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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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Why is the 2s orbital lower in energy than the 2p orbital when the electrons in 2s are usually farther from the nucleus?

My chemistry book explains that even though electrons in the $\mathrm{2p}$ orbital are closer to the nucleus on average, electrons from the $\mathrm{2s}$ orbital spend a very short time very close to ...
Gordon Gustafson's user avatar
56 votes
5 answers
10k views

How can antibonding orbitals be more antibonding than bonding orbitals are bonding?

In molecular orbital theory, the fact that a bonding and antibonding molecular orbital pair have different energies is accompanied by the fact that the energy by which the bonding is lowered is less ...
stochastic13's user avatar
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44 votes
3 answers
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What is the inert pair effect?

I was reading about the p-block elements and found that the inert pair effect is mentioned everywhere in this topic. However, the book does not explain it very well. So, what is the inert pair effect? ...
radiantshaw's user avatar
43 votes
2 answers
290k views

Difference between shells, subshells and orbitals

What are the definitions of these three things and how are they related? I've tried looking online but there is no concrete answer online for this question.
Amuna's user avatar
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37 votes
3 answers
6k views

Can hot food ever emit x-rays or gamma rays?

I was just wondering, if heating food up is the result of increasing the energy of bends and stretches in the bonds of the molecules, is it ever possible for tiny amounts of x-rays and gamma rays be ...
user314901's user avatar
36 votes
3 answers
3k views

Pauli exclusion principle and resonance

I've always been a bit uncomfortable with the concept of more than two electrons in a single orbital-like region(probability-wise) which occurs in resonance. This seems to disobey Pauli's exclusion ...
ManishEarth's user avatar
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36 votes
1 answer
4k views

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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35 votes
1 answer
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Acidity of substituted phenols

Phenol has a $\mathrm pK_\mathrm a$ approximately equal to $9.9$. When one studies the acidity of chlorophenols, one notices the following: First of all, chlorophenols are more acidic than phenol, ...
mannaia's user avatar
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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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33 votes
2 answers
16k views

Sp5 hybridization in cyclopropane?

I have never before heard/read about something as a $sp^5$ hybridization. Today, Henry Rzepa's blog post made me aware of the existance of such a bonding system. That made me search a little bit and I ...
pH13 - Yet another Philipp's user avatar
31 votes
3 answers
12k views

What is the physical basis for Hund's first rule?

According to Hund's first rule, a set of degenerate orbitals are singly occupied first, before the second slot in any of the orbitals are populated. This is quite intuitive because electron-electron ...
Aniansh's user avatar
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30 votes
3 answers
65k views

Why are covalent bonds directional?

It is said that covalent bonds are directional, while ionic bonds are not. Why? Is it because of the orientation/directional properties of the overlapping orbitals?
Abhirikshma's user avatar
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30 votes
3 answers
15k views

Why are noble gases stable

I was recently asked the question "Why are noble gases stable? with the expectation of providing an answer beyond the general explanation of "they have full valence layers" and I couldn't think of one....
Chris's user avatar
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30 votes
2 answers
42k views

Why do atoms "want" to have a full outer shell?

Okay, so I know that this is about filling the orbitals of the atom, and I understand that. What I don't understand is why? For example, an Oxygen atom has 8 protons and 8 electrons spinning around it....
Mertcan Ekiz's user avatar
30 votes
1 answer
633 views

Imaginary Bonding Interactions

Usually in chemistry, we deal with bonding interactions. That is, if I have the diatomic A-A molecule or A-B molecule, there's a favorable interaction (i.e., a bond) and a prototypical MO diagram like ...
Geoff Hutchison's user avatar
29 votes
3 answers
6k views

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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28 votes
2 answers
38k views

What do the quantum numbers actually signify?

I know how to calculate them and such stuff, but I wanted to know what they actually signify. I have a vague idea that they have something to do with an electron's position in an atom but what do all ...
Jyotinder Singh's user avatar
28 votes
4 answers
1k views

How do quantum software packages Work?

