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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168 views

Molecular dynamics (Mac OSX user): what software can I use to plot RDFs for NPT simulations? VMD only works for NVT (constant volume)

I would be very grateful if someone could recommend a way to plot RDFs for NPT ensemble molecular dynamics simulation results. I use CP2K to run NVT simulations and usually use VMD to process ...
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117 views

Interaction of the oscillating magnetic field of electromagnetic radiation with a molecule or atom

Whenever we read rotational, vibrational and electronic spectroscopy, we find details on the interaction of the electric dipole moment of the molecule with the oscillating electric field of ...
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1answer
230 views

Molpro - Connection between atomic term symbols and system symmetry

Let's say I have a carbon atom with the electron configuration $1s^22s^22p^2$. We know, that using atomic term symbols we can describe the following states: $${}^3P_0, {}^3P_1, {}^3P_2, {}^1D_2, {}^...
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112 views

Wave-particle duality in electron [closed]

Electrons show wave-particle duality Particle (atom) containing a wave (electron) on Quora Atoms contain electrons. We consider atoms as particles but electrons which are inside the atom as waves (...
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1answer
883 views

Why do the 4f and 5d orbitals have similar energies?

I've come to know this phenomena when learning about the exceptions to the aufbau principle. But there is no explanation of why 4f and 5d have similar energies (as I've read in some websites).
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723 views

How does the screening effect work for orbital in the same shell?

Energy of an orbital in a single electron system depends solely on n, while energy of orbital in multi electron system depends on n and l, why is that? I understand that the screening effect is at ...
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49 views

Is it possible to define the coordinates of an atom using other coordinates while optimizing in gaussian 16?

I know it is possible to fix the value of composite expressions using generalized internal coordinates (gic) in gaussian 16. But is it possible to assign that value to the coordinate of another atom. ...
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1answer
90 views

Does the probability function for a molecule to have energy in a certain mode depend on the minimum required quantum energy level of that mode?

I understand that the probability density for a particular velocity v+dv in one dimension for a molecule in a system is: $$f(v) = \sqrt{\frac{m}{2\pi k_B T}} \cdot e^{\frac{-mv^2}{2k_B T}}$$ I'm ...
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1answer
458 views

Radial probability for orbitals [closed]

Questions Question F-2: I think question f1 requires one to relate $\mathrm{\psi^2}$ or. Probability density with radius but I am just in high school and have not learnt about Hamilton ...
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1answer
458 views

Do atoms in a molecule actually keep rotating freely in case they have sigma bond? [duplicate]

While reading about cis-trans isomers I found the concept of free rotation of atoms.it says 2 atoms connected by sigma bond do not have trans sis isomer because of free rotation.while in case of pi ...
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149 views

Balmer's formula and lagurre polynomials [closed]

How can you explain the Balmer's formula using the properties of Laguerre polynomials? It's a homework question but I really don't get it. 1.What is the property of the Lagurre polynomials in the ...
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204 views

PIB wave functions and the effects of e- delocalization on UV absorption in conjugated systems

I'll try to keep this short and concise. What made me start thinking about this question was the material we have been covering regarding PIB wave functions in my quantum chemistry class, and material ...
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1k views

Why does greater orbital overlap mean a stronger bond?

According to valence bond theory, orbital overlap produces a bond. However, I don’t understand why having greater orbital overlap renders a bond stronger. It’s intuitive, I suppose, but I haven’t been ...
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487 views

Does a wavefunction have to be an eigenfunction of an operator in order to obtain an expectation value?

I am reviewing an intro quantum chemistry book and I came across the line "If $\hat{A}$ corresponds to a physical property of the system, whether or not $\psi$ is an eigenfunction of $\hat{A}$, the ...
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481 views

Why are wavefunctions with same principal quantum number and different azimuthal quantum number orthogonal

The value turns out to be zero . This is understandable since they are completely independent of each other and also perpendicular in real space. But also turns out to be zero . How do I justify the ...
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1answer
87 views

Ignoring spin, consider an electron in a hydrogen 2p orbital, what is its orbital magnetic moment?

