Quantum chemistry is a subfield of quantum mechanics. Like the parent it focuses on physical phenomena that occur at an atomic scale, usually specifically aiming at chemical reactions.

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Meaning of natural orbitals occupation numbers [on hold]

What is the meaning of natural orbitals and occupation numbers? Example of output from Molcas: Natural orbitals and occupation numbers for root 1 sym 1: 1.961698 1.932462 1.972469 1....
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Why are correlation consistent basis sets used with DFT calculations?

Reading some papers, there is a bit of a curious observation that does not exactly make a lot of sense to me. In Quantum Chemistry, different methods exist to carry out geometry optimisation and ...
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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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Why the intensity vs. frequency graph initially increases for black-body radiation?

Max Planck had said that $E = n h \nu$ in his quantum theory. Then according to it, as the frequency of the light emitted increases, so the light's intensity should decrease. But it is not in the ...
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phase differences between conical intersections and light-induced conical intersections?

I've read recently about light-induced conical intersections, a phenomenon where conical intersections can be artificially introduced to molecules, and that this can be observed even in diatomic ...
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Why does each element only react with certain elements? [closed]

Please, describe the above question on the context of atomic and molecular vibrational energy.Is there any link between molecular vibration and occurance or existence of a chemical reaction?
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Calculating Optical Absorption Spectra using Ab Initio Computations

My main question is, How does one compute the optical absorption spectra of a molecule by using say, Configuration Interaction energy and associated transition dipole moments. I am trying to use an ...
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Does quantum mechanics play a role in protein folding?

Protein folding takes a very long time (relatively speaking) when thinking of quantum mechanical effect. However, for the initial micro-steps of folding, when an atom, or a configuration of atoms, can ...
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H2 STO-3G molecule in second quantization

Does anyebody know where to find an explicit expression for the $\ce{H2}$ molecular Hamiltonian in the STO-3G basis in the second quantization formalism (in Chemist's notation)?
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Tunneling corrections in reaction rates

I've studied Transition State Theory since I asked this a while ago, but so far I have not been able to find a detailed explanation of how tunneling corrections are implemented into reaction rates ...
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How does entanglement explain myoglobin's preference for O₂?

I was reading an article about the myoglobin's unusual preference for O2 than CO, in which I found: It turns out that some electrons in the myoglobin involved in binding CO and O2 exhibit a ...
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Request for a graph of the combined probability density of the three p-orbitals

This started when i was wondering why p-orbitals have an unsymmetrical shape (which isn't clear to me yet). But then i came across Halliday/Resnick/Walker's Fundamentals of Physics. In it, they have ...
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Can hot food ever emit x-rays or gamma rays?

I was just wondering, if heating food up is the result of increasing the energy of bends and stretches in the bonds of the molecules, is it ever possible for tiny amounts of x-rays and gamma rays be ...
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Why don't equivalent hydrogens cause splitting in NMR?

When doing NMR spectroscopy, it is an observed fact that equivalent hydrogens do not split one another. Why don't equivalent hydrogens split each other's signals? For example, why is the NMR spectrum ...
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Why are spherical compounds with a half filled HOMO concidered aromatic?

In planar annulenes, huckel's rule effectively states that compounds with a full HOMO for their conjugated $\pi$ systems are considered aromatic while those with a half filled outer shell are ...
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Clarification on absorption spectra and crystal field theory

My textbook, Chemistry: The Central Science, gives this figure to explain the purple color of the hexaaquatitanium(III) ion: Such absorption spectra are common to chemistry and biochemistry. ...
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271 views

Why are full and half filled orbitals the most stable?

Why are degenerate orbitals (restricted to a single spin) less stable when neither fully filled nor completely empty? Why, in most molecules, are half-filled and fully-filled shells more stable than ...
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Is there wavefunction interference between alternate reaction pathways?

I'll give a specific example to clarify my question. If we look at the approach of chloromethylenecarbene's electron pair to cis-2-butene in the reaction of creating 1-chloro-2,3-dimethylcyclopropane, ...
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How to calculate probability of finding an electron at a point?

In school i learned how to calculate probability of finding electrons in some volume but how can we calculate the probability of finding a electron at a particular point. Point here ...
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DFT-D3 calculation in Gaussian 09

Can anyone help me to run geometry optimization and frequency calculation using B2PLYPD3 functional including the DFT-D3 dispersion correction? I went through Gaussian manual, however, it was not very ...
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Supramolecules in Spartan

Maybe someone here is familiar with supramolecules calculations using Spartan? I am new in this area and trying to figure out some things: First - what's the best way to draw them? I tried to draw one ...
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Spin of electrons about it's own axis, release of energy & COLLAPSE of atom? [closed]

An electron spins about it's own axis. So it has rotational accln. Thus, according to Maxwell's Electromagnetic Theory , Rotational Kinetic energy must be released. Thus, at some pt. of time , ...
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Density Functional Definition

Is it wrong to say that Density functional means that Electron Density is a function of the wave function of all electrons in 3 dimensions, if so, why ?
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What is the reason why protons and electrons do not collide?

can someone give me an intuitive picture of why electrons don't collide with protons? I know that electrons move in a sort of cloud, which is our 'orbital', and that they mainly behave like ...
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What are the unsolved problems in quantum chemistry? [closed]

The question is not necessarily limited in quantum, but also computational and theoretical chemistry. Wikipedia listed a few https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_chemistry ...
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How do i make an molecular orbital diagram step by step?

