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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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Rationalisation of stabilising dispersion interactions in HgS via QM

It is well-known that mercury(II) sulfide, HgS, is extremely inert- for example, it has one of the lowest solubilities of nearly all known compounds in water(check Wikipedia). There are two well-known ...
Kanghun Kim's user avatar
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1 answer
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What does it mean and what are the implications of individual electrons being identical? [closed]

In Jeremy Harvey's Computational Chemistry, I found the following statement: [...] all electrons need to be treated as being very strictly identical to each other, so the wavefunction cannot treat ...
PassionFruitChemistry's user avatar
-2 votes
2 answers
744 views

Why silicon doesn't have the electronic configuration [Ne] 3s¹ 3p³?

I have read that half or fully filled orbitals provide more stability to the element due to symmetry and exchange energy.It is the reason for electronic configuration of Cr-[Ar]4s¹3d⁵. Then, why doesn'...
j sivesh's user avatar
1 vote
1 answer
39 views

Solid state chemistry current in tunnel diode [closed]

I am interested in this subject: Applied Mathematics in Electrical Engineering. I am interested in finding an expression for the tunnel current of a tunnel diode in the negative differential ...
Jun Seo-He's user avatar
6 votes
1 answer
508 views

Physical interpretation of transition density matrix

I am studying natural transition orbitals (NTOs) and I am following the Martin's article [1]. I understand the mathematics, but I am a bit confused about what exactly is the transition density matrix ...
AlfredV's user avatar
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Provide a mathematical explanation of why the non-relativistic Schrödinger equation for a Molecular system, cannot be solved analytically?

The non-relativistic Schrödinger Equation is: $\widehat{H}|\psi\rangle=E|\psi\rangle$ Where $\widehat{H}$ is the Molecular Hamiltonian in Atomic Units and has the following terms: $$ \widehat{H} =...
Guri's user avatar
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-4 votes
1 answer
767 views

What Are Angular Nodes , Radial Nodes, Nodal Plane and Nodal Surfaces? [duplicate]

Actually, i am confused between all of them. some says that angular node is same as nodal planes and radial nodes is same as nodal surfaces... and what's the difference between nodal planes and nodal ...
Piyush Baweja's user avatar
0 votes
2 answers
361 views

Molecular orbital mixing in second period elements

Why do the $\pi_u(2p)$ and $\sigma_g(2p)$ diatomic MO mix within the second period elements as it's shown in the image below?
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Analog of chemical accuracy in vibronic energy calculation [closed]

Is there any analog of chemical accuracy in vibronic energy calculation? I understand that the unit is /cm however, probably, I shouldn't expect a given number in that unit as the analog. I am ...
Omar Shehab's user avatar
2 votes
1 answer
556 views

Why does Hartree-Fock overestimate the energy gap?

Everybody knows that Hartree-fock overestimates the energy or band gap, but it is difficult to have a clear explanation of the origin of this large deviation from the experimental values. I know that ...
M06-2x's user avatar
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3 answers
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Why are there so many metals in the periodic table?

Brief Background: I was studying about the 'classical electronic configuration' of the first 20 elements of the periodic table today and was bewildered by the fact that there are an equal number of ...
Chinmay Krishna's user avatar
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Problem on the wavelength of the electromagnetic radiation needed to break 1 mol of gas [closed]

Problem. To break the bonds in 1 mol of Cl$_2$ we need E=243.4 kJ of energy. The bonds on this amount of gas can be broken by electromagnetic radiation of a certain length. Determine the wavelength. ...
utobi's user avatar
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1 answer
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Where does the exchange energy released in an exchange interaction come from?

Two electrons in different orbitals can exchange with each other and if this happens they release a certain amount of exchange energy. But where does this energy come from? And wouldn't the atom ...
dotmashrc's user avatar
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4 votes
1 answer
213 views

How is a Barrierless Reaction Confirmed in Quantum Chemistry?

Background: Some quantum chemistry papers explore potential energy surfaces by characterizing critical points with an ab initio method. Reactants, products, intermediates, and transition states are ...
Daylight's user avatar
-1 votes
2 answers
434 views

Solving two-electron integrals

I have been trying to teach myself Quantum Chemistry through a book, and I came across this problem and I am not sure how they solve a certain problem. Essentially, they applied first-order ...
M.L's user avatar
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3 votes
1 answer
654 views

Fundamental doubts about energy levels vs. shells vs. subshells vs. orbitals

First in school I learnt that when supplied enough energy, the electron in a hydrogen atom will jump energy level(s), denoted by $n=1$, $n=2$ and so on for integral values of $n$. Then I learnt about ...
AVS's user avatar
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Will the total transition from ground to excited state decrease when two potential energy surface come near to each other?

