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)

4
votes
1answer
159 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 ...
1
vote
0answers
23 views

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 (...
2
votes
0answers
24 views

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 ...
1
vote
1answer
25 views

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 ...
0
votes
0answers
43 views

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 ...
1
vote
0answers
26 views

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 ...
3
votes
0answers
87 views

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 ...
0
votes
0answers
16 views

Is the exchange interaction always less in value than coulomb interaction? (Absolute values of both.)

I wonder for 2 electron interactions whether exchange interaction (K) is always less than the coulomb interaction(J) value in all cases (HF or DFT) or there is no such necessity? Does it depend on the ...
3
votes
2answers
81 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 ...
4
votes
3answers
90 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 ...
3
votes
0answers
53 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 ...
5
votes
0answers
72 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 ...
10
votes
2answers
143 views

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,...
1
vote
0answers
80 views

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, ...
4
votes
1answer
58 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 ...
6
votes
0answers
297 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: ...
1
vote
0answers
42 views

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 ...
3
votes
1answer
129 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 ...
1
vote
0answers
19 views

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. ...
0
votes
1answer
59 views

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 ...
2
votes
0answers
65 views

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 ...
6
votes
1answer
327 views

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 ...
2
votes
0answers
24 views

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 ...
1
vote
0answers
31 views

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 ...
2
votes
1answer
51 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 ...
6
votes
1answer
139 views

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...
4
votes
1answer
812 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 ...
2
votes
1answer
73 views

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 ...
3
votes
1answer
160 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 ...
2
votes
1answer
149 views

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 ...
2
votes
0answers
31 views

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 ...
1
vote
1answer
46 views

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. ...
3
votes
0answers
99 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: ...
13
votes
3answers
745 views

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 ...
2
votes
1answer
69 views

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 ...
2
votes
1answer
450 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 ...
1
vote
0answers
211 views

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 ...
3
votes
1answer
192 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 ...
1
vote
1answer
55 views

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: ...
6
votes
1answer
289 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 ...
2
votes
0answers
66 views

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 ...
9
votes
1answer
1k views

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. ...
3
votes
1answer
515 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 ...
7
votes
1answer
921 views

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 ...
0
votes
0answers
19 views

Scan type of DFT calculations adding Freq=Raman keyword [duplicate]

I´m doing a scan calculation by adding Freq=Raman keyword. My objective is to obtain the Raman spectrum of each conformation in my compound. However, when i write the Freq=Raman Keyword, only the ...
10
votes
1answer
197 views

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}) \...
7
votes
1answer
316 views

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-...
1
vote
1answer
86 views

How to reconcile conflict between assumed oxidation state and the most stable multiplicity from electronic structure calculations?

Consider either of the small, neutral molecules containing two Zn atoms below. If I were to determine the oxidation state of Zn from this molecule, I would formally assume each Zn to be Zn(I) in order ...
14
votes
1answer
566 views

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 ...
12
votes
3answers
4k views

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 ...