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 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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How one can calculate Si-O-Ca/Mg valence angle energy?
I want to perform the relaxed scan (or relaxed surface scan) for the valence angle energy of Si–O–Ca, Si–O–Mg, Al–O–Ca, and Al–O–Mg. I will use DFT (ORCA), probably B3LYP and triple zeta basis set (...
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Guide to DFT functionals [duplicate]
Having recently begun studying Quantum and computational chemistry in depth... I am looking for a manual, text, or guide of any form that could explain the basic formulations and potential of the ...
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Limitations of extremes for the exchange-correlation functional - DFT
In DFT the exchange-correlation function is critical for the correct description of the system. I have a question about the applications and shortcoming of some functionals.
LDA delocalizes electrons ...
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Getting radial Kohn-Sham orbital from the output of an electronic structure code using gaussian basis sets
As we know that in solving the Schrodinger equation (Kohn-Sham equation), we expand the wavefunction in terms of some basis sets and them optimize the energy (the expectation value of Hamiltonian) by ...
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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 an easy explanation of MP2 calculation [closed]
for my master thesis I calculated some ring strains with MP2 calculation. It was not hard to calculate these things but I'm struggling to understand what the theory is about. Now I'm defending my work ...
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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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DFT Exchange-Correlation Two Electron Integrals
Are there any electronic structure programs that print the exchange-correlation (XC) integrals for a given DFT functional? These integrals are of the form $$\langle pq|w|rs\rangle$$
where $w$ is an XC ...
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How to know about radical character of a diradical molecule?
I understand that the radical character is quantified by the occupation number of the lowest unoccupied natural orbital obtained from UHF calculations or from CASSCF calculations.
My issue is that I ...
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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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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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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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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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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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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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Natural orbital population analysis in Gaussian
Within (restricted) density functional theory and Hartree-Fock, respectively the Kohn-Sham orbitals and spin orbitals are the natural orbitals, with occupation numbers either 2 or 0.
As I understood, ...
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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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How is occupation number of lowest unoccupied natural orbital defined?
I have run some calculations to analyse the occupation of natural orbitals. Since it is needed to run in triplet state, I get two orbitals with occupation number 1. Now, which orbital occupation ...
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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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Finding the weighted parameters of the vertex adjacency matrix for a derivative of azulene
In my last question ( Numbering in a derivative of azulene ), I asked about the numbered of a molecule in order to find the Vertex Adjacency Matrix (VAM) of the same molecule. To explain my question ...
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Relationship between the 2nd theorem of Hohenberg and Kohn and the Variational Principle
In Density Functional Theory, the 2nd theorem of Hohenberg and Kohn states:
The electron density of the system that minimises the ground-state energy is the exact electron density of the system.
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Q-chem equivalent solvent settings for Gaussian NMR ab inito methods
I'm attempting to use Q-Chem 5.0 to replicate CHESHIRE-esque http://cheshirenmr.info/ ab initio calculations of chemical shift values. Unfortunately Gaussian seems to be the predominant tool in this ...
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Hartree product and the Slater determinant
I am very new to theoretical chemistry so my question may sound very basic to most. It is known that the Hartree product does not satisfy the antisymmetry principle for electrons while the Slater ...
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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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How to calculate the magnetic exchange coupling in a charged radical complex?
I have a neutral complex with two magnetic centers. For this case it is easy to calculate the exchange coupling as the broken symmetry state is converged properly. But when I am making the complex ...
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Why density functional theory favors a uniform electron density?
Why density functional theory (DFT) favors a uniform electron density when we use it to calculate ground state electronic structure self-consistently?
For example, when we use DFT for calculating ...
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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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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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How does MP2 level of theory affect the natural orbital picture of the system?
I have performed CASSCF calculations for a series of molecules with varying input orbitals. I have used UNO, UHF, ROHF and MP2 based natural orbitals to perform the CASSCF calculations. For my system ...
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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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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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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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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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Determining global minimum structures for ion-water clusters
What is the best way to find the global minimum structure(or get close to it) for an ion and N waters? (i.e. ion cluster in vacuum)
For example, consider Flouride ion cluster with N=5 waters.
I want ...
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DFT calculation of solids with different periods
I would like to simulate a layer of graphene on a solid substrate, say Cu or quartz. I'm interested in their binding energy and other physical or chemical properties. However, their periods do not ...
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Is it possible to define the coordinates of an atom using other coordinates while optimizing in gaussian 16?
I know it is possible to fix the value of composite expressions using generalized internal coordinates (gic) in gaussian 16. But is it possible to assign that value to the coordinate of another atom.
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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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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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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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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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Understanding the basics of DFT [closed]
I have recently started doing DFT calculations as part of my PhD. The general approach to DFT in my department appears to be that it is regarded as a tool for doing calculations, which need not be ...
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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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Small but modest displacements at negligible forces from DFT
When looking at a Gaussian log file today, I noticed the following information in the convergence criterion section after an analytical frequency calculation was performed:
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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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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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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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What exactly does setting the net charge in a DFT calculation do?
In the implementations of DFT using Gaussian basis sets, it is common to set the net charge. What exactly does this do? The Gaussian (program) manual, for instance, says that it introduces a ...
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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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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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