I am trying to study Tully simple avoided crossing model via mean-field dynamics. I see when two surfaces are near when value of parameter C is 0.0005 (Unavoided/trivial crossings) there is less ...
johnny123's user avatar
1 vote
1 answer
81 views

What does it mean that a state belongin to a given irrep transforms like $Rx$, $Ry$ or $Rz$

The present question is related to this other question I did few days ago. Given a point group and the list of the irreps (see for example here) the meaning of an irrep which transforms like $x$ or $x^...
Davide Sangalli's user avatar
0 votes
1 answer
196 views

Confusing statement about the Pauli exclusion principle

While reading "Francis A. Carey, Richard J. Sundberg - Advanced Organic Chemistry Part A. Structure and Mechanisms-Springer (2007)", I came across the following: The paragraph says that ...
ananta's user avatar
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-1 votes
1 answer
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Calculation of canonical orthogonalized basis from overlap matrix?

From Equation [3.169] in David Cook's Book , I understand that we use eigenvalues and eigenvectors of the overlap matrix to calculate an orthonormal matrix using the formula $$X=Us^{-1/2}$$, where $U$ ...
Hemanth Haridas's user avatar
3 votes
1 answer
981 views

Probability density and radial distribution function of finding the most probable distance of electron in 2p orbital in hydrogen atom

Referring to the answer by DSVA (Most probable point for finding an electron in the 1s orbital of a Hydrogen atom) There's a maximum of finding the electron at a certain distance away from the core (...
user4723's user avatar
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0 votes
2 answers
113 views

Violation of pauli's exclusion principle? ( During formation of neutron star or black holes) [closed]

Sorry it's a bit long so i was watching Action labs video on how neutron stars are formed and in that he says " if there's enough mass, then the mass will keep on collapsing in on itself in the ...
Nnn's user avatar
  • 25
0 votes
2 answers
368 views

Proof for equation of radial probability distribution curve

So my textbook says the equation for this is $$P(r) = 4\pi r^2\Psi^2 $$ It also gives the volume of the shell formula $$\mathrm{d}V = \frac 43 \pi(r+\mathrm{d}r)^3 - \frac 43 \pi r^3$$ which I ...
G.S.'s user avatar
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7 votes
1 answer
404 views

How do I show that a transition is electric dipole allowed with group theory/symmetry?

This is actually a follow up of this question The follow-up is not because of the electric instead of magnetic dipole (this is trivial). It is because I'm interested in extra info. Suppose I have a ...
Davide Sangalli's user avatar
-2 votes
2 answers
106 views

If two atoms join, do they lose their original spectral lines? [closed]

Each element and even isotope of element has its own set of unique spectral absorption lines. What happens to these lines after two or more atoms join together into molecule? Are all of the lines that ...
Soliton's user avatar
  • 25
-3 votes
1 answer
328 views

Which quantum numbers does the orbital angular momentum depend on? [closed]

I learned from a question that I recently solved that the orbital angular momentum depends on both the azimuthal and magnetic quantum numbers. I did not expect this because the formula for the orbital ...
user avatar
1 vote
1 answer
195 views

Estimating the energy required for dissociation from visible absorption spectra

I have been having trouble solving the following problem. The question calls for an estimation of the energy (in kj/mol) required to dissociate ClO in its excited state when it is excited from the v = ...
aqollo's user avatar
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3 votes
1 answer
163 views

Modelling barrier-less reactions

I am currently exploring a few dissociative reactions whose PES is barrierless (no first order saddle point). I have found a few approaches in literature like flexible nudged elastic band theory, ...
Arun's user avatar
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1 vote
0 answers
127 views

How to calculate <ab|ab> integrals to carry out Schwarz pre-screening of molecular integrals

During writing my own two-electron integration library I faced an issue of dealing with integrals that have very small values beyond the C/C++ double data type. ...
Dmitry Govorov's user avatar
-1 votes
1 answer
213 views

If you are doing an ONIOM calculation in Gaussian, how do you add both custom MM parameters and a custom basis set using GEN and Amber=SoftFirst?

I am trying to calculate a biological system using AMBER forcefields as my low-layer in ONIOM3. I also want to use a non-native basis set (aug-pc-1 for opt+freq, aug-pc-2 for SPE) that requires custom ...
Runa's user avatar
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0 votes
0 answers
74 views

Calculating zero-point energy for the Varshini potential

As per my understanding, for a harmonic oscillator, we can find the zero-point energy by the relation below: $$ E_0 = \left(0 + \frac{1}{2}\right)\hbar \omega$$ The equation for the potential in ...
D.Kang's user avatar
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2 votes
0 answers
35 views

How do the spin-matrices change in systems without spherical symmetry?

The (non-relativistic) spin matrices of a free electron, with $z$ as the quantization axis, read: $$ \boldsymbol{S}_{x}=\frac{\hbar}{2}\left[\begin{array}{ll} 0 & 1 \\ 1 & 0 \end{array}\right],...
Logi's user avatar
  • 181
7 votes
1 answer
172 views

Why do sp³ orbitals have tetrahedral symmetry when they are linear combinations of orbitals with octahedral symmetry?

