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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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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53 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
157 views

What are the steps of Hartree-Fock Theory?

Outline the steps involved in the Hartree–Fock method for the calculation of molecular electronic structure. I understand the basics of HF, in that it does not account for electron correlation (...
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28 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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36 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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65 views

Available datasets to study using Hückel theory

I'm trying to do some tests using graph Laplacians based on Hückel theory. This, however, is said to be only applicable for conjugated hydrocarbons. My questions are: I have been working with QM9 ...
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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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46 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
72 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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16 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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41 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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30 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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78 views

What will happen if electron as a wave undergo destructive interference while travelling in its Orbit?

Today I came across a justification of Bohr's angular momentum quantization using the De- Broglie's hypothesis. The justification said that since the electrons are having wave nature also then while ...
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2answers
391 views

What does Brightness/ intensity of spectral lines depend on?

My book states that it depends upon "The number of photons of same frequency or wavelength ABSORBED or EMITTED" From what I understood, if many hydrogen atoms within the discharge tube EMIT the same ...
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61 views

What will be the form of coulomb and exchange operators for electron m in an n-electron molecule?

I know that the Fock operator for electron $m$ in $n$-electron molecule would be: $$\hat{f}\!(m)u_i(m) = \varepsilon_iu_i(m)$$ $$\hat{f}\!(m) \equiv -\frac 1 2 ∇_m^2 - \sum_α\frac{Z_α}{r_{ma}} + \...
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56 views

How To Calculate “İntermolecular Energy” as QM and MM? [closed]

In this article: http://ambermd.org/antechamber/gaff.pdf, in Table-7, authors have calculated "intermolecular energy" of a molecule set. I wonder how can I calculate this energy by using gaussian09 ...
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192 views

Hartree Fock for an atom (Beryllium)?

I'm working on solution of Hartree-Fock equation for Beryllium in configuration $1s^2 2s^2$, expanding the orbital with STO-3G basis $$\phi_k=\sum_{p=0}^3 C_{kp}e^{-\alpha_{kp}r^2}$$ I know the values ...
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85 views

GAMESS Gaussian qst2 method alternative

I've heard about the qst2 Gaussian method but I don't have any licence so I use the free GAMESS-US. Nevertheless, I didn't find any Gamess method which allows a TS finding like qst2. Have you heard ...
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77 views

Antiaromaticity can be explained by Hückel method?

Pentalene is one of the most famous antiaromatic molecules. But I obtained its energy of $\pi$ system as $8\alpha+10.46\beta$ by Hückel method (with a bit complicated but straightforward calculation). ...
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1answer
139 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 ...
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73 views

How is occupation number of lowest unoccupied natural orbital defined?

I have run some calculations to analyse the occupation of natural orbitals. Since it is needed to run in triplet state, I get two orbitals with occupation number 1. Now, which orbital occupation ...
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37 views

Relationship between the 2nd theorem of Hohenberg and Kohn and the Variational Principle

In Density Functional Theory, the 2nd theorem of Hohenberg and Kohn states: The electron density of the system that minimises the ground-state energy is the exact electron density of the system. ...
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71 views

STO 3G basis expansion of p orbitals

I am expanding $2p_{x},2p_{y},2p_{z}$ (slater type )orbitals in gaussian STO-3G basis as follows, $$|2p_{x}\rangle=C_{1}x\exp(-\alpha_{1}r^{2})+C_{2}x\exp(-\alpha_{2}r^{2})+C_{3}x\exp(-\alpha_{3}r^{2}...
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37 views

Constructing two body Hamiltonian for Helium atom in hydrogen slater orbitals single particle basis [duplicate]

I am trying to construct a two body states to calculate the matrix elements of electronic Hamiltonian of Helium atom. $$H=-\frac{1}{2}\nabla^{2}_{1}-\frac{1}{2}\nabla^{2}_{2}-\frac{2}{\vec{r}_{1}}-\...
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38 views

In 1 dimension, the curvature of Ψ = K.E. of wave. Is this the case for 2 dimensions and 3 dimensions also?

In 1 dimension, the curvature of Ψ = K.E. of wave. Is this the case for 2 dimensions and 3 dimensions also? Moreover, is curvature of Ψ = K.E. of wave = 2nd derivative of Ψ?
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44 views

What is the intuition of using “dimer method” for searching transition states?

I learned dimer method for searching transition state in this paper: A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. However, I still ...
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566 views

Calculation of average bond length from force constant and IR absorption?