If one wants to calculate a moderate size Alkane (with say 10-15 carbons, assuming 100 electrons, with Restricted Hartree Fock based methods) we can simply say that the electron-electron part will be $...
Aug's user avatar
  • 2,141
28 votes
2 answers
5k views

How to find the second order perturbation to wave function?

Today, I'm looking for how to find the 2nd perturbation to the base in Rayleigh Schrödinger Perturbation Theory (RSPT). SETUP Starting from the 2nd order perturbation in Dirac's notation: \begin{...
Another.Chemist's user avatar
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
  • 525
28 votes
0 answers
2k views

How to calculate transition dipole moment from two known wavefunctions

I'm interested in calculating the transition dipole moment (TDM) from the information from two wavefunctions of different states. This is somewhat similar to calculating the molecular dipole moment ...
brose's user avatar
  • 322
27 votes
4 answers
8k views

Why don't equivalent hydrogens cause splitting in NMR?

When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum ...
Niels Kornerup's user avatar
27 votes
1 answer
40k views

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
  • 273
26 votes
5 answers
18k views

Physical intuition behind negative values for wave function?

So a positive and a positive wave function create a bonding orbital where the probability of finding an electron is summed while a positive and a negative create an anti-bonding orbital with a lower ...
norlesh's user avatar
  • 553
26 votes
2 answers
7k views

Why is chemical accuracy defined as 1 kcal/mol?

"Chemical accuracy" in computational chemistry, is commonly understood to be $1~\mathrm{kcal\over mol}$, or about $4~\mathrm{kJ\over mol}$. Spectroscopic accuracy is $1~\mathrm{kJ\over mol}$, and that ...
Samuel Tan's user avatar
26 votes
1 answer
5k views

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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25 votes
4 answers
39k views

Why does the energy gap for π - π* transitions shrink with the size of the pi-conjugated system?

Quoting from this site: As conjugated pi systems become larger, the energy gap for a π - π* transition becomes increasingly narrow, and the wavelength of light absorbed correspondingly becomes ...
user4717's user avatar
  • 487
25 votes
3 answers
2k views

What is charge shift bonding?

Can someone explain to me charge shift bonding? I recognize that it is proposed as another domain of bonding - one different from ionic and covalent bonding. I am also told that $\ce{C-F}$ exhibits ...
Dissenter's user avatar
  • 19k
25 votes
2 answers
8k views

What exactly is meant by 'multi-configurational' and 'multireference'?

Some topics here have touched on this before (see 1, 2, 3), but I haven't found a clear definition yet. I would like to know what exact property of the wave function these terms refer to. It would ...
snurden's user avatar
  • 1,459
25 votes
3 answers
2k views

Symmetry lost in orbitals?

I've always thought that orbitals lead to a loss of symmetry, and have never been able to give myself a satisfactory answer to this. I'll explain via an example: Let's take an $\ce{N^3+}$ atom. It's ...
ManishEarth's user avatar
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25 votes
0 answers
1k views

What is d-orbital collapse?

When I was perusing the works of Schwarz on atomic structure, I came across the unfamiliar term of d-orbital collapse. He describes it as a variation in energetic sequence from group 1 to 3 elements ...
C. WJ's user avatar
  • 251
23 votes
2 answers
25k views

Derivation of the Heisenberg uncertainty principle

The Heisenberg uncertainty principle states that $$\Delta x \Delta p \geq \frac{\hbar}{2}$$ where $\Delta x$ is the uncertainty in the position, $\Delta p$ is the uncertainty in linear momentum, and ...
orthocresol's user avatar
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23 votes
3 answers
14k views

Is Hydrogen Bonding a Type of Dipole Dipole Interaction?

I understand that dipole dipole forces is due to the attraction of the different partials charges of atoms in different molecules due to their different electro-negativities. For hydrogen bonding, ...
Nanoputian's user avatar
  • 7,412
23 votes
1 answer
9k views

What is behind of the attraction between a burned matchstick with a magnet?

I do not know how to understand the reasons of why a burned matchstick is attracted by a magnet (most probably) of Neodymium. Here are some screenshots of a video in which this natural phenomenon is ...
Another.Chemist's user avatar
22 votes
3 answers
20k views

When are two orbitals orthogonal?

