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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Why are covalent bonds directional?

It is said that covalent bonds are directional, while ionic bonds are not. Why? Is it because of the orientation/directional properties of the overlapping orbitals?
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How does the magnetic quantum number work?

I am really confused. When writing an orbital you write it as $2p_x ^e$ where x is the orbital's space orientation and e is the number of electrons in that orbital but but what is space orientation ...
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Relationship between the first and the second quantum number

Does the secondary quantum number tell how many subshells a specific principal quantum number shell has? E.g., if the principal quantum number is $n$, there are ($n-1$) subshells.
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What is the reason behind exchange and pairing energy? I mean how do they contribute to stability? [closed]

I have seen that when electrons exchange their positions , stability is obtained and so half filled orbitals are also quite stable but what is the reason behind this?
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1answer
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How does molecular collision excites the electron?

I have this question for a long time that in LASER when we do the pumping by heating how does molecular collision causes the atoms(molecules) to reach in excited state. E.g. in $\ce{He-Ne}$ laser when ...
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Why carbon is more versatile than silicon [duplicate]

The valence of carbon and silicon is same. Carbon has a number of allotropes such as graphite, diamond, carbon nanotube, graphene, fullerenes, nano cones etc., However, silicon does not exhibit such ...
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Why is MP2 failing to calculate proper IR frequencies for [Co(CO)4]^- and [Fe(CO)4]^{2-}?

I was doing some IR calculations on different 3d transition metal carbonyl complexes during some practical courses where we were introduced to effective core potentials. While doing this we found that ...
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Similarities and Differences between Resonance and MCSCF treatments

Recently, there has been a question by Voldemort concerning different resonance structures of $\ce{NCO-}$, requesting an explanation why one resonance structure would be more preferred than another. ...
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65 views

Radical Computational Calculations [closed]

I am trying to calculate the total energies for a particular organic molecule that undergoes radical decomposition. I was able to calculate the neutral molecule with the exact results of a paper I ...
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41 views

How come the value of principal quantum number is an irrational number?

My textbook says that for a d-electron, the orbital angular momentum is $$ \frac{\sqrt{6}h}{2\pi} $$ But since:- $$ mvr = \frac{nh}{2\pi}$$ $$ n \in N$$ where N is the set of natural numbers. So how ...
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Can one relate the frequencies of vibrational modes within a molecule to the molecule's HOMO/LUMO energies?

Can one relate the frequencies of vibrational modes within a molecule to the molecule's HOMO/LUMO energies?
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How many electrons can have the quantum number set n=5, l=3, ml=-1? [closed]

How many electrons can have the quantum number set $n=6,\ l=3,\ m_l=-1$? Also, please explain why. I know that n describes the number of shells in an atom but what do n, l, and ml have to do with ...
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60 views

Calculation of fluorescence quantum yield

The fluorescence quantum yield of a sample $\Phi$ is defined as the fraction of photons emitted per number of photons absorbed. The quantum yield is often calculated by comparing a sample with a ...
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32 views

Which mechanics or law is predominately seen in nanoscale

Nanotechnology and sciences are the study or the manipulation of matter on an atomic, molecular, and supramolecular scale especially at nano meter range. We know that at macro molecular level a system ...
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1answer
110 views

Complexity in computing the integrals involving higher angular momentum

I am quite familiar with Hartree Fock theory. However, writing it into program is an entirely different task, and that is where I have a couple of questions. I wrote a HF routine in Python a couple ...
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1answer
80 views

Why replace c with v in Broglie hypothesis

I am in XI and I am currently studying atomic orbitals, I stumbled up and am stuck at the following step of de Broglie's hypothesis. I did a little search on-line and found almost same derivation ...
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Is it possible to calculate energy of electron in any orbital and atom in Schrödinger wave model theory?

Is it possible to calculate the energy of electron in any orbital and atom in the Schrödinger wave model theory? If so, how? E.g. energy of the $3s^2$ electron of the $\ce{Na-}$ ion.
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Why are real orbitals used to define the orbital of hydrogenic atoms instead of complex orbitals?

On the physics site, I asked the difference between real & complex orbital. Real orbitals are the superposition of complex orbitals having definite magnetic quantum number states. \begin{align} ...
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Where exactly do the orbital angular momentum get their numbers e.g. +1/2

I was thinking about orbital angular momentum today and wondering how exactly do the numbers come about that quantize them. I was doing a little research on how they come about and I came across ...
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Question about quantum mechanical model of atom and the quantum numbers

I studied Bohr's model of atom and then the drawbacks of it and then quantum mechanical model of atom. Now quantum model is according to uncertainty principal and dual nature of matter and it says we ...
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Subtracting wave functions?

In molecular orbital theory bonding of two atoms can be seen as addition or subtraction of the wave functions of these atoms. Addition seems pretty logical and straight-forward - atoms overlap their ...
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Why does the transverse magnetization in Fourier Transform NMR consist of Ix+iIy ($\hat{I}_x+i\hat{I}_y$)?

More specifically, the expression for the average expectation value of the transverse magnetization has the following form: $$M^+(t)=N\gamma \hbar \text{Tr}(\hat{\sigma}(t)\hat{F}^+)$$ with ...
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1answer
359 views

Element with No Proton?

On seeing this graph closely I noticed the there a element with no proton. But with no proton there will be no electron too and don't think that an lone neutron would form an element. So is that ...
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electron passing nodes?