I know that a magnetic dipole moment is given by $$\mu=\frac {-e}{2m}I$$ and that the z component of angular momentum is $$m_j\hbar.$$ However, I have also seen that angular momentum $I$ is given by $$...
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112 views

Conical intersections - phase

Can anyone explain why at a conical intersection the wavefunction changes sign? My understanding is that it is a test to see if the crossing IS indeed a conical intersection or just a coincidence of ...
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29 views

Maximum number of Spectral Lines “A better quantum model shows that there will be n^2 transitions”?

I was parsing the following post What is the maximum number of emission lines when the excited electron of a H atom in n = 6 drops to ground state? and came across with the reply from @porphyrin. cite ...
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32 views

Physical interpreation of coulomb and exchange integral

When trying to solve the Schrodinger equation for the electronic hamiltonian: $$H_{el} = \sum_{i=1}^{N} \bigg( - \frac{1}{2}\nabla_i^2 - \sum_A \frac{Z}{r_{i_A}} \bigg) + \sum_{i>j=1}^{N}\frac{1}{...
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20 views

How to retrieve excited state molecular orbital coefficients in Gaussian?

Currently I am doing TD-DFT calculations in Gaussian and want to calculate transition dipole moment for $S_1$ transition manually. However, I do not manage to retrieve molecular orbital coefficients ...
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28 views

Angle made by orbital with y-axis

What is the hybridization and angle (in degrees) with respect to the y-axis for the following hybrid orbital $ψ=\frac{1}{√3}c×(s-\frac{1}{√3}×p_x+\frac{√3}{√2}×p_y)$ The answer given is 30 degrees but ...
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ORCA: meaning of numbers in ORCA.out of MOLECULAR ORBITALS for STO-3G Lithium

For Lithium STO-3G basis set ...
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32 views

Direction of valence bond and Unsöld's theorem

Is it true that the atomic orbitals of separate atoms already have directionality, which ensures the directionality of the valence bond? Or another words, what is the real spatial distribution of the ...
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35 views

What common observations in chemistry were unexplainable by an early twentieth century chemistry who had no knowledge of relativity

I was reading quite a well-known paper titled 'Relativistic Effects in Structural Chemistry' by PEKKA PYYKKO, and in it, he states that 'it was not until the 1970s that the full relevance of ...
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86 views

What causes little spikes in a flame? [closed]

I have a lighter at home which everytime I use it, generates little spikes (see picture below) which eventually disappear. The Spikes are not an effect generated by the camera. I can confirm that ...
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26 views

How to calculate core electron SCF Fock Matrix Elements?

I have searched everywhere but there are no worked examples of how to calculate the $\langle\chi_i |\hat{h}| \chi_j \rangle $ values for the Fock Operator using the SCF HF Roothaan method. I am given ...
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65 views

Do different eigenstates of total angular momentum have necessarily different energies?

Let $H$ be the Hamiltonian of a specific atom and $J$ the total angular momentum. Since $H$ and $J$ commute, they have common eigenstate. So we can label the atomic states by their energy and total ...
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64 views

Why does the Most Stable State of an Atom Tend to be One with Full s and p Subshells?

I'm new to posting on stack exchange, although I've read a lot of it before. This question seems like it might end up being marked as a duplicate, but I've looked through a lot of the similar ...
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220 views

Comparison between Molecular Orbital Theory and Valence Bond Theory

I have been reading about introductory Molecular Orbital Theory lately. Till now, I have used Valence Bond Theory to evaluate bonding. I have a few questions about it:- In VBT, we used the idea that ...
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54 views

Exchange energy

The half filled and full filled configuration is considered to be most the stable among the other configurations. But d7 or d8 configuration has a greater exchange energy mathematically. So why isn't ...
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35 views

spatial degeneracy in the states of atoms and molecules

I would like to know if the exact ground state of a closed-shell atom or molecule can have a spatial degeneracy. If we ignore relativistics effects, the Hamiltonian of the system $H$ commutes with the ...
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25 views

Approximate Hamiltonian in variational method (Example of Helium atom)

As one know, the exact Helium Hamiltonian \begin{equation} \hat H = \left( -\frac{\hbar}{2m_e}\Delta_1 - \frac{Ze^2}{r_1} \right) + \left( -\frac{\hbar}{2m_e}\Delta_2 ...
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63 views

How to plot the wave function after a Hartree-Fock calculation?

