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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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Schrodinger Equation : TDSE and TISE [on hold]

How can one particular Schrodinger equation be both time-dependent and time-independent at the same time? I mean its wrong by the very notion to say both simultaneously. We encounter many instances ...
2
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How to derive a state symmetry in D2h point group from its wavefunction?

Let's have an atomic carbon with the following electron configuration: $$ 1s^2 2s^2 2p^2 $$ One of it's levels is ${}^1S_0$, which is corresponding with the following state: $$ \begin{align} \left| ...
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Potential energy curve for dissociation of cation of carbon dioxide

Can anyone suggest me some literature where I can find the potential energy curve or the electronic states for singly ionized $\ce{CO2}$ ($\ce{CO2+}$) dissociating into singly charged oxygen ($\ce{O+}$...
4
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1answer
33 views

How to specify atomic carbon terms in the coupled and uncoupled representation?

So, we know, that the atomic carbon in the electronic configuration $1s^22s^22p^2$ has the following terms $${}^1S, {}^1D, {}^3P$$ My question is - how can I correctly specify these terms in the ...
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1answer
27 views

Difference between adiabatic and nonadiabatic tunneling for molecule

What is the difference between adiabatic and nonadiabatic tunneling for a molecule in a laser-field? Does nonadiabatic tunneling necessarily relate to the Conical intersection or curve-crossing?
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How often does the Strong Interaction happen? [on hold]

Like does it happen all the time? Does whenever a pion is transferred, the interaction takes place again? Is it only whenever a force is acted on it? Is it a second?
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1answer
24 views

Roothaan matrix equation equation orbital coefficients

To go from the Hartree_Fock $f\psi{_i}=\epsilon\psi{_i}$ to the Roothaan equation $FC=SC\epsilon$. equation we expand the orbitals as $\psi{_i}=\sum C_{\mu i}\phi_i$ But for Helium atom we ...
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1answer
57 views

What happens if the electron in the lowest energy level of an atom loses its energy?

What happens if the electron in the lowest energy level of an atom loses its energy? Excited states spontaneously give of energy and transition to a lower energy state. Why can't an electron in the ...
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28 views

Generating density matrix of a wavefunction from common quantum chemistry software

I am looking for density matrixes for a couple of closed shell and open-shell systems, and I wonder if there is a trivial way to make e.g. Gaussian just generate an output for me. I may be wrong but ...
5
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1answer
47 views

Why do people add H(core)+F in Hartree-Fock?

I'm trying to implement my own HF code, and it seem usual that, after building the Fock matrix $F$, they add that to the pre-existing $H_{core}$, from the two-point interactions, when computing final ...
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25 views

Putting high symmetry points labels to a band structure plot

So far I've got this plot using Quantum Espresso. I want to put gamma, X, L, etc labels to the k-path. Quantum Espresso's ouput states the following: ...
3
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1answer
108 views

What is the exact definition of the radial distribution function?

I have been very confused by the radial distribution function which is often used in chemistry to predict the probability of finding an electron at a distance from the nucleus. From Atkins' Physical ...
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36 views

Photoelectric effect — Expected observations for waves

I understand why treating light as particle-like photon packets leads to the observed results. I seek to understand why one would expect different results if light were treated as a wave (and thus ...
5
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1answer
74 views

Raman spectra in Gaussian [closed]

I want to simulate Raman spectra of benzene in Gaussian 09. Calculated spectra for a single ring Benzene ($\ce{C6H6}$) in Gaussian differs about $\pu{40 cm^{-1}}$ for each peak, compared with ...
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31 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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Calculate molecular orbitals of high-lying or Rydberg states in GAMESS (US)

I am trying to calculate the high-lying or Rydberg orbitals (i.e. unoccupied orbitals with their energies just below the ionization energy) of a nitrogen molecule. I use GAMESS(US) with the following ...
2
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1answer
22 views

Calculate the transition dipole moments between the 1s inner shell and the high-lying states of a molecule

I am trying to calculate the transition dipole moments between the 1s inner shell and the high-lying states of N2, namely, $<\psi_{1s}|{\bf r}|\psi_{high-lying}>$. For this, I installed ...
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32 views

Can LCAO be seen as superposition?

