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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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DFT simulations of UV-VIS spectroscopy for Semiconducting Heterojunctions [migrated]

I have a course project that involves DFT. Does anyone know of any tutorials or resources to simulate UV-Vis spectroscopy of semiconducting heterojunctions using ORCA, Quantum Espresso, or any other ...
-1 votes
0 answers
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Hartree-Fock accurate for energy but not other observables [duplicate]

The Hartree-Fock approximation tries to minimize the energy, so in many cases the energy value it yields is reasonably accurate. But how about other observables? Is there any case such that the energy ...
2 votes
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49 views

Why is the translational motion left out of the energy eigenvalue in the hydrogen atom?

When solving the hydrogen atom, since we have two particles, the wave function has the form $\psi(\mathbf{q}_1,\mathbf{q}_2)$. Generally, textbooks proceed to demonstrate that an interacting two-...
4 votes
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Helium ground state with Hartree–Fock using angular dependent functions

To compute the helium ground state (singlet) in Hartree–Fock theory we have to solve this equation: $$ \left[ - \frac{\Delta}{2} - \frac{Z}{r} + \frac{1}{2} \int d^3 r' \frac{\vert \Psi(\boldsymbol{r'}...
3 votes
1 answer
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Why there are nodes in orbital?

I know that because Schrödinger equations solution for probability of that region is 0, but I was looking for an intuitive explanation. How can you intuitively explain the existence of nodes in an ...
-1 votes
1 answer
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How does an electron's distance from the nucleus depend on angular quantum number?

My school teacher told us this and I couldn't get his reason: The average distance of an electron (belonging to the same orbit) from the nucleus decreases with the increase in the value of angular ...
3 votes
2 answers
99 views

Why aren't there different peaks for different IE in PhotoElectron Spectroscopy (PES)?

The way I understand PES is that you shine a light on a bunch of atoms and measure the kinetic energy of photons to determine ionisation energies (or energies of electrons... is that different?). I ...
8 votes
4 answers
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When does Hartree-Fock fundamentally fail?

The Hartree-Fock approximation (HFA) works by assuming each electron sees the effective electric field from all the other electrons as some self-consistent field. The HFA is known to give pretty ...
3 votes
0 answers
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Integrating out spin in 2-electron energy function

I am just starting to learn Quantum Chemistry for my research. I am currently reading Modern Quantum Chemistry by Szabo and I am a little confused by a concept. The two-electron ground state energy is:...
5 votes
3 answers
382 views

Can orbitals in atoms be compressed?

Consider an atom (carbon) with a ground state valence electron configuration of: $$ \underset{2s}{[\uparrow \downarrow]} \underset{2p}{[\uparrow \vert \uparrow \vert \; \; ]} $$ In some molecules such ...
1 vote
1 answer
47 views

Frequency to induce nitrogen inversion

I would like to know how to compute the frequency at which nitrogen inversion happens in ammonia gas at STP. The activation energy for such process is 24.2 kJ/mol, therefore I thought that it is ...
1 vote
0 answers
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How to perform LC-BLYP calculation with GAMESS? [migrated]

I am currently working on time-dependent density functional theory (TDDFT) calculations for both singlet and triplet states using the LC-BLYP functional in GAMESS. I believe I have correctly followed ...
1 vote
1 answer
159 views

Wave function in Schrodinger's model

I am trying to wrap my head around the Schrodinger's quantum mechanical model of an atom. According to the NCERT$^1$, the Schrodinger's equation is given by: $$\hat{H}\Psi=E\Psi$$ where $\hat{H}$ is ...
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Why is the concept of penetration power only applied to Be and B, and not to Be and C, N, O, F?

