Questions tagged [quantum-chemistry]

Quantum chemistry is a subfield of quantum mechanics. Like its parent field, quantum chemistry focuses on understanding physical phenomena occuring at the atomic scale. Quantum chemistry however is more focused on providing useful descriptions of electronic structure to aid in understanding chemical problems (e.g. reactions, spectra, dynamics, ...).

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Is the quantum theory a violation of heisenberg uncertainty principle?

Here's what's written in my textbook: quantum theory of atomic structure: the energy states of atoms and molecules are discrete. This means that it is similar to the Bohr's model, which is a ...
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Quantum Espresso, phonon band dispersion

I am having a trouble calculating the phonon band dispersion of graphene at $K$-point using Quantum-Espresso, using normconserving+LDA and normconserving+PBE, ecutwfs=60, Ecutrho=400.0, forc_conv_thr=...
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The quantum mechanics behind periodicity of elements

Especially in high school/first-year undergraduate chemistry courses, we learn with great dedication the periodicities along groups and periods. There are various useful and interesting trends. I ...
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Potential energy expectation value and virial theorem

I have been trying to solve this problem Part a) can be proved using the formula . For part b), the expectation value of the potential energy can be calculated from the integral , where Therefore, ...
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Regarding the probability density of finding an electron in between a bond

I'd found the image attached on a website explaining the molecular orbital theory. My question is, shouldn't the graph have a maximum at the middle of the two hydrogen atoms? since my intuition tells ...
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Why do electrons jump back after absorbing energy and moving to a higher energy level?

Electrons in a shell absorb energy and move to higher energy levels, but they release their energy and jump back to the shell they originally were in. Why do they jump back? Why can they not keep ...
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are the exact and kohn-sham electron densities totally symmetric?

For any molecule with open or closed shells considering the electronic state to be the ground state: Is the exact electron density totally symmetric? is the Kohn-Sham electron density totally ...
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Confusion about the second ionisation enthalpy of elements of 3d series of d block in periodic table [closed]

I am posting this question again from another device in the hope of making it more accessible to people. This time I have included pictures instead of the links. Fortunately got it right this time! ...
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Confusion about the trend of second ionization enthalpy of 3d series of d block elements in the periodic table

link for the paragraph Link for numerical data related to the paragraph I don't know where to start but I m really confused about this. Firstly how does shielding (screening effect) of electrons ...
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How to determine which levels correspond to each other in LS coupling and jj-coupling?

In the LS coupling scheme, L, S and J are good quantum numbers; whereas in jj-coupling scheme, j1,j2 and J are good quantum numbers. I have learnt how to get the term symbols for LS coupling, and for ...
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What does orbital mean, exactly? [duplicate]

My teacher told me that orbital is the probability distribution data of the electron around nucleus which is amplitude data in a way. An example of how my teacher actually told what it means involves ...
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What causes little spikes in a flame? [closed]

I have a lighter at home which everytime I use it, generates little spikes (see picture below) which eventually disappear. The Spikes are not an effect generated by the camera. I can confirm that ...
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Heat of reaction for the decomposition of AgCl means?

Certain sunglasses have small crystals of $\ce{AgCl}$ incorporated into the lenses, on exposure to light of appropriate wavelength produces a grey colour to reduce the glare following the reaction: $$\...
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Does electron mass decrease when it changes its orbit?

I have studied this in my chapter atomic structure that when an electron changes its orbit from lower energy to higher energy state , it does not state in my book that it moves there but that it ...
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Is the correct zeroth-order wave function $\phi^{(0)}$ an eigenfunction of the perturbation Hamiltonian $H'$?