When one say that the electron goes from 1 orbit , say 2S, to another (higher) orbital, say 5P, it mean that the electron is excited to that orbit by some mean. In doing so it also it will also cross ...
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Electron vs electron density

I recently read somewhere that electrons do not exist. it's just the electron density. Is it true that there are no electrons (particles) but only in the form of electron density? And does this ...
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Why does the quantum number n limit the value of l, and why does l, likewise, limit the value of m?

Any quantum number that represents the size of an orbital is a positive integer, $n$. Any quantum number that represents the shape of an orbital is a non-negative integer, $l : l < n$. Any ...
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Photograph of Light as Wave and Particle [closed]

What is this? Actually its the first photo of light as wave and a particle The bottom "slice" of the image shows the particles, while the top image shows light as a wave. I have questions: How ...
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Bohr's model of atom

"the most remarkable success of the Bohr's theory is that it provides a satisfactory explanation for the line spectrum of hydrogen." But what is the problem with line spectrum of hydrogen which ...
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Is it wise to use ECPs on light atoms for electronic structure calculations?

So, I have a big organic molecule consisting of $\ce{H}$, $\ce{C}$, $\ce{O}$, and $\ce{N}$ atoms and the goal is to find its equilibrium geometry and US/Vis spectrum. The molecule is so big that I was ...
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Change of electron spin in Carbon after Hybridization?

During bond formation in Carbon, one of the electrons from the 2s orbital jumps to one the empty p orbitals creating a hybrid orbital as seen in the figure below. Now clearly one electron from the ...
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What do the quantum numbers actually signify?

I know how to calculate them and such stuff, but I wanted to know what they actually signify. I have a vague idea that they have something to do with an electron's position in an atom but what do all ...
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219 views

What is GAMESS and Gaussian?

I've been seeing a lot of GAMESS and Gaussian questions around here, and most of them seem very hard to answer. I extrapolated that they might be some sort of Chemistry programs, but what do they do ...
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303 views

Born-Oppenheimer adiabaticity

Born Oppenheimer aproximation is said to be adiabatic. What does adiabatic mean? What would be the case of a non adiabatic approximation to the Schrödinger equation?
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How to interpret orbital transition in TDDFT?

Consider the following TDDFT run with GAMESS: ...
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Should chemistry students worry about quarks and such?

Even chemistry textbooks less than five years old start by describing atomic structure based only on protons, neutrons and electrons. Is an understanding of chemistry fundamentals likely to become out ...
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Why Acetone does not behave like its computational values?

I am trying to simulate the excitation state of acetone. I ran TDDFT for it both in gas phase and solvated state in water (both implicit and explicit water). The experimental data say that acetone ...
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How to calculate molecular dipole moment from a known wavefunction?

Say I have a molecular wavefunction as a set of molecular orbitals and want to calculate the molecule's dipole moment, but don't know how ! I searched a lot but couldn't find any practical example. ...
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1answer
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How to interpret TD-DFT results for finding λmax and amplitude

I am trying to understand how to interpret the TDDFT results ( I know there is another post for this but it does not answer my question ). Here is part of the result from GAMESS: I downloaded ...
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problem regarding hydrogen spectrum [closed]

The wavelength of the $\ce{H}_\alpha$ line of the Lyman series for the $\ce{H}$ atom is $\lambda$. What will be the transition for the $\ce{He+}$ ion which will have the same wavelength?" How do ...
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Why does Thallium form TlF and TlF3 but aluminium only forms AlF3?

Does this have something to do with the inert pair effect?
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Exchange-correlation hole

I did a google and chemistry.exchange search and found several technical definitions of the exchange correlation hole. The questions that I want to proposes are: What the exchange-correlation hole ...
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How important are those fields of chemistry in biotechnology and medicinal chemistry

At my university there is some fields of chemistry that i don't know if i should focus on them... How important are in biotechnology and medicinal chemistry those fields of chemistry? : Quantum ...
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Are molecular optimizations in quantum chemistry really in vacuum?

I know that Gaussian (or other quantum chemical packages) assumes the molecule is alone in vacuum, that is, the molecule doesn't interact with anything, when optimizing the molecule. But I've found ...
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Theoretical model for the hydrogen bond

I am curious how, from the perspective of quantum chemistry, the hydrogen bond is modeled. In particular I am interested in using such a model to estimate the amount of energy released per $\ce{H2O}$ ...
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Quantum mechanical explanation for Le Chatelier's principle?

We know from basic chemistry that a reaction reaches equilibrium as described by Le Châtelier's principle. What's the quantum mechanical explanation to this principle? How a change in ...
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Charge consistency in fragment qm/mm methods

I'm calculating the multipole moments on several fragments of a molecule. What are some procedures to ensure that the charges etc are representative of the complete molecule? How do I ensure I'm not ...
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What is the relation between surface tension and initial molecular harmonic?

Initial harmonic is measured as Hartree/Bohr^2 ( like IHarmonic=n in Gaussian Software ). As I am from physics background I am used to look at features from their dimension point of view: Actually ...
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Why are 2 ways to calculate the energy of the ground state?

Setup I found in Szabo, Quantum chemistry page 88, these two ways to calculate the energy of the ground state: $$E_0= \sum_a^N \langle a|h|a\rangle + \frac{1}{2} \sum_a^N \sum_b^N \langle ...
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installation error of dalton 2015 in ubuntu 12 [closed]

what are the answer for CONFIRMATION OF REGISTRATION when register in dalton forum ? install got error, how to install dalton 2015 wonder@wonder-VirtualBox:~/dalton/DALTON-Source$ uname -r ...
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Atomic number $Z$

If electrons were spin-$\frac {3}{2}$ instead of spin-$\frac {1}{2}$ , what would be the atomic number $Z$ for the first noble gas ?