In wave physics, when waves interact, the principle of superposition applies. It states that the displacement of the resultant wave is made up of the sum of the individual displacements of the ...
4
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1answer
48 views

How can f-f transitions happen?

The question might seem quite easy to answer but still I am unable to find any useful information. I am aware of laporte- or symmetry-forbidden transitions and for f-f also spin-forbidden transitions ...
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1answer
45 views

How to write a cube file?

This is a technical question. I have a real-space representation of a potential in a grid of points, however I have the grid in the format: x y z w v meaning: the coordinates of each point, the ...
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1answer
49 views

Can the oxidation of hydrogen peroxide be reversed? [closed]

Hydrogen peroxide is said to be unstable, for it undergoes auto-oxidation on standing/heating: $$\ce{2H_2O_2 -> 2H_2O + O_2}$$ where $\Delta S=\pu{70.5 J {mol}^{-1}K^{-1}}$ and $\Delta H^{\Theta} ...
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29 views

Why has na 1S0 term Ag symmetry?

This question is a continuation of Molpro - Connection between atomic term symbols and system symmetry. In the Feodoran's answer I found it somehow confusing, that ${}^1S_0$ term should be totally ...
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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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35 views

Vanishing integral rule for two-electron integrals

I understand the vanishing integral rule for a one-electron integral to be: $$\langle i | \hat{O} | j \rangle = 0 \hspace{1cm} \mathrm{if} \hspace{0.2cm} \Gamma_O \neq \Gamma_i \otimes \Gamma_j$$ ...
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1answer
69 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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1answer
68 views

Getting the pressure of a system by series expansion

An average number, $N$, of bosons of spin $S = 0$ is conned to a two-dimensional domain with surface $A$. The gas is ultrarelativistic with a single particle energy $\epsilon = cp$, where $c$ is the ...
5
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2answers
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What is the sense of electronic configuration of an atom if an electron can be in any superposition of energy eigenfunctions?

I recently started learning abou the quantum mechanical explanation of hydrogen atom. In the derivation of the wave functions of hydrogen atom we find its energy eigenfunctions. But according to the ...
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2answers
107 views

Connection of term symbols with specific microstates for atomic carbon

I'm currently studying atomic term symbols. I wanted to try it on a simple atomic carbon with the electron configuration $1s^22s^22p^2$. I know, that only open-shell electrons are involved in the ...
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1answer
35 views

Can Hartree-Fock mix more orbitals than there are electrons?

I'm trying to understand Hartree-Fock well enough to write my own implementation. A point that I seem to be coming to that I'm not sure is correct: the method only produces one-electron orbitals that ...
5
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1answer
40 views

What determines the coefficients in the wavefunction of a hybrid orbital?

I was studying the hybridization of carbon atoms and I came to figure out that the $sp$ hybridization wavefunction is given as $$|sp_{+}\rangle =\frac{1}{\sqrt{2}}\left(\psi_{2s}+\psi_{2p_x}\right)$$ ...
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Are there any selection rules for collisional excitation/relaxation?

There are well-known selection rules for possible optical transitions of an isolated atom (molecule) between different electronic states. But are there any rules, limiting possibilities of collisional ...
3
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1answer
44 views

Molecular orbitals symmetry - states notation

I'm reading some papers and I'm repeatedly seeing the following notation of system states: Could you, please, explain to me the meaning of it? E.g. the state $B^2 \Sigma_u^+$ - I'm aware, that $\...
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How would vicinal coupling of two 1H with intervening 12c nuclei look with a naive Fermi contact model?