I'm trying to understand how orbitals hybridize. What I understand—or at least think I understand—is the following: The "standard" $\mathrm{s}, \mathrm{p}, \mathrm{d}, …$ orbitals are ...
Bernhard Werner's user avatar
4 votes
0 answers
115 views

How to obtain excited state energies from configuration interaction? (Helium example)

A theory of CI method For going beyond the Hartree–Fock (one determinant) approximation one introduce configuration interaction (CI). Interaction means the mixing (interaction) of different electronic ...
Sergio's user avatar
  • 925
3 votes
0 answers
113 views

Hund's Rule for Determining Term Symbol Energy Order

In order to determine the energy order based on J-value for term symbols we employ Hund's rule: In a less than $\frac12$-filled subshell Lowest J-value is Lowest energy In a more than $\frac12$-...
Allod's user avatar
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1 vote
0 answers
43 views

Electronic vibrational rotational transition of electrons -- for a transition from $\Sigma$ to $\Pi$ how are P and R branches possible?

Suppose we have a diatomic molecule in a $^1\Sigma$ state, and it transitions to an excited $^1\Pi$ state. Note that the total spin of the electrons remains unchanged ($\Delta S=0$) as we can assume ...
Garf's user avatar
  • 163
14 votes
1 answer
620 views

Why are Slater-type orbitals used for atomic calculations instead of hydrogen-like orbitals?

Originally, Hartree–Fock atomic calculations were done by using numerical methods to solve the Hartree–Fock equations, and the resulting orbitals were given as tables of the radial functions for ...
Sergio's user avatar
  • 925
5 votes
1 answer
329 views

How to compute the electronic energy reported by Gaussian from the AlphaOrbitalEnergies?

I followed the instructions on Wolfram Language & System Documentation Center — FCHK (.fchk) to analyze the *.fchk file. I ran an energy calculation with ...
mostafa100000's user avatar
3 votes
0 answers
463 views

Shielding vs electron-electron repulsion

Example of shielding: (source) The last electron in the 6s subshell of $\ce {Cs}$ is shielded from the nucleus by the inner electrons. Example of electron-electron repulsion: The electron affinity of ...
user avatar
3 votes
0 answers
133 views

6-31G basis set of Silicon and the p-orbitals

Following is the 6-31G basis set of silicon. The numbers in the bracket denotes power raised to 10, e.g. $1.231(+3)=1.231 * 10^3$ Now silicon is $1s^22s^22p_x^22p_y^22p_z^23s^23p_x^13p_y^1$. Now I am ...
user3001408's user avatar
4 votes
2 answers
227 views

Spontaneous reaction between chlorine and a hypothetical atom

Suppose, my hypothetical element is $\ce X$, whose first ionization energy is $\pu{200kJ/mol}$. An atom of this element will react with an atom of chlorine (assume that I've already dissociated a ...
user avatar
2 votes
1 answer
502 views

My book's claim about the shielding effect of s,p,d and f electrons

It's a relatively unknown Bangladeshi book. It's called "Chemistry-First Paper (Class XI-XII)" by Professor Haradhan Nag According to my book, "Electrons of s and p orbitals produce ...
user avatar
1 vote
1 answer
139 views

Gaussian type orbitals and p-type orbital

The Gaussian orbital for $p_x$ is defined as: $$\psi_{p_x}=C(x-x_A)e^{-(\vec r - \vec R_A)^2}$$ Where $C$ is a constant. Now if we multiply two such orbitals with two different centers, then $$C_1(x-...
user3001408's user avatar
5 votes
1 answer
458 views

Why do DFT calculations output molecular orbitals?

My understanding is that DFT finds the electron density which minimizes some energy functional. How does it make the connection from this optimized density to molecular orbitals?
greatscissors's user avatar
0 votes
0 answers
80 views

A question regarding hydrogen emission and absorption spectrum

I have read in many articles as well as my textbook that when an electric discharge is passed through a sample of hydrogen gas it excites the electrons and glows emitting light.When this light is ...
AJknight's user avatar
3 votes
0 answers
100 views

What is the physical significance of the damping function in calculation of vdW interaction energy?

In order to take care for the singularity (also removing the double counting of exchange-correlation) obtained with this expression, $$E_\mathrm{vdW} = \frac{C^6}{R^6}$$ many type of damping functions ...
Pro's user avatar
  • 181
3 votes
1 answer
560 views

2p wave function radial distribution

The following graph shows the values of the wave function $\psi_{210} $ (i.e. the $2\mathrm{p}_z$ orbital) versus the radius (divided by $a_{0}$). I don't understand it, however, since the $2\mathrm{...
niobium's user avatar
  • 239
7 votes
1 answer
261 views

Variational (Rayleigh-Ritz) method and multielectron wavefunction

I am studying Rayleigh-Ritz method and I have some problem to understand what the final wavefunction looks like. Suppose that we have a system with $N$ electrons, atom or molecule. We are interested ...
Anton's user avatar
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-1 votes
1 answer
497 views

A question regarding excitation of electrons in atomic orbital [closed]

In Bohr's model of an atom, the formula used to find the energy between the 2 orbits and wavelength of emitted photon was valid only for single electron species like hydrogen.In the case of a multi-...
AJknight's user avatar
3 votes
0 answers
84 views

State-of-the-art results on molecular vibronic energy calculation

Could someone kindly point me to the latest records in scale, accuracy, or timescale for molecular vibronic energy calculation? I tried to start with Google Scholar: https://scholar.google.com/scholar?...
Omar Shehab's user avatar

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