When are two orbitals considered to be orthogonal? It will be helpful if you can provide an example molecule (or molecular orbitals in a molecule) since I can't really think of a scenario where ...
CPak's user avatar
  • 451
22 votes
2 answers
2k views

Why do n AOs only form n MOs? [duplicate]

The textbooks I have read introduce LCAO by considering the H2 molecule. In this example, there are only two possible combinations of the two 1s orbitals (in phase and out of phase). When there are ...
Adroit's user avatar
  • 780
22 votes
2 answers
31k views

Why is a singlet state called singlet and a triplet state called triplet?

I kind of get the idea of singlet and triplet states. But why are they called singlet and triplet (what is the single and what is the triple in these cases)? I feel that I am missing something ...
hat's user avatar
  • 427
22 votes
3 answers
8k views

What is a virtual state?

In talking about Raman spectroscopy, one finds the Stokes line is simply the difference between the energy of an incoming photon and an emitted photon. This energy corresponds to a vibrational ...
jheindel's user avatar
  • 13.1k
22 votes
3 answers
1k views

Non-integer hybridization

I've known that hybridization in distorted geometries is not exactly $sp^3$ or $sp^2$ or whatever. For example, $\ce{PH3}$ has nearly pure $p$ orbitals in the $\ce{P-H}$ bond, and the lone pair is in ...
ManishEarth's user avatar
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22 votes
1 answer
579 views

Is the Springborg 6D phase space model used in modern molecular orbital modeling?

In various papers written in the 1980s, Michael Springborg explored an interpretation of the Wigner phase space function as an electron density in a six-dimensional $(q,p)$ phase space. He applied it ...
Terry Bollinger's user avatar
22 votes
1 answer
565 views

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
22 votes
1 answer
393 views

f-electrons in chemistry of heavy transition metals

In one lecture at a recent MCR-X conference I was puzzled by a side-note "f-hole is critical to describe DoS of $\ce{IrO2}$ correctly." The context was DFT periodic plane-wave calculations ...
permeakra's user avatar
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22 votes
1 answer
428 views

Mechanistic Explanation of Circular Dichroism

I'm wondering if someone can give me a physically motivated reason for why chiral molecules interact with left and right circularly polarized light (LCP) (RCP) differently. Particularly, a quantum ...
jheindel's user avatar
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22 votes
1 answer
968 views

DFT Code for Atoms : Sources

Some time ago I implemented the restricted and unrestricted Hartree-Fock methods. I had a lot of fun and I learned a lot. It goes without saying that Szabo and Ostlund's book "Modern Quantum Chemistry"...
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
  • 68.3k
21 votes
2 answers
3k views

How to calculate wavenumbers of normal modes from the eigenvalues of the Cartesian force constant matrix?

I would like to get the wavenumbers (Frequencies -- in Gaussian) from the eigenvalues of the force constant matrix (...
Roloka's user avatar
  • 339
21 votes
2 answers
6k views

Jahn-Teller Distortions in Square Planar Complexes?

A Jahn-Teller distortion is predicted whenever a non-linear symmetric molecule has degenerate orbitals and has unequal electron occupation in those degenerate orbitals. Of course, this most often is ...
Geoff Hutchison's user avatar
21 votes
1 answer
1k views

What is occuring on the quantum level when a molecule rotates plane polarized light?

What is occuring on the quantum level when a molecule rotates plane polarized light? Also, why do enantiomers then rotate light in opposite directions? I would think that the electromagnetic waves ...
Joe's user avatar
  • 884
20 votes
3 answers
6k views

How to calculate molecular dipole moment from a known wavefunction?

Say I have a molecular wavefunction as a set of molecular orbitals and want to calculate the molecule's dipole moment, but don't know how! I searched a lot but couldn't find any practical example. $$\...
Aug's user avatar
  • 2,141
20 votes
1 answer
881 views

Is it possible to compute the colour of *any* molecule?

Above are some spectral emission lines for hydrogen, helium and neon. Using the Schroedinger equation, it's possible to derive the colours that hydrogen will emit when light is fired at it. A little ...
Meow's user avatar
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