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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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Connection of term symbols with specific microstates for atomic carbon

I'm currently studying atomic term symbols. I wanted to try it on a simple atomic carbon with the electron configuration $1s^22s^22p^2$. I know, that only open-shell electrons are involved in the ...
Eenoku's user avatar
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Letter codes in molecular term symbols

I'm trying to find an explanation for the letter codes (X, A, B, C, etc) when you get a term symbol like this $$X\,{}^3{\Sigma}^-_g$$ Can anyone point me to some literature that explains these ...
Arno van der Weijden's user avatar
7 votes
3 answers
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How is Negative Temperature Hotter than Infinite Temperature?

Recently I have read articles stating that negative Kelvin has been achieved. I was a bit speculative at first as I though how can you get cold than 0 Kelvin, but after doing some research it makes ...
Nanoputian's user avatar
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7 votes
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Calculate the wavelength of the radiation released when an electron moves from n= 5 to n=2

Calculate the wavelength of the radiation released when an electron in a hydrogen atom moves from $n = 5$ to $n = 2$. How can I find this wavelength?
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How much energy does it cost to have electron configurations that are not in accordance with Hund's rules?

What is a ballpark figure for the difference in energy for an atom that follows Hund's rule vs one that has two electrons with opposite spins? I'd be interested to know carbon and nitrogen. Is there ...
Brinn Belyea's user avatar
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4 votes
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Pauli-forbidden term symbols for atomic carbon

Carbon has a $\mathrm{p^2}$ configuration, and within the Russell-Saunders coupling scheme, we have $$\begin{align} s_1 = s_2 &= \frac{1}{2} & S &= 1,0 \\ l_1 = l_2 &= 1 & L &=...
orthocresol's user avatar
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Predicting bond-strength of metal carbonyls

The metal carbonyls (and similar organometallic compounds) involve a combination of sigma bond, a pi bond and backbonding. The bond strengths under consideration are the metal-carbon bond and the ...
stochastic13's user avatar
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36 votes
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Is there an energy cost associated with flipping the spin of an electron?

THE STORY: A common example used to illustrate the limitations of restricted Hartree-Fock (RHF) theory is the H$_2$ dissociation energy ($D_e$) curves. RHF enforces electrons to be paired into spin ...
LordStryker's user avatar
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29 votes
3 answers
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Do electrons have some volume, area or shape?

I am in 8th grade now and when I was in 6th grade, my science book had diagrams of the electronic configuration of atoms. The electrons were round like spherical balls. Is it true that the electrons ...
Jimgao's user avatar
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What's the difference between PBE and B3LYP methods?

I can't find an answer to that question. I was told that in B3LYP, more variables implemented in the method are empirical, but I can't find anywhere if it's true, and I'm sure it's not the only ...
Basia's user avatar
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How dependent are computed charges using the quantum theory of atoms in molecules on the used level of theory?

The quantum theory of atoms in molecules is based on the topology of the electron density. This mathematical analysis allows to find critical points and hence has a unambiguous way of separating a ...
Martin - マーチン's user avatar
21 votes
3 answers
13k views

Why is manganese(II) coloured although the transition should be spin-forbidden?

In every basic coordination chemistry class, at some early point the crystal field theory and LFSE will be taught, explaining that there will be an energy difference between d-orbitals (typically ...
Jan's user avatar
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20 votes
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508 views

Observability of Orbitals and Orbital Energies

This question comes from some thoughts I had after reading this question. First of all, is an orbital an observable? I know the answer to this question is no because there is no "single-orbital ...
jheindel's user avatar
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Is it correct to talk about an empty orbital?

Professor A. J. Kirby mentions: The properties of an orbital are those of an electron contained in it. It is normal practice, illogical though it may sound, to talk of 'vacant orbitals'.The ...
Tan Yong Boon's user avatar
16 votes
1 answer
3k views

Explaining the Relative Energies of Various Vibrational Modes

Specifically, I'm wondering why it is that the asymmetric stretch in most (although there are exceptions to this I believe) molecules is higher in energy than the symmetric stretch. Similarly, I ...
jheindel's user avatar
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Are there any datasets containing molecules with more than 38 heavy atoms?

