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 Car Parrinello Molecular Dynamics doesn't need an SCF calculation at every time-step?
Reading the various terms of the Lagrangian in CP-MD I'm trying to understand why there is not an SCF procedure hidden there.
In fact, it seems to me like the second term of the Eulero-Lagrange ...
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Why can we see color during d-d transition of electron? [duplicate]
As we know that when a ligand approaches the transition metals, the d-orbital loses its degeneracy and in this case electron can jump from lower orbitals to higher orbitals adsorbing the visible light....
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Why do we only use the principal quantum number to calculate the energy of an electron in an H atom? [duplicate]
According to my high school textbook the energy of an electron in a hydrogen atom is $E = -13.6/n^2~\pu{eV}$, where $n$ is the principal quantum number of the orbital in which the electron is present.
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Gaussian Program Error for Linear Triatomic Molecule
I am trying to calculate the Hessian and vibrational frequencies of OCS (and eventually some anharmonic properties) with Gaussian09. However, the program always fails and I end up with the following ...
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Why d orbitals oriented along $z^2$ axis assigned zero magnetic quantum number?
I have been studying quantum mechanics recently and I have come across this:
d orbitals oriented along the $z^2$ axis is assigned zero magnetic quantum number
I am wondering why specifically the $dz^...
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Why we seldon use Fermi Surfaces to explain chemical behaviour? [closed]
Chemistry basically involves studying the behavior of electrons in atoms and their consequences. Solid-state physicists deeply study Fermi surfaces, analyzing the three-dimensional behavior of atoms. ...
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Is the $2p$ RPD defined along the z-axis?
In MIT $5.111$ class (lecture $6$), we are to compare the most probable radius of a $2s$ orbital and a $2p$ orbital. The graph used by the professor represents the RPD versus $r$:
My question is: in ...
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Doubly occupied orbitals in quantum chemistry? (openfermion.org)
I am interested in diagonalising the fermionic Hamiltonian of small molecules, such as LiH in second quantisation.
The Hamiltonian is
\begin{align}
H(R) = \sum_{ij} h_{ij}(R) a_i^\dagger a_j + \frac{1}...
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Finding the relative position of the molecule in the adsorbate and adsorbent by minimizing the Hamiltonian
I am trying to model an adsorbing process (or more precisely, Heterogeneous catalysis process between solid catalyst and gas reactants). One of the step is to minimize the Hamiltonian by optimizing ...
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Do Electron-Withdrawing-Groups give an hypsochromic shift or a bathochromic one? and why?
I'm not familiar with this topic,but i know that bathochromic shifts occur when the energy gap between energy levels of the substituted molecule, are lower than the unsubstituted one;viceversa ...
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Absurd result for the selection rules in the three-dimensional PIB
By using time-dependent perturbation theory and some simplifying assumptions, we know that a transition for a charged particle $q$ from a quantum-mechanical state $m$ to $n$ is allowed if and only if
\...
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In the RDF plot of a particular orbital, why don't peaks come with radius greater than that of the max peak?
While studying radial distribution function (RDF) plots I see that orbitals (3s, 2s, ... 3p etc) often have peaks at radii less than that of the maximum peak. Assuming the probability of finding an e-...
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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-...
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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'}...
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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 ...
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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:...
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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 ...
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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 ...
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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(...
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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)...
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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. ...
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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 ...
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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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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 ...
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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 '...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 ...
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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 $|\...
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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 ...
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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 ...
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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 ...
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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 ...
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Why does graphite have a higher standard molar entropy than diamond?
It is known that graphite (5.7 J/K mol) has a higher standard molar entropy than diamond (2.4 J/K mol) (data from Physical Chemistry by Atkins). I understand that this must be due to the structural ...
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How to compute solvent reorganization energy in Marcus' non-equilibrium approach?
I try to solve the following equations for some system solvated in the water. The goal is to obtain value in kcal/mol. Unfortunately, I cannot reach the desired output. May someone help me?
The ...
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How to display atomic charges from MOPAC calculation?
MOPAC (http://openmopac.net/) is nice semiempirical code. It also calculates charges on atoms.
Is there a free GUI to display them please ? What I know, Avogadro diplays vibrations, JMol orbitals......
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Determination of ground state energy that has the same L and S values
Determination of the energy of a multielectron atom in the ground state can be estimated using the atomic term symbol which involves the values L, S and J. If an atom has the same L and S values ...
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Can geometry of a molecule change when the spin-orbit coupling is taken into account? [closed]
Cross-posted on MMSE.
Is there any evidence that spin-orbit coupling (SOC) leads to significant changes in atomic structure? Using DFT, I am getting different relaxed lattice constants when I take SOC ...
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How to perform NBO calculations at each point while scanning the bond length?
For a given bond, I would like to perform an NBO calculation for each structure while scanning the bond lengths.
Can I use Opt=ModRedundant in combination with pop=nbo? Simply specifying both ...
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How to calculate the ground state molecular potential curve of Lithium in different spin configuration?
I am a physics student and interested in Atom, and molecule optical physics. I am studying Feshbach resonance and I need to know the potential curve of the molecule, then I ask question in physics ...
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Critical point from ab initio calculations
I was wondering if it is possible to calculate the critical point (e.g., temperature, pressure, and density) other than via experiments or equations of state like ab initio calculations.
Would anyone ...
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Mulliken Labels: How do I tell apart E, doubly degenerate, symmetry apart from T, triply degenerate, symmetry for a molecular orbital?
With A and B, it's pretty simple. If you rotate the molecule about its principle axis to where the orbitals line up, and the signs change, it's Mulliken label has a B. otherwise, it's A.
I heard E ...
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Why is the statement regarding the strength of ionic bonds reasonable? [closed]
In largely ionic compounds (e.g. NaBr and NaI), it seems to be generally true that, the greater the differences of electronegativity between the forming substances (i.e. The compound has more "...
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Band structure of solids in general
As my last question (Semiconductors and their electronic bands) was badly structured, I decided to elaborate my questions a bit.
As I now know, every solid/liquid forms a band structure, so all ...
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What can be inferred from overlap matrix S if coefficient matrix C is unitary in Roothaan equation?
I was going through the Hartree-Fock algorithm, transforming the spatial molecular orbitals {$\psi _i$} to be a linear combination of Roothaan basis set {$\phi_i$}. From my knowledge {$\psi_i$} is set ...
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Effective Core Potentials and their "corresponding Pople" basis set
Reading through a handout from Yale shows the author correlating the ECP with Pople basis sets. The author of this says the corresponding basis set for SDD is 6-31G, Def2TZV as 6-311G, LANL2MB as STO-...
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Why is cyclohexane more polarizable than benzene?
Looking at figure 4.2 here, we can see experimentaly that cyclohexane is more polarizable than benzene.
Why is this the case?
Intuitively, I would think that because the orbitals in benzene are more ...
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