going through ...
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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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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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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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439 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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243 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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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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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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255 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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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 ...
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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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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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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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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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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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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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715 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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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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Laplacian of the Hartree Fock second-order reduced density matrix

I am attempting to replicate and understand the Colle-Salvetti method for approximating the electron correlation energy.[1] I have successfully replicated the method, but there is one part of the ...
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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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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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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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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 ...
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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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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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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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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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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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Would there be need for periodic DFT if we are able to get all the forcefields of all the elements in the periodic table in ReaxFF?

Since ReaxFF combines the advantages of DFT and Classical MD, would there be any need for periodic DFT calculations if we can have all the forcefields of all the elements of all the possible systems?
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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 ...
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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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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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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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240 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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DFT: Can the calculated enthalpy of two systems that aren't isoelectronic be compared?

As a simple example, say I wish to calculate the product of a reaction between methyl iodide and triethylamine, then compare it with the product of benzyl iodide and triethylamine. My goal is to ...
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407 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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81 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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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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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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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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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 ...
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855 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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I'd like to model some compounds with DFTB, but some elements don't have parameter files in the official site. Is there a workaround?

I was reading a bit about this technique, Density Functional based Tight binding (DFTB). It's claimed it can provide a speed improvement of several fold when compared to plain DFT, still giving ...
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302 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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239 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 ...
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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 ...
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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 ...