I have been testing a machine learning approach for molecular energy prediction. The current dataset that I have is QM9, which is consist of molecules with up to 9 heavy atoms. I was wondering if ...
Blade's user avatar
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15 votes
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Difference between radial, planar, angular and spherical nodes

What is the difference between them? I think radial nodes and spherical nodes are the same, and angular and planar nodes are the same. Reference Finally, how many spherical nodes are there in ...
pikachu's user avatar
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13 votes
5 answers
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Is coupled cluster variational for two electrons?

I know that coupled cluster (CC) is not variational for the general case. However, if we only have two electrons with one nucleus, CCSD should be exact for this system like full configuration ...
James LT's user avatar
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11 votes
2 answers
3k views

Non-bonded orbitals in water

General Chemistry perspective: Looking at the molecular orbitals of water, we can see that the oxygen is $sp^3$ hybridized. Oxygen forms two sigma bonds with hydrogens, and there are two lone pair ...
user3786990's user avatar
11 votes
1 answer
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Significance of phase of atomic orbitals

I am learning about orbitals and bonding and antibonding MOs. So far, I know that when you combine s orbitals, you form both bonding and antibonding MOs- bonding when the wavefunctions are added in ...
Meep's user avatar
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11 votes
2 answers
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Irreducible representations and system states connection

Let's say I have the molecule $\ce{N2+}$. Its symmetry point group is clearly $D_{\infty\mathrm h}$. But I'm confused by its irreducible representations. I know there are 8 of them, but as I ...
Eenoku's user avatar
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11 votes
3 answers
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Significance of 4π in the uncertainty principle

What is the significance of the quantity 4π in the Heisenberg uncertainty principle? $Δp \times Δx = \frac{h}{4π}$ The focus of the question is to learn why the ratio of a circle's circumference to ...
TheStarVoyager's user avatar
11 votes
1 answer
437 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}) \...
Christopher's user avatar
10 votes
1 answer
560 views

Negative Kelvin Temperature

I remember my Physical Chemistry Professor saying that very tiny negative Kelvin temperatures have been achieved on the quantum level. Is this true?
user 85795's user avatar
10 votes
1 answer
7k views

What is an orbital boundary surface?

Could anybody explain the following statement: "The boundary surface is a common way to represent atomic orbitals, incorporating the volume in which there is about a 90 percent probability of finding ...
user14840's user avatar
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9 votes
1 answer
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How to choose proper active space in method such as CASPT2 and CASSCF?

I am confused about other research paper which use CASPT2 method. I am not clear about how to choose active space. What can I refer to? (tutorial reviews, papers, or any other things)
Chao Song's user avatar
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9 votes
3 answers
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Does partial charge violate the law of quantization of charge?

Well, we know that charge is quantized. Quoting wikipedia below, Charge quantization is the principle that the charge of any object is an integer multiple of the elementary charge. And we know that ...
user3459110's user avatar
9 votes
2 answers
2k views

Why are correlation consistent basis sets used with DFT calculations?

Reading some papers, there is a bit of a curious observation that does not exactly make a lot of sense to me. In Quantum Chemistry, different methods exist to carry out geometry optimisation and ...
DetlevCM's user avatar
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9 votes
2 answers
430 views

How was the quantum three-body problem solved?

What method was used to solve the quantum three-body problem? I heard it was solved and was not sure about the status of the problem.
user4884's user avatar
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1 answer
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How strict is the "to excite electrons the energy must equal the energy state difference" fact?

We are always told that to excite an electron from one state to a higher energy states, for example from the valance band to the conduction band, the energy must equal the energy difference between ...
Yoda's user avatar
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8 votes
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How to obtain the radial probability distribution function from a quantum chemical calculation?

In the book "Quantum Chemistry" by Ira Levine I found a plot of the radial probability distribution function of argon: The figure appears in the context of the following section: Electron ...
Sergio's user avatar
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8 votes
1 answer
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how are the frequencies at a local maximum of PES like?

On the potential energy surface, if you find a local maximum and calculate its frequencies in Gaussian or something like that, will you get all negative frequencies or all positive frequencies? I know ...
OhLook'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
7 votes
1 answer
3k views

Why cannot the Schrödinger equation be solved exactly for systems in which more than two particles interact? [duplicate]

Is it so because it is impossible to solve mathematically? If so, why? Or is it possible to solve the equation mathematically, but no physical interpretation of the solutions is possible? Or is it ...
Yoda's user avatar
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6 votes
1 answer
498 views

Just how quantized are energy levels?