My professor explained me, but i'm still unable to do a correct molecular orbital, for a diatomic molecule
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Why do ionisations from non-bonding orbitals result in little/no vibrational structure in a photoelectron spectrum?

I understand that ionisations from bonding and antibonding orbitals create vibrational structure from the Franck-Condon principle, resulting in a greater overlap integral between different vibrational ...
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Why Balmer series exist?

I was reading about Bohr's atomic model in our textbook when came up with this question: Since the electron in hydrogen atom is most stable in the first orbit, why should it descend from higher orbits ...
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Estimating stability and mean life of anomalous tautomers

A pair of bases in the DNA, say, A-T, have a tautomerized form A*-T* (resulting from switching the sides of both protons along the hydrogen bridges). I have studied how, by means of DFT computations, ...
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radial wave function [closed]

Well i just started to learn the basics of quantum mechanics and i came across a graph between the radial wave function plotted against the distance r from the nucleus. Could someone please explain to ...
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Why are antiaromatic compounds unstable?

Antiaromaticity is so destabilizing that it can cause compounds such as cyclobutadiene to elongate or manipulate their orbitals so that the pi system is no longer aromatic. I understand that aromatic ...
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Why is no dynamical correlation recovered in the CASSCF method?

Configuration interaction is the only certain way to get a calculation to converge toward the exact solution of the many-body Schrödinger equation (within the approximations used for the framework of ...
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Is HF the only method for trying to solve the many-electron Schrödinger equation?

The Schrödinger equation can only be solved analytically for the smallest of "molecular" systems. The Hartree-Fock method is a method of obtaining approximate solutions to the many-electron ...
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Is the reasoning of spin contamination correct?

The common explanation of spin contamination is like the following: The non-relativistic electronic Hamiltonian commutes with $S^2$ and $S_z$. Hence, the exact eigenfunction should be an ...
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Why is oxygen paramagnetic?

Paramagnetic molecules are molecules that have single electrons. When I draw the lewis structure of $\ce{O2}$, it appears to be a diamagnetic structure. What makes it paramagnetic?
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Why is the CASSCF method multi-configurational, while the CI method is not?

The CASSCF method is perhaps the most commonly used theoretical method for studying difficult chemical systems exhibiting multi-reference character or non-dynamical/static/strong correlation. CASSCF ...
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From a Quantum Chemistry Point of View how can I break Bonds without Heating?

Is it possible to dissolve/break bonds using strong alternating magnetic fields at high frequency? (Perhaps from tesla coils?) Could you use resonance to cause the bond to weaken over time? If so how ...
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DFT Code for Atoms : Sources

Some time ago I implemented the restricted and unrestricted Hartree-Fock methods. I had a lot of fun and I learned a lot. It goes without saying that Szabo and Ostlund's book "Modern Quantum Chemistry"...
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What should I use as the starting structure for stacking interaction energy calculation using Gaussian?

I have a crystal structure (single crystal, organic molecule- pyranone carboxamide derivative) that shows pi-pi stacking, and I want to do theoretical calculations. I am using Dispersion-correcting ...
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Raman Spectrum of carbon chains: How to identify the vibrational modes

So I am using NWChem to calculate the raman spectra of C2H2 and C4H4. For example for the first I obtained the following frequencies: ...
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Physicist notation for spatial orbital

In quantum chemistry, the two-electron integrals often denoted as physicist's and chemist's notations. For spin orbital, the physicist's and chemist's notations are $$ \langle pq | rs \rangle = \int ...
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What is the purpose of Gaussian, itself?

I have been doing some research into computational chemistry, having never looked into the area before, and have read about a popular, paid-for software "Gaussian", and free alternatives such as "...
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Conical intersection using GAMESS

I would like to know how to locate conical intersection using a GAMESS calculation. I really appreciate an example alone with this since I am a novice I do not have a clear idea among bunch of ...
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If d-electrons are such poor shielders, why do trends increase more gradually across the d-block than the s or p-block?

If I understand correctly, the shielding effect of d- (and f-) electrons seems to be much poorer than those of s- and p-electrons, due to the fact that they are less penetrating, have less electron ...
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Are electron orbitals in a helium atom degenerate?

In a hydrogen atom, the presence of only one electron allows various orbitals' energy states to be dependent only on the principal quantum number and not on angular momentum. Orbital degeneracy and ...
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Do Moore's tables really exist?

I recently have a lesson about quantum chemistry, and in one lesson we spoke about hyperfine stucture of elements. The teacher give us something which looks to be an extract of Moore's tables here ...
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How dependent are computed charges using the quantum theory of atoms in molecules on the used level of theory?

The quantum theory of atoms in molecules is based on the topology of the electron density. This mathematical analysis allows to find critical points and hence has a unambiguous way of separating a ...
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Basic Intro to Quantum Chemistry. Two Slit Experiment and Bohr Model

We just learned about the 2 slit experiment in Quantum Chemistry today, where electrons behave as waves when nobody is looking and behave as particles when they are being observed. So, what would ...
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The time scale of calculations in computational chemistry

Is it possible to determine or get a range of the time required for a specific calculation with knowledge of the type (MM or QM), class (semi-empirical, ab initio or DFT), the degree of theory (MP, CI,...
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What does Molecular Orbital Theory get wrong?

I am learning about MO theory in my advanced inorganic chemistry course and am starting to realize that it is truly the most accurate representation of how molecular orbitals look like, where they are ...