Consider the perturbation treatment of He $1s2s$ excited state configuration. The quantum mechanical Hamiltonian for the He-atom is: \begin{aligned} \hat{H} = \sum_{i=1}^2 h_i + \frac 1 {r_{1 2}} \\ ...
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Interpretation of Coloumb operator in Hartree Fock equation

I have read in a textbook (Modern Quantum Chemistry Szabo and Ostlund) that the Coulomb operator of the form \begin{equation} \mathcal{J}_{j}\left(\mathbf{x}_{1}\right)=\int d \mathbf{x}_{2}\left|\...
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Negative coefficients for DIIS in SCF procedure

I am trying to implement the direct inversion of the iterative subspace (DIIS) algorithm in my simple HF program. After some problems it now seems to work. I can calculate the correct energies in ...
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1answer
77 views

What does Pauli’s exclusion principle mean in atomic or fundamental way? [closed]

It means is that no electron can have same n , l and $m_l$ but can have two different spin quantum number. I want to know why is this rule valid?Means there must be some other things happening also ...
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Which quantum chemistry approximation to use?

I'm just getting started with quantum chemistry using some simple test problems. I have a few of the software packages installed (ASE, Psi4, GPAW etc) and everything is working - no issues with the ...
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1answer
139 views

Is the quantum number $L$ a combination of the azimuthal and magnetic quantum numbers $(\ell, m_\ell)$?

Somehow, I am getting confused about this... In the $J=L+S$ equation about total angular momentum, which of the four quantum numbers used to describe the electrons and their states is included in the $...
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1answer
131 views

Is it rigorous enough to change a DFT functional after geometry optimization to calculate excited state properties?

I have done the geometry optimization with the B3LYP functional, however, there is a need to calculate the excited state energy, so the CAM-B3LYP functional should be better suited for that. Do I need ...
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energy difference vs absolute energy

When dealing with determining the ground state energy of a system, people often say that it is better to know the energy difference rather than the absolute minimum energy. I understand that the ...
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1answer
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Origin of Virtual Molecular Orbitals in Hartree-Fock equations

Let's look at scheme Do I understand correctly, the origin of Virtual Orbitals is a solution of (for, exampre, restricted) HF equation eigenfuncions?
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Stress-Strain Graph Under Uniaxial and Biaxial Strain

I was going through a recently published journal paper by ACS (Applied Nano Materials) about gas separation using nanoporous Graphene. DOI No. - https://dx.doi.org/10.1021/acsanm.0c02340 Supporting ...
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What is the form of the Hamiltonian for solids?

For atoms, and even molecules, I can understand how the Hamiltonian would be constructed, but what of solids (as in, for electronic structure calculations)? Thank you for any help.
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What is the ground-state electron configuration of V2+? [duplicate]

Why do you get rid of the 4s^2 electron subshell first rather than 2 electrons from the 3d subshell?
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What is the definition of energy-independent basis?

I am reading this article by Zatsarinny and Fischer [1]. At some point they span a wave function $\Psi_{E}$ in terms of energy independent basis $$\Psi_{E}=\sum_{k} A_{E k} \Psi_{k}$$ For example, The ...
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Resources to learn second quantization formalism and how to implement it in code for quantum/computational chemistry research

I want to know what are some useful and practical resources to learn the formalism of second quantization as it pertains to fermions to be used in solving Quantum Chemistry problems. I am particularly ...
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Do changing opt=modredundant to opt in Gaussian makes geometry optimization not to take into account frozen angles?

I am a newbie to Gaussian and just generated an input for the geometry optimization for some molecules with multi ring system. However, in the article that was a reference for those calculations, some ...
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1answer
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Why do Rydberg atoms have electrons that can transition above the first ionization energy?

Why do Rydberg atoms have electrons that can transition above the first ionization energy? This can only happen in a multi-electron system but I don't see how the Rydberg formula would even work for n ...
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1answer
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Electron configuration of non-hybridized carbon

I'm currently learning about orbital hybridization in carbon. I see that carbon has an electron configuration of 1s2 2s2 2p2. However, there are actually three p-orbitals in the second shell, namely ...
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How do I maintain a charge on one of my species in Quantum Espresso for an NEB calculation?

I am trying to run an NEB calculation that reacts a sulfite anion with an organic molecule. I've gotten it to converge, and I've added necessary the fcp_tot_charge_first = -2 and fcp_tot_charge_last = ...
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Dissociation probability vs ionization probability

Suppose we have a diatomic molecule whose ionization energy is $I$ and dissociation energy is $D$ such that $I>D$. Now suppose an electron with energy $E>I>D$ collides with this molecule. ...
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How to calculate core electron SCF Fock Matrix Elements?