One thing which has bothered me for a while about the idea of quantized energy levels is that it's clear just how quantized they are. Let me give an example. If I was looking the energy some ...
jheindel's user avatar
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6 votes
2 answers
2k views

Quantum mechanical model of atom and the quantum numbers

I studied Bohr's model of atom and then the drawbacks of it and then quantum mechanical model of atom. Now quantum model is according to uncertainty principal and dual nature of matter and it says we ...
Shashank Kumar's user avatar
6 votes
2 answers
4k views

How can two electrons lie together in an orbital?

Two electron of opposite spin can lie in a single orbital.. But what about the electron-electron repulsion. Okay! I got that the nuclear charge rather the large Z-effective overcome this repulsion by ...
sedflix's user avatar
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6 votes
4 answers
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Why do solutions of electron in a box (and in a ring) predict coefficients for LCAO (linear combination of atomic orbitals) in 1D systems?

The solutions to the 1D particle in a box quantum mechanic system are standing waves (zero at both ends of the box) with 0,1,2... nodes for increasing energy (zero for the ground state). If I look ...
Karsten's user avatar
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6 votes
1 answer
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GAMESS crash course?

I'm still learning about MO theory – and I thought that I would do some calculations with GAMESS to become more familiar with the concept. Even though I look forward to delving into the realm of ...
Ravaru's user avatar
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6 votes
2 answers
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Why are the radial wavefunction and radial distribution function different?

I'm having trouble understanding the following two graphs, which are the radial wavefunction of the hydrogen 1s orbital and the corresponding radial distribution function: Specifically, why is the ...
Guhan Purushothaman's user avatar
3 votes
1 answer
518 views

Justify the value of the product of the uncertainty in position and momentum

The Heisenberg uncertainty principle states that: $$\Delta x\Delta p_x\ge\frac12\hbar$$ But more generally for operators $A$ and $B$: $$\Delta A\Delta B=\frac12|\langle[A,B]\rangle|$$ However, $[x,p_x]...
RobChem's user avatar
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3 votes
1 answer
936 views

Solving two electron integral numerically

Is there any software package in python or C to evaluate two electron integral repulsion integral in quantum chemistry of atoms. I am trying to solve the helium atom by constructing many body basis....
user135580's user avatar
3 votes
5 answers
2k views

Why is the molecular structure of water bent?

Water has the formula $\ce{H2O}$ and we can draw a Lewis structure with two lone pairs on the central oxygen. As a physics student and not a chemist, I think to myself, "Okay, there are two lone pairs,...
BooleanDesigns's user avatar
1 vote
1 answer
382 views

How to determine the shape of hybridized atomic orbitals in VB theory?

From diagrams, it's rather obvious how $sp$ orbitals are hybridized - the hybrids are just a composite of the $s$ and the $\pm p_{(x)}$ orbitals. However, $sp^2$ orbitals are not just composites of $s,...
John Lou's user avatar
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1 vote
3 answers
403 views

I am trying to picture how electrons move around in atomic orbitals

Are they thought to continuously pop in and out of existence at various points inside the orbital defined by probabilities or do they follow definite paths that are made fuzzy by the Heisenberg ...
user2609404's user avatar
-1 votes
1 answer
1k views

Potential energy of electron in excited hydrogen atom

The angular momentum of electron in an excited H atom is $\frac{h}{\pi}$. The potential energy (PE) of electron is? Let $\frac{h}{\pi}=\frac{nh}{2\pi}$, therefore $n=2$. So, $$E=\frac{-13.6}{4}=\pu{-...
Aditya 's user avatar
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-2 votes
2 answers
1k views

What is the magnetic quantum number -1 vs +1?

In my book it states the magnetic quantum number “$m_\ell$" is the spatial orientation of the orbital with respect to a standard set of coordinates. I get what $n$ and $\ell$ is, but what does it mean ...
user528911's user avatar
33 votes
2 answers
8k views

What is the exchange interaction?

As the wikipedia article for the exchange interaction so aptly notes, exchange "has no classical analogue." How wonderful. Exchange shows up essentially while enforcing the condition that two ...
jheindel's user avatar
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28 votes
1 answer
3k views

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/...
ARaf's user avatar
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26 votes
1 answer
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Canonical MOs vs. Localized MOs: Do both represent reality in the same way?

In my understanding localized molecular orbitals (LMOs) are equivalent to "standard" molecular orbitals, often called canonical orbitals (CMOs—by the way, why are they called canonical?). We can ...
DSVA's user avatar
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