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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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2answers
739 views

Is there any difference between a completely filled orbital and an half-filled one?

Is there any reduction in size of the orbital for a half-filled orbital? Is the probability at any point of finding an electron doubled if there are two electrons instead of one? Is there any ...
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200 views

Why search new elements?

I've read this today. From the little I know, they discover new elements by bombarding some atoms in particle accelerators (I guess) and these new elements last a few miliseconds. So what's the ...
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Which computational method/basis set should I use for small molecular ions?

I apologize for a perhaps newbie question. I would like to compute the potential energy surface for several small molecular ions, like $\ce{H2+}$ and $\ce{H3+}$, using quantum chemistry software. ...
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1answer
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Special electron configurations in p block on periodic table?

Gold and Chromium have weird electron configurations, because electrons are differently divided : eg an electron jumps from a s-orbital to a d-orbital to have a half-filled s orbital and a fully/half ...
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2answers
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Have we ever created artificial metals?

I really don't know anything about chemistry, but I was wondering about the following vague question: has any substance been created that does not contain any metallic elements, but is metallic? In ...
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2answers
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How to find the number of valence electrons?

I need to understand the following: Considering an element Sulfur - S which has 16 electrons. How do we calculate the number of valence electrons of S? Please correct me if I am wrong:2+2+6+2+4=16. ...
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Wave function for particle in a infinite well located at -L and +L

The wave function for a particle in a infinite potential well located at $0$ and $L$ is $$\psi_n=\sqrt{\frac2L}\sin\frac{n\pi x}L$$ I want to find our wave function for the same situation where the ...
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2answers
203 views

Helium empirical wavefunctions

I know that analytic forms of the Helium wavefunctions are not known. However, are there empirical expressions for the low-lying states of Helium? I'd like to use them to calculate some transition ...
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1answer
347 views

CI without Hartree-Fock SCF

Usually textbooks on CI takes the SCF Hartree-Fock orbitals as a starting point. Can one use the configuration interaction method starting directly from the single-particle Slater determinants ...
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2answers
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Degeneracy of orbitals?

Why is that in an external magnetic field(uniform) the degeneracy of d,f orbitals is lost but the degeneracy of p orbitals remain intact if the main cause of losing degeneracy is the difference in ...
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2answers
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Approximation of electron density of a small molecule

Starting point for Quantum Theory of Atoms in Molecules (AIM) is the electron density $\rho(r)$. It could be determined experimentally (X-ray diffractions). I want to know how can we estimate them ab ...
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516 views

Did Mendeleev predict the existence of neutrinos?

According to this Wikipedia page, Mendeleev originally believed that the inert gases belonged in Group 0 (to the left of the alkali metals). Thus, helium would be placed in the second period to the ...
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5answers
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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 ...
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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.
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1answer
448 views

Database for molecular dipole transition moments?

I'm looking for the dipole transition matrix elements for electronic transitions between the ground and excited states of the carbon dioxide cation, $\text{CO}_2^+$, that is, objects like $$\langle\...
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1answer
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Quantum dot band gap energy equation

I have been looking into certain approximations for band gap energies in quantum dots and I found a paper [1]. However, I noticed that the equations which they obtain are not useful to the situation ...
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1answer
605 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 ...
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2answers
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Bonding and anti-bonding orbitals in the light of time-dependent Schrödinger equation?

In organic chemistry, people draw 2p orbitals like this: and then they explain how the orbitals combine to non-bonding (π*) or bonding (π) molecular orbitals, like this: depending on whether the ...
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1answer
829 views

Intrinsic Reaction Coordinate - What does 'Connectivity' really mean?

I am currently in the process of connecting transition state (TS) structures to other minima in order to determine a reaction pathway. Now I understand the basic concepts of this method. You start ...
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3answers
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Why are these molecular orbitals invalid for hexatriene?

Here are the MOs (more accurately, the breakdown of the MOs) for conjugated hexatriene: Now, when first asked to draw the MOs myself, I drew this one for $\psi_3$: It still has two nodes, and is ...
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2answers
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Why are there quantised energy levels in the vibrational energy of a molecule?

Why are the energy levels for vibrational energy in a molecule discrete as opposed to continuous? I don't understand how a vibration can't have continuous amounts of energy that depend on the ...
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3answers
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Why can an s orbital only hold two electrons?

According to Pauli’s exclusion principle, an $s$ orbital contains at most two electrons with the opposite spin (up and down). Why can't an $s$ orbital contain a third electron whose state is the ...
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1answer
479 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?
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Can quantum entanglement affect the chemistry of molecules?

I recently heard a talk from a physicist about experiments on simple two-atom molecules in vacuum that showed that certain behaviour of the molecules was explained by quantum entanglement. Now this ...
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1answer
1k views

Naming of quantum chemistry basis sets

For some DFT calculations I performed with localized (Gaussian) basis sets, I wonder how the basis sets I used are properly named. The names I could gather are: H, C and O: 6-311G(d,p) Al: 8-511G* ...
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387 views

Energy of the n-th level for an atom

Going through the Bohr's model and his assumptions, I came across with this formula to find the energy of the n-th level of any atom: $$E = - \frac{Z k_e e^2}{2r_n} = -\frac{Z^2(k_e e^2)^2m_e}{2\...
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1answer
507 views

Does this set of quantum numbers have a unique solution?

I received the following question: Complete the missing values for the four quantum numbers: $n=?$, $\ell = 2 , m_\ell = 0$ , $m_s= ? $ . The problem is that I think $n$ can be any number ...
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1answer
236 views

What is the 'ancestral term' in the LS coupling scheme?

On the physics.nist.gov site: http://physics.nist.gov/Pubs/AtSpec/node06.html The table titled "Atomic structural hierarchy in LS coupling and names for the groups of all transitions between ...
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465 views

multielectron spin-orbit coupling and energy level

For multi-electron atoms for which the Hamiltonian (including the spin-orbit coupling) reads $$H=\sum_i T_i-Z\sum_i V_i+\sum_i V_i^{\text{s.o.}}+\sum_{i>j}V_{ij}$$ The $T_i$ are the kinetic ...
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1answer
3k views

Moment of inertia for a triatomic molecule

What does the moment of inertia for a molecule mean? How is it useful? Most importantly, how would I go about solving it for a triatomic molecule?
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1answer
210 views

Modeling perturbations of a quantum mechanical system

I'm wondering how to properly perturb a quantum mechanical system. I'm looking for a way to express the return to equilibrium of such a perturbed state, where the perturbation is in the Hamiltonian. ...
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1answer
498 views

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

In a series of papers in the early 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 ...
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1answer
119 views

Has QTPIE ever been applied to 1D metals such as polyacetylene?

QTPIE is a fluctuating charge model that substantially improves modeling of polarization and charge transfer. I was curious whether it has ever been applied to 1D metals such as polyacetylene to ...
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3answers
2k 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 ...
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3answers
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 ...
21
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3answers
936 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 ...
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7answers
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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 ...

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