I need some help beginning a problem set problem (just want hints, I'd like to figure it out myself). The problem gives me the IR absorption for a diatomic, as well as the force constant for the ...
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56 views

Is the energy of the orbital an electron “resides” in, considered to be a factor of that electrons ionization energy? [duplicate]

The answers to a Quora question mainly refer to the factors as generally being, the electrostatic forces of attraction between the electron and the positively charged nucleus and all other events, by ...
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227 views

The nature of Chemical Bonds [closed]

It was nearly a year ago when I appeared for my first interview through the KVPY (a research oriented scholarship exam) when one of the questions the panel asked me was : What is a chemical bond? I ...
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36 views

How on the quantum level do the notch filters used in Raman spectroscopy remove only a select wavelength of light?

From what I have gathered regarding the schematics of Raman spectrometers, the notch filter is used to remove a narrow range of light corresponding to that of the laser used to excite the sample. How ...
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48 views

Probabilities of Electrons in Orbitals [duplicate]

I've heard both of the following: An orbital is a region where there's a 90% probability of finding an electron An orbital is a region where an electron spends 90% of its time Are these essentially ...
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201 views

Quantum Mechanical Model of An Atom, Shells, Subshells [duplicate]

I've just been introduced to the quantum Mechanical Model of an Atom, so please bear with me on this one. Are Subshells and shell physical things, like do they really exist, or is it a concept only to ...
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903 views

Difference between Covalent and non-Covalent interactions

It is a general concept that non-covalent interactions are a result of electrostatic interaction while covalent interactions arise due to sharing of electron density. This, however, is not true since ...
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155 views

Why must the ground state of hydrogen molecule be gerade?

As we know, the electronic ground state of hydrogen molecule is $\Sigma_g^+$. My question is, how do we know it from the theoretical perspective? The nuclear potential in the hamiltonian of hydrogen ...
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35 views

rovibrational spectroscopy: why do intensities of rotational lines increase with J? [closed]

how and why do the intensity of rotational lines change with J value? in the simplest terms possible please
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74 views

How does the electron configuration correlate with the emission spectrum of an ionic compound? [closed]

trying to find a way to correlate the electron configuration of an ionic compound (ie. Cupric sulfate, potassium chloride, Cupric chloride, sodium carbonate, strontium chloride) with its emission ...
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87 views

Nodes of orientation of electron orbitals [duplicate]

As you can see the orientation of 2 p orbital there is a nodal plane in between the electron density pictures.... So doesn't the electron move from left to right side in2 p X orbital. I think that ...
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83 views

Should the normal mode of vibration leading to the transition state be removed?

I would like to know whether the normal mode of vibration of the reactant should be deleted when calculating $\Delta ZPE$ and $\delta G_{\text{corr}}(T)$, where $\Delta ZPE$ is the zero-point energy ...
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57 views

How to determine which normal mode leads to the transition state (numerically)? [closed]

I would like to get the normal mode of vibration which leads to the transition state. Do you know any software for this problem? If no, could you offer an algorithm to generate it? By the way, is ...
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37 views

What is meant by 'penetration of atomic orbitals'? [duplicate]

While studying atomic structure, I have faced this term very often. I assume this is a very detailed type question! But if anyone can please provide just the preliminary idea about this, it will be ...
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43 views

how to understand to what subshell is related basis function in the Eigenvectors block (GAMESS output)?

So, each subshell of some atom is described by one or combination of few BF's. But I can't find correlation between representation of subshells in eigenvectors block and, say, basis set or something ...
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81 views

Spatial form of an atom and type of chemical bond

As we know, the first excited states of Hydrogen atoms is $2p$. The wave functions in cartesian basis we can represent as $$ p_x = \frac{1}{\sqrt{2}} \left( Y_{1, +1} + Y_{1, -1}\right) =\...
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25 views

What is the significance of increasing (and decreasing) the number of ionic releaxation steps in a plane wave DFT calcuation? [closed]

The number of ionic relaxation steps is one of the adjustable parameters while setting up a DFT calculation using a commercial code such as VASP. I was wondering what's the significance of this value?
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44 views

quantum capacitance of supercapacitor electrodes, high or low needed?

quantum capacitance of supercapacitor electrodes should be low or high? to achieve high specific capacitance. The quantum capacitance forms among the layers of low DOS materials when used as an ...
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107 views

How fast do electron's quantum spin flip?

In a recent lecture it was taught that carbenes with their electrons in a triplet configuration do not undergo concerted reactions as one of the electrons must first undergo a spin flip; on what ...
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56 views

Computational chemistry: is k a quantum number

I am trying to use the natural band offset method to calculate band offsets of junctions. In order to do so, I need to find the core electron eigenstates. I tried using FP-LAPW software Elk. But in ...
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469 views

How does symmetry of half filled and fully filled orbitals lead to stability

I googled a lot about why symmetry of half filled and fully filled orbitals decreases their energy but every time it is repeated that symmetry leads to stability. So the question is why symmetry leads ...

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