I have written a code in Mathematica and have obtained the final energy of the hydrogen molecule. I have used Gaussian basis functions for this calculation. Now I need to plot the ground state wave ...
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225 views

How do you optimize an organo-metalic compound containing cerium in in Gaussian?

I am trying to perform optimisation of the molecule attached. I need to get partial charges and frequencies. The issue is that it doesn't converge with following input: ...
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48 views

Would the 1st excited state of Li2 be vibrationally excited?

I'm quite torn about an interesting past paper question that's come up during my revision, shown below. On the one hand, you're promoting an electron in a bonding orbital. In general, since this ...
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52 views

Wave function of a harmonic oscillator

In a question it is given that consider $H-X$ chemical bond and find $H-X$ bond distance for which there is zero probability density of finding the proton. It is easy to solve we have just to ...
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34 views

Number of nodes in Hartree-Fock solution

The Hartree-Fock equation for atoms is of the form $\left(\frac{d}{dr^2}+f(r)-\epsilon\right)P(r)=g(r) \tag1$ Usually algorithms to solve this equation assumes that the number of nodes of $P(r)$, ...
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55 views

Why does ψ^2 ( square of orbital wave function ψ) give the probability of an electron in a given region? [duplicate]

Today I was introduced to the Orbital Wave Function for electrons. $\Psi$ is a mathematical function for coordinate of electrons and has no physical meaning. But $\Psi^2$ gives probability of an ...
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37 views

How to assign initial and final states of iodine transitions in its spectrum?

In an experiment, I gently heated iodine crystals to create vapors and then observed it absorption spectrum with a spectrometer. The temperatures throughout were moderate (maximum $40^\circ\text{C}$). ...
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1answer
410 views

Hartree-Fock vs. DFT vs. Hohenberg-Kohn vs. Kohn-Sham

What is the difference between these four approaches? My current understanding is that Hartree-Fock uses only 1 slater-determine (accounting for antisymmetry) but neglects correlation between ...
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30 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 ...
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38 views

Stabilizing donor-acceptor interaction between orthogonal orbitals in the SN2 transition state?

For the $\mathrm{S_N2}$ reaction between chloroacetone and iodide, my professor has drawn a donor-acceptor interaction between the iodine lone pair attacking the Cl-substituted carbon and the π ...
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42 views

Force on an electron in a given orbital [closed]

I was wondering whether there is any quantum mechanical equation to describe the force on a given electron in a given orbital of an atom (lets say, for the simplest case, for hydrogen atom). Also if ...
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50 views

Calculating the Molecular Orbitals of a Molecular State [closed]

I am performing ab initio calculations on a heteronuclear diatomic compound using the MRCI method on the MOLPRO quantum chemistry package. I obtained the molecular states of the compound but I was ...
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1answer
146 views

Why does the Heisenberg uncertainty principle work in an atom? [closed]

Where is my logic wrong? An electron can only assume quantized energies in an atom. If an electron is localized in space in a hydrogen atom, its radius is known. If its radius is known, its ...
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0answers
18 views

What is mass-independent isotope separation/fractionation?

How can any separation/fractionation of isotopes be mass-independent, if the only difference between two isotopes is mass? The only clue(s) on Wikipedia mention molecular symmetry, 'forbidden' spin-...
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29 views

Properties that probe electron kinetic energy

This post is inspired by a question regarding the meaning of off-diagonal elements of the KE matrix (in some AO basis). One answer suggests that a diagonalized KE matrix might not be very useful. I ...
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37 views

Why is the iron inter-system crossing non-negligible at high temperatures?

Inter-system crossing (ISC) is a radiationless process involving forbidden transitions between singlet and triplet state. These are more common with heavy atom molecules, so I understand that ISC is ...
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45 views

Which DFT functional is best for predicting macroscopic properties of molecular liquids?

Which DFT functional (KS-DFT, to be more specific) performs best at predicting macroscopic properties (density, surface tension, boiling point, relative permittivity etc.) of molecular liquids (...
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32 views

Applying the constant number of electrons in DFT density optimisation

(Previous related question: Finding mathematically the ground state density in DFT) I am studying the density optimisation procedure (in particular for Orbital-Free DFT) this thesis. The derivative ...

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