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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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Critical point from ab initio calculations

I was wondering if it is possible to calculate the critical point (e.g., temperature, pressure, and density) other than via experiments or equations of state like ab initio calculations. Would anyone ...
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What can be inferred from overlap matrix S if coefficient matrix C is unitary in Roothaan equation?

I was going through the Hartree-Fock algorithm, transforming the spatial molecular orbitals {$\psi _i$} to be a linear