In my book, it is stated that the ionization energy of Be is greater than that of B because the electron removed in Be is in the 2s orbital, while the electron removed in B is in the 2p orbital. This ...
0 votes
1 answer
50 views

Degeneracy of Orbitals

(Sorry, in advance if my question is silly or low quality but I want to ask this to someone.) When an electron enters an orbital, it should technically have an electric Field and also magnetic field(...
3 votes
3 answers
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Challenging Orbital Restrictions

According to the n-l-1 formula for nodal count, wouldn't 1p, 1d, and 1f orbitals be possible since l (angular momentum quantum number) can be 1, 2, and 3 respectively for a principal quantum number (n)...
25 votes
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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 ...
28 votes
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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 ...
3 votes
1 answer
156 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, ...
-4 votes
1 answer
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Stability of atomic/ionic quantum states in relation to the full octet rule

We have started our Chemistry Unit and the very first statement the teacher says is that "All Atoms want to be stable. They achieve this by having a full valence shell, which they achieve by ...
2 votes
2 answers
394 views

Why does aqeous fluorescein shine when exposed to UV light?

I know that (aq) fluorescein "absorbs" UV light and releases light in the visible spectrum , but how does it do that. It is a really interesting and useful organic chemical and I just wanted to know ...
0 votes
0 answers
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Why aren't other elements with valence 6s electrons liquid like mercury?

I found out that mercury's 6s electrons undergo relativistic effects that make their pull to the nucleus greater, which makes it so that they can't bond with other atoms of mercury to form a solid. ...
9 votes
1 answer
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Computing energy of Gaussian basis set

I'd like to compute the decay rate using $$ \Gamma = 2\pi \ | \langle ab|V|cd \rangle|^2 \delta\ (\epsilon_\mathrm{f}-\epsilon_\mathrm{i}), \tag{1}$$ where $a,$ $b,$ $c,$ $d$ are the MOs, $V$ is the ...
12 votes
1 answer
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How to choose the best method and basis set for a calculation in computational chemistry?

In Quantum chemical calculations, it is not always the case that the more complex (correlated) the method or the basis set the more accurate the results. For example, it's sometimes found that MP2 ...
12 votes
1 answer
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How to simulate chemical reactions in GAMESS?

I am rather a newb in the GAMESS field but finally, learned to make input files for most molecular simulations and could even use TD-DFT for excited states (special thanks to Geoff Hutchison for his ...
2 votes
1 answer
202 views

How do the orbitals in atoms synchronize their phases during the formation of a bond?

Suppose I have two hydrogen atoms, each with one electron in their 1s orbital. Then we can represent the wave functions of these two as say $|\Psi\rangle_1$ and $|\Psi\rangle_2$ for each of the two ...
1 vote
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Resonance stabilization : lowering of kinetic energy or electron repulsion?

Resonance stabilization, or delocalization stabilization, occurs when the overall energy of the molecule is lowered compared the localized "resonance structures". In solid state physics, or ...
4 votes
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The sign of the lobes in an atomic orbital

In atomic orbital diagram of an atomic orbital, the lobes are represented as either positive or negative. It is said that the wave function is 'positive' where the lobe is depicted positive and '...
1 vote
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How to find the global minima of a supramolecular complex in computational chemistry?

So this is more of a complex problem, and my masters thesis could greatly benefit if I find an answer to this. Essentially, I have a supramolecular inclusion complex. There is this molecule A (the ...
4 votes
1 answer
866 views

Angular momentum of an electron in an atom

The Orbital Angular momentum of an electron in an atom is given by $\hbar \sqrt{l(l+1)}$ and the Spin Angular Momentum is $\hbar \sqrt{s(s+1)}$. So what is the resultant Angular momentum of an ...
1 vote
1 answer
78 views

Does ionisation change the physical/chemical properties of an atom?

From what I have learned, the number of electrons on the outermost shell (called the valence electrons), determines the chemical properties of an atom, this is why elements in the same group have ...
13 votes
3 answers
12k views

Why is the $\mathrm d_{z^2}$ orbital so different from the rest?