Multiple sources I've found talk about vicinal J-coupling between two hydrogen-1 nuclei with two intervening carbon-13 isotopes as in: 1H-13C-13C-1H Which would be (electron spin of covalent bonds ...
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2answers
84 views

Why are the radial wavefunction and radial distribution function different?

I'm having trouble understanding the following two graphs, which are the radial wavefunction of the hydrogen 1s orbital and the corresponding radial distribution function: Specifically, why is the ...
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0answers
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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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0answers
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Implementing the projected atomic orbital (PAO) localization in Gaussian

I am currently attempting to learn how to setup the active space for performing CASSCF calculations. The textbook I am using as a reference is: Roos, Björn O. "Multiconfigurational quantum ...
3
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1answer
55 views

Can someone tell the me the data bank which has all the STO-3G basis sets for many electron atoms?

I am trying to express the slater orbitals of Helium atom in terms of 3 gaussian functions (STO-3G basis set). Is there any data bank or reference table in which I can find the exponents and ...
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53 views

Significance of this unusual matrix element

Recently, I was solving some questions on Rigid Rotor. I found this question very amusing. Evaluate the integral i) <$Y_{l,m+2}$ | $L_{x}^{2}$ | $Y_{l,m}$ > I evaluated it, found that the value ...
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107 views

Chemistry: What's the difference between “binding energy” and “ionization energy”?

In respect to PhotoElectron Spectroscopy (PES), we are analyzing graphs with "binding energy (MegaJoules)" on the x-axis, and with "relative number of electrons" on the y-axis. I have also seen the x-...
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0answers
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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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2answers
618 views

Is the notion of orbitals different in theoretical chemistry?

I am just an average chemistry student that is currently exposed to theoretical chemistry. And I am seeing very puzzling things, such as 1p, 2f, 3g orbitals. In my attempt to reconcile this, I was ...
2
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0answers
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Are any other orthonormalizations schemes besides Lowdin normally used when solving SCF equations?

When solving equations of the form: $$F(C)C = SC\epsilon$$ (e.g. Hartree-Fock-Roothan or Kohn-Sham-Roothan) where $F$ is the fock matrix, $C$ is the coefficient matrix and $S$ the overlap matrix, ...
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1answer
57 views

what is the issue in evaluating two electron integral?

What sort of issue is there in two electron integral in quantum chemistry ? Is it a problem of convergence when we do numerically? Can someone suggest any review article or reference articles which ...
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How to calculate the magnetic exchange coupling in a charged radical complex?

I have a neutral complex with two magnetic centers. For this case it is easy to calculate the exchange coupling as the broken symmetry state is converged properly. But when I am making the complex ...
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1answer
46 views

According to MO theory, do 1s orbitals in dioxygen interfere?

According to MO theory, are the 1s orbitals in $\ce{O2}$ too radially contracted to interfere with each other and thus cannot form bonding and antibonding orbitals, or do they still do that?
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1answer
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Why is the active space for the dinitrogen cation defined this large in my example?

I have a Molpro script which computes $\ce{N2^+}$ potential energies. The significant part is the input for the wave function: ...
2
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1answer
44 views

Shape of the container and the number of possible energy states

A question came up when studying the formula for calculating the total number of possible energy states of a particle within a container. Why is the number of possible energy states independent of ...
2
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0answers
41 views

Slater Determinant in Python package

Is there any library functions which I could use in python to construct slater determinant? Can someone suggest me something to fix this issue ? Is it really possible to determinant with functions as ...
3
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1answer
89 views

Solving two electron integral numerically

Is there any software package in python or C to evaluate two electron integral repulsion integral in quantum chemistry of atoms. I am trying to solve the helium atom by constructing many body basis....
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1answer
93 views

What exactly is meant by 'multi-configurational' and 'multireference'?

Some topics here have touched on this before (see 1, 2, 3), but I haven't found a clear definition yet. I would like to know what exact property of the wave function these terms refer to. It would ...