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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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What can visual observations of pi-bonding MOs explain about LCAO expansion coefficients?

For examples of CO and N2, what can their MOs tell us about their expansion coefficients? I've thought that in N2, its symmetric so the expansion coefficients would be equal from the pi-bonding MO due ...
Audrix's user avatar
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Why is solubility affected by temperature? [closed]

Why when we raise temperature generally a bigger concentration of a soluble gets dissolved in a inorganic solvent?Is there a quantum mechanical reason why this is happening?
Volpina's user avatar
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What does the χ (chi) symbol represent in a Slater determinant? [closed]

Context: Computational Chemistry So, I am looking at the molecular orbital approximation, which allows the assumption that the overall wavefunction can be written as individual molecular orbitals Ψ(r1,...
PassionFruitChemistry's user avatar
1 vote
2 answers
305 views

Molecular orbital picture in conjugated molecules - double bonds

I am a not a chemistry student but a physics student. Nevertheless, I am quite familiar with molecular orbital theory and similar quantum chemistry concepts. However, I have problems understanding the ...
Lockhart 's user avatar
2 votes
0 answers
225 views

What is the origin of the spin selection rule?

Why is the change in total spin ∆S = 0 during an absorption of a photon, in contrast to the ∆L = ±1, where the photon transfers its angular momentum? I am aware that spin-orbit coupling breaks this, I ...
Furrier Transform's user avatar
4 votes
3 answers
130 views

Choice of Basis for solution space of a Hamiltonian

I'm quite new to the field of Quantum chemistry, but I can't wrap my head around the following postulate of Quantum Chemistry, stated as Postulate 3 in [1]: Postulate 3: In any measurement of the ...
student7481's user avatar
2 votes
0 answers
79 views

How to determine the unitary transformation from atomic orbitals to symmetry-adapted atomic orbitals of Ethylene

I am trying to find the unitary transformation matrix U from the atomic orbitals $|\mathbf{\chi}\rangle$ of ethylene to the symmetry-adapted atomic orbitals $|\mathbf{\chi}'\rangle$ of Ethylene, i.e $|...
curiousrabbit13's user avatar
1 vote
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Does a free radical localized at one atom in a compound make sense or could it be?

In most of our cases, a dot that represents a radical is always considered affiliated to an atom in a molecule, especially when we talk about a reaction mechanism. However, Levine's Quantum Chemistry [...
Seonsam's user avatar
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How does an electrons's wave function change when it moves between energy levels?

I'm taking a class on QM and we're simulating the wave function of an electron in a box at the lowest energy level and I'm supposed to change the simulation to show the wave function for the next ...
Mikayla Eckel Cifrese's user avatar
1 vote
1 answer
248 views

In which cases do we use the the reduced mass formula?

Edit: Karsten added a description of reduced mass: "It is a quantity which allows the two-body problem to be solved as if it were a one-body problem." I do not really have a strong Physics ...
Galen N. P.'s user avatar
5 votes
3 answers
229 views

Why is C a matrix and not a vector in Roothaan Equations?

I'm trying to figure out why $C$ is a matrix and not a vector in the Roothaan Equations. Below, I have a derivation which I have taken from this PDF in which, to me, $C$ appears to be a vector. But ...
Frank's user avatar
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Why must the beta term in the rigid-rotator PDE take the form J(J+1)? [duplicate]

Here is the partial differential equation for which the solutions Y are are the wave functions for the rigid-rotator (Equation 5.5 in McQuarrie and Simon's Physical Chemistry). $$\sin\theta\frac{\...
Eli Jones's user avatar
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361 views

Energy of a molecule using MOs diagram (case of CH4)

I'm trying to find the ground state energy of CH4 in order to compare it with simulations I made. I came across this diagram of energy (not sure it gives the true energies though, if you could confirm....
QMLSorbonne's user avatar
4 votes
1 answer
2k views

Term Symbol for Carbon

Currently studying how to "compute" term symbols. My book gives the example of carbon. Carbon has the electron configuration $(1s)^2(2s)^2(2p)^2$. We can ignore full orbitals, so we only ...
xotix's user avatar
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Why do nuclei move considerably slower than electrons

I've been trying to learn quantum chemistry at an introductionary level. While reading I've found out that the Born-Oppenheimer approximation seems to be the reason for the basic and crucial model of ...
Atsjo's user avatar
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4 votes
1 answer
705 views

Length of a 1D box in hexa-1,3,5-triene

Problem From Hayward's Quantum Mechanics for Chemists [1, p. 36] 2.3. Calculate the wavelength of light that will be absorbed when a it electron in hexa-1,3,5-triene, $\ce{CH2=CH—CH=CH—CH=CH2},$ is ...
Karina Maria Piotrowska's user avatar
1 vote
0 answers
52 views

Spin-preserving excitations

I'm currently working on quantum chemistry with quantum machine learning and one of the operations considered for the calculation is the excitation (and desexcitation) of an electron from an orbital ...
QMLSorbonne's user avatar
6 votes
3 answers
530 views

Can one calculate the polarizability of Xe isotopes using Gaussian 09 and the 3-21G basis?

While trying to understand this paper, I read the following method for calculating the polarizability of different Xe isotopes: Quantum chemical calculation of the exact polarizability of each xenon ...
Ryan's user avatar
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-2 votes
1 answer
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If you give an electron in a subshell more energy, does it simply "jump" to the next energy level?

Suppose you have an electron in the $\ce{2s}$ subshell of an atom. If energy is given to it, does it simply jump to the next energy level (into the $\ce{3s}$ subshell), or does it move into $\ce{2p}$?
Shane's user avatar
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2 votes
1 answer
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Is there a difference between Hartree-Fock method and LCAO?

I have to dive in some quantum chemistry for a quantum machine learning project and I came across the so-called Hartree-Fock method. In one of the reference I used, they considered electrons as ...
QMLSorbonne's user avatar
-3 votes
1 answer
106 views

Can it be said that an electron transfer between two neutral atoms comes from the neutral atomic electric field? [closed]

Let us consider the example of Na and Cl. Both atoms are called "neutral" simply because they have the same total number of protons as electrons, yet they have a very weak electric field ...
iwab's user avatar
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1 vote
1 answer
145 views

How do we know the orbitals of multielectron atoms look like the orbitals of hydrogen?

How do we know that the solutions of the time-independent Schrodinger equation for multielectron atoms are the same with the solutions for the hydrogen atom if we add a effective potential due to the ...
Volpina's user avatar
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3 votes
0 answers
148 views

Derivation of the Hartree-Fock equation

I have to do some tutoring about basic quantum chemistry and the main purpose is to give a deep insight into Hartree-Fock theory. My plan is to start from expressing the many-electron wavefunction as ...
C_Swann22's user avatar
  • 133
1 vote
1 answer
174 views

Show that if H is hermitian, then <Ψ|H|Φ> = <HΨ|Φ>

Since H is hermitian, it is equal to its complex conjugate (shown below), a property which I used to do the proof: I had to prove that <Ψ|H|Φ> = <HΨ|Φ>, which I did as below: My question ...
Bartholomew696969's user avatar
7 votes
2 answers
100 views

Point group of harmonic oscillator

The book Molecular Quantum Mechanics by Atkins and Friedman [1] says the point group of a harmonic oscillator is $C_\mathrm{s},$ composed by the identity operator $E$ and a reflection $\sigma_\mathrm{...
AlfredV's user avatar
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1 vote
0 answers
57 views

How can one obtain the same-spin and opposite-spin terms of the MP2 energy by integrating out the spin?

Just as specified on the psi4 website here: https://psicode.org/psi4manual/master/dfmp2.html, the Moller-Plesset Perturbation Theory states that the second order correction to the ground state energy, ...
Myxo123's user avatar
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1 answer
276 views

Would a hypothetical Og2 +235 form a chemical bond?

In a hypothetical (?) Og2+235 we would have a simple sigma bonding orbital occupied by one electron (leading to a bond order of 1/2). But how to take into account the giant repulsion of the two nuclei?...
user avatar
3 votes
4 answers
317 views

Why don't we consider quantum effects when thinking about reactions and their mechanisms?

When we're reasoning about chemical reactions and their mechanisms (in organic chemistry in particular), the way we model the behaviour of molecules is almost in a “common sense” way. In terms of ...
xasthor's user avatar
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-2 votes
1 answer
370 views

Average molecular distance between hydrogen gas [closed]

The whole question goes like this: Calculate the de Broglie wavelength of hydrogen molecule at 300K (Take the translational energy to be 1.5kT) Compare this to the average intermolecular spacing for ...
Sj2704's user avatar
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3 votes
0 answers
48 views

Computing molecular integrales in STO-3G basis with the Obara Saika recursion formulas

I'm trying to implement the Hartree-Fock algorithm on a water molecule using the STO-3G basis. This far I have only computed the overlap integrals with the Obara-Saika formulas but I'm running into ...
Marco Mendívil Carboni's user avatar
-2 votes
1 answer
395 views

How do I determine the hybridization of terminal atoms? [duplicate]

I've read that terminal atoms do not undergo hybridization because there is no need to. For example, CF4, C undergo sp hybridization, but fluorine do not hybridize. How about CO2? Why is oxygen sp2 ...
Bryan Foong Zhi Chuan's user avatar
1 vote
1 answer
254 views

Integral Prescreening in calculation of Electron Repulsion Integrals?