$\mathrm d_{z^2}$ orbital appears to be special: Although degenerate with other d orbitals, it has no nodal planes, instead it has two nodal "cones". Instead of having four lobes, it has ...
13 votes
5 answers
2k views

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 ...
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What are some general chemistry textbook with good problems? [duplicate]

I’m currently in a General Chemistry course and am wondering if you guys can recommend me a textbook or resource where I can find good problems to work through and further my understanding. We’re ...
5 votes
0 answers
153 views

N2 HOMO-LUMO Gap calculation

I am thinking about calculating the HOMO-LUMO energy gap for $\ce{N2}$ molecule. I thought I could model this as a particle in a box problem to find $E_\text{HOMO}$ and $E_\text{LUMO}$. The formula I ...
2 votes
1 answer
89 views

UV-Vis absorbtion spectra of a radical cation/anion. Is it different compared to its even counterpart?

I am studying gas-phase Uv-Vis spectra of radical molecules. If one electron is removed/captured from the neutral even electron molecule, what can I expect from the absorption spectra? Will new peaks ...
0 votes
0 answers
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How does more s character shorten bonds and strengthen bonds?

I'm aware that having more s character means that electrons are in the lowest energy state (?) and very close to the positive atomic nucleus. But then what’s the connection between having electrons ...
9 votes
2 answers
384 views

Software to draw one-dimensional PES including vibrational energy levels

I am trying to represent the difference between the an anharmonic and harmonic treatment of vibrations. The easiest way to do this is to present an example of a harmonic and anharmonic potential ...
1 vote
4 answers
229 views

What is a degenerate orbital

I don't really get what degenerate orbitals are, I mean I know that orbitals are degenerate when they have the same energy level, and I know that the energy levels depend on n, but does that mean that ...
2 votes
1 answer
93 views

Grade 12 chemistry question about Schrodinger's standing wave?

this is my first time ever using a website like this but I have something I'm curious about. My chemistry teacher showed this today (img below) and discussed how quantum of energy that hits the ...
0 votes
0 answers
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Difference between HOMO and LUMO and corresponding spectral gap in Hamiltonian

Coming from a Quantum Computing background, I am familiar with the singular value definition of spectral gap, I am currently trying to run Quantum Chemistry simulations and I would like to know if $|\...
4 votes
3 answers
128 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 ...
14 votes
2 answers
2k views

Transforming AO basis electron repulsion integrals into molecular spin orbital basis?

I am attempting to do project 5 on Daniel Crawford's Group's GitHub. I'm having some trouble reproducing the MP2 energy, which is given in project 4. In project 4, the MP2 energy is given as: $$ E_{...
5 votes
1 answer
442 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?
0 votes
1 answer
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Quantum Mechanics expectation value help

I am working on the following question: I've so far done the first part of a, finding $H_{so}$ in terms of J L and S. I'm struggling to see how to relate that to $E_{LSJ}$ as it involves the ...
2 votes
1 answer
148 views

Mechanism of adhesion of particles on solid surfaces [closed]

What is the mechanism of adhesion of particles on solid surfaces? How does anything stick to anything, for that matter? Why does cigarette smoke stay so stubbornly on our bodies? Why does perfume also ...
5 votes
0 answers
423 views

What is the theoretical justification for state averaging in SA-CASSCF?

According to the molpro manual, section 7.3: In order to compute excited states it is usually best to optimize the energy average for all states under consideration. This avoids root-flipping ...
1 vote
0 answers
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Theoretic (QED) Atomic Ionization Energies Database?

The 2019 paper "QED tests with highly-charged ions" contains a survey of QED-calculated atomic ionization energies. I can manually copy the values of interest therein, but it seem that ...
4 votes
2 answers
537 views

What's the meaning of "wavefunction must be finite"?

In quantum mechanics it's important that the wavefunction has to be finite everywhere, including infinity. It's a criteria for acceptable wavefunction. What exactly does this statement mean? And how ...
30 votes
3 answers
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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....

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