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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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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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164 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 ...
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79 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 ...
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35 views

How to find a full electron basis set that approximate a given ECP basis set?

In computational chemistry of transition metals, there are many popular basis sets, from pople+LANL2DZ mixed basis sets, to Ahlrich and Karlsruhe basis sets like def-SVPD, def2-TZVPPD. Often the ECP ...
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47 views

How to account for solvation of a molecule explicitly in quantum calculation?

I'm looking for a way to account for hydrogen bonds formed with my molecule in aqueous solution explicitly in DFT calculation. I'll do it in order to compare stability of two structures in water. As ...
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30 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 ...
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112 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 ...
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134 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, ...
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78 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 ...
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27 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 ...
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35 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 ...
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96 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 ...
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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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378 views

Units on rate constants from transition state theory and DFT

I am using density functional theory (DFT) to compute the energy profile and, by extension, kinetic parameters for a catalytic process. I know that the rate constant from transition state theory (TST) ...
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247 views

PAW vs USPP: Pseudopotentials in commercial DFT codes

What is the difference between USPP (ultra-soft pseudopotential) and PAW (projector augmented wave) in commercial DFT codes?
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314 views

Which Basis Set is suitable For Mercury-complex in DFT calculations?

Which Basis Set is suitable For Mercury-complex in DFT calculations? Please provide the answers with any journal reference.
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228 views

How to model iso-surface of molecular orbitals of singlet and triplet excited states

I've modelled the iso-surface of the highest occupied molecular orbital of a molecule with DFT in Gaussian.If I were to model the iso-surface of the molecular orbital of the singlet and triplet ...
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48 views

Where can I find a collection of density functional parameterizations?

I want to implement a solution to the electronic Schroedinger equation using DFT. I know there are many, many functionals to choose from but don't know where to start. I'd like to start with a couple ...
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59 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: ...
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1answer
28 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 ...
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91 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 ...
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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. ...
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1answer
58 views

Building a solid-liquid interface

I am trying to build a solid-liquid interface to be simulate using molecular dynamics. Since, I would be using ab initio molecular dynamics (cpmd or cp2k), I need the minimum cell dimensions. Please ...
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40 views

Which DFT functional is best for predicting macroscopic properties of molecular liquids?

Which DFT functional (KS-DFT, to be more specific) performs best at predicting macroscopic properties (density, surface tension, boiling point, relative permittivity etc.) of molecular liquids (...
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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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57 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 ...
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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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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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237 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 ...
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88 views

How do I get a value of the Gibbs energy of formation at 0 K and 0 bar?

The standard Gibbs free energy of formation for copper(II) oxide, $\ce{CuO}$, is $\Delta_\mathrm fG (\pu{298.15 K},\pu{1 bar}) = \pu{-129.7 kJ mol-1}$. How can I get $\Delta_\mathrm fG (\pu{0 K},\pu{...
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What is the significance of increasing (and decreasing) the number of ionic releaxation steps in a plane wave DFT calcuation? [closed]

The number of ionic relaxation steps is one of the adjustable parameters while setting up a DFT calculation using a commercial code such as VASP. I was wondering what's the significance of this value?
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525 views

Calculation correct binding energy for a system?

I am doing DFT calculations (VASP) to calculate correct binding energy for an amorphous silica surface and silver adatoms. From MD simulation, we found that a binding energy of 0.2 eV gives results ...
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33 views

Is There an Acceptable Potential Which Can Represent the Interaction between Finite-sized Ions?

The question is as mentioned in the title. Take for example the interaction between $\ce{Ar^+}$ and $\ce{Ar^+}$. In general, is there an acceptable potential which can represent the interaction ...
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How can I find the smaller symmetric structure from big crystal unit cell?

I have Pyrope (Mg3Al2(SiO4)3) crystal structure, downloaded from materialsproject: https://materialsproject.org/materials/mp-6073/# It appeared as triclinic cell, but if you download CIF file and ...
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1answer
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Question about the setting of DFT (Density Function Theory)-NEB (Nudged Elastic Band) calculation

I'm attempting to calculate Mg ion vacancy migration inside Pyrope unit cell. System has total 159 atoms, beginning and end geometry is optimized. For intermediate image, I'm using one coordinate that ...
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1answer
173 views

Optimization and DFT-D3 energy calculation in GamessFireFly version 8.1.1

My input was ...
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19 views

Convert to NASA polynomial format for chemical kinetic calculations

I have calculated thermodynamic properties of a metal oxide through DFT calculations, I want to convert it to NASA polynomial format and use those thermodynamic properties along with rate constants ...
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34 views

How can I obtain the wave function of the dihydrogen system, by hand using Self Consistent Field DFT calculations? [duplicate]

After doing some energy calculations for the $\ce{H2}$ system in Gaussian, using ωB97X-D/STO-3G level of theory, and generating a .wfn file, I wanted to understand better how the computer does ...
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91 views

What is meaning of LAV2P basis set?

I am working on energy optimization of molecular structure using DFT with Schrödinger software and I found that job is running on LAV2P basis set. I want to know what is the meaning of this basis set ...
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30 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 (...
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49 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 ...
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19 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 ...
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1answer
77 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 ...
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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 ...