I am trying to write a basic Hartree-Fock program that can calculate the total energy and molecular orbitals of any molecule at arbitrary basis sets largely as a self-taught effort. I am able to ...
Hemanth Haridas's user avatar
-1 votes
1 answer
137 views

Which is faster? Electron close to the atom's nucleus or the one far from it and why? [closed]

As we know when the distance between the atom's nucleus and the electron increase the energy of it increase but what about the speed, as physics say that the smaller the radius is the faster the body ...
Hossny's user avatar
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1 vote
0 answers
57 views

Reproducing Hydrogen Molecule Hamiltonian in OpenFermion

I am learning quantum chemistry at the moment and I'm trying to understand the Hamiltonian generated by the OpenFermion package. I'm now stuck at understanding how openfermion calculates the ...
QF2QP's user avatar
  • 11
4 votes
1 answer
263 views

Self Consistent Field method and LCAO

I am reading about the Self Consistent Field Method and Linear Combination of Atomic Orbitals. Suppose we have one electron and one nucleus, then we can solve the Schrodinger equation explicitly. If ...
Iti's user avatar
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-1 votes
1 answer
28 views

How do I predict the spectrum when a magnetic field is applied on a specific transition from a term symbol to another? [closed]

More specifically the question pertains to physical/quantum chemistry, and to the transition 2D (with J = 3/2) -> 2P (J = 1/2) where the numbers before the letters are multiplicities (these are ...
Michael Zhang's user avatar
4 votes
0 answers
85 views

Notation for spectral density in experimental community

In an open quantum system setup where the system is coupled to infinite harmonic oscillators as bath (as in the Caldeira leggett model, for example), through the Hamiltonian, $$H = \frac{\hat{p}^2}{2m}...
Prem's user avatar
  • 91
0 votes
2 answers
116 views

Is the periodic table ordered by bonded atoms and does that change the electron configurations and ordering? [closed]

In my book (Mortimer, The Basic Knowledge of Chemistry) the electron structure of the elements is introduced, there the relative energies of the atomic orbitals are shown: 1s < 2s < 2p < 3s &...
iwab's user avatar
  • 383
2 votes
0 answers
74 views

Fermi Contact Interactions and resultant coupling contants

I am currently learning about spin-spin coupling mechanisms and have become very confused about the mechanisms by which spin-spin coupling happens in NMR. I understand how we can draw diagrams where ...
FFerreira's user avatar
1 vote
0 answers
27 views

How to obtain reliable ligand binding energy in QM manner?

I wish to compute the binding energy of the ligand to the known cavity of the receptor. My idea was to: Perform docking to the recognized binding site using AutoDock Vina Export bound ligands and ...
farmaceut's user avatar
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0 votes
0 answers
604 views

How do longer chains and more cis-double bonds affect melting point and viscosity of fatty acids?

In fatty acids, longer chains increase the melting point (and viscosity) by increasing London dispersion forces. Inversely, more double bonds decrease the melting point. What happens when both chain ...
George Ntoulos's user avatar
6 votes
2 answers
241 views

How is geometry optimization of small molecules different from the protein folding problem?

My layman's understanding is that finding the ground state geometry of small molecules is "hard". I don't have a good sense of how hard though. I suppose there are applications where methods ...
theQman's user avatar
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1 vote
1 answer
88 views

On the energies of molecular orbitals

I found a useful (though simplified) scheme illustrating the energies of atomic orbitals, in crescent order: I wonder if there was an analogous (simplified) scheme or rule for molecular orbitals, at ...
ric.san's user avatar
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0 votes
0 answers
47 views

Terms used to describe the Pauli exclusion principle

This is a very simple question. Today, someone explained to me that the Pauli exclusion principle says that "no two electrons can occupy the same energy state". However, I believe that this ...
user avatar
-2 votes
1 answer
124 views

How can Planck’s equation and the wave equation simultaneously coexist with contradicting each other? [closed]

The equations: E=hf (Planck’s equation, where E is the energy, h is the Planck’s constant and f is the frequency) and V=fλ (where v is the speed (in a vacuum), f is frequency and λ is the wavelength) ...
Kirandk's user avatar
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2 votes
0 answers
86 views

Hartree Fock calculations on F2 don't match the traditionally taught MO diagram

I ran some Hartree-Fock calculations on F2 to visualize the MOs, and the energy levels don't match the trend commonly taught (shown here: https://chem.libretexts.org/Courses/...
Brian's user avatar
  • 21
0 votes
1 answer
150 views

What is the difference between exciting an electron to a higher bound state and removing it completely (ionizing the atom)? [closed]

We are reviewing quantum mechanics and orbitals in my class and I'm confused about the energies to move an electron to an excited state vs. remove it from an atom. Even though the electron on a H atom ...
htho's user avatar
  • 11
-1 votes
2 answers
463 views

Why is the d subshell only stable when all of its orbitals are filled? [closed]

I was learning about the Aufbau principle and quantum electronic configuration. Here are some of the yet-unanswered queries that I wish to know the answers to. I discovered that the d subshell is ...
Chinmay Krishna's user avatar
1 vote
3 answers
175 views

Can two wavefunctions of 1s electrons in He atom have opposite phase?

Suppose we have a He atom. I understand that its wavefunction can be written as product of 2 hydrogenic orbitals: $$\Psi(r_1,r_2) = \psi(r_1)\psi(r_2),$$ which in this case, are two 1s wavefunctions ...
bzd's user avatar
  • 21
-2 votes
1 answer
92 views

Hund’s Rule - Please help explain below using Hunds rule? [closed]

Why is it easier to excite a 2s electron in a carbon atom to a 2p orbital than it would be to do the same in nitrogen?
Röss Lynch's user avatar

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