I have searched everywhere but there are no worked examples of how to calculate the $\langle\chi_i |\hat{h}| \chi_j \rangle $ values for the Fock Operator using the SCF HF Roothaan method. I am given ...
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Can we observe the shapes of MOs? [duplicate]

Condensed question formulation: Is there an experimental method to directly visualise the 3D form of a MO wave function, or at least the electron density associated with it? Full statement: ...
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How to construct spin free coefficient of 2-hole 1-particle configuration?

It is clear that we can expand a cationic state of a molecule in terms of configuration interaction (up to double excitation). It means: $$|I \rangle = \sum_j c^{(I)}_j a_j|\phi_0 \rangle \ + \ \sum_{...
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Why do molecules showing a pure rotational spectra require a permanent dipole?

I understand that pure rotational spectra only exist for molecules which have a permanent dipole moment. The common explanation is "so that they can interact with the E-field of the incoming ...
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1answer
59 views

Spin orbital coupling and total angular momentum

The Sodium emission spectrum has 2 bright yellow lines called the Na-D lines. Superficially I understand the cause; there's spin-orbit coupling and the 3p orbital is split into 3p(3/2) and 3p(1/2). ...
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Question on symmetry-adapted, possibly antibonding combinations of core orbitals

In this question, it is said that the core electrons do mix and form delocalised, symmetry-adapted orbitals. The boron trifluoride molecule is often said to have no anti-bonding orbitals filled, with ...
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1answer
51 views

Do different eigenstates of total angular momentum have necessarily different energies?

Let $H$ be the Hamiltonian of a specific atom and $J$ the total angular momentum. Since $H$ and $J$ commute, they have common eigenstate. So we can label the atomic states by their energy and total ...
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1answer
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Azimuthal Quantum Number [closed]

In the radial equation of hydrogen atom the differential equation is described by But why is l taken to be integer. I know the principal quantum number n correspond to energy levels so that's why it'...
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1answer
233 views

Fundamental vibrational modes for QM9 molecules

QM9 dataset contains about $134000$ small organic molecules with following properties computed using DFT approaches: ...
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How does an MRI work? [closed]

This is my interpretation of an MRI: There's an external magnetic field and most of the hydrogen nuclei will align themselves with that field (which is the lower energy state) but some are not aligned ...
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Why does the Most Stable State of an Atom Tend to be One with Full s and p Subshells?

I'm new to posting on stack exchange, although I've read a lot of it before. This question seems like it might end up being marked as a duplicate, but I've looked through a lot of the similar ...
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How is it possible to draw a radial wave function for the 2p orbital?

Below is a picture of the radial component of the wave function distribution for the 1 through 3s orbitals. It makes sense to me that there are points where the wave function is 0, since by definition,...
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How are class II force field cross-terms derived?

In some class II force fields, there are cross-terms that characterize bond-bond, bond-angle interactions and others. Example of equations below: $$\sum_{bond-bond}^{}k_{bb}(b-b_{0})(b^{'}-b^{'}_{0})$$...
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How does a body lose electrons? [closed]

If there is a Na and cl in solid form , There will be atoms inside of them.How do they lose electrons ?.We know solid body has a structure and covering.Just like you can touch is the covering of table ...
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105 views

Is there any theory more advanced than MOT?

I am currently going through molecular orbital theory(MOT), I should appreciate it's a very nice theory and it's helping me to fill the gaps of normal hybridization concepts and valance bond theory(...
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Comparison between Molecular Orbital Theory and Valence Bond Theory

I have been reading about introductory Molecular Orbital Theory lately. Till now, I have used Valence Bond Theory to evaluate bonding. I have a few questions about it:- In VBT, we used the idea that ...
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nodes and probability density

The square of the wavefunction gives probability density of finding an electron somewhere in the orbital. The text I'm referring to says that the value of probability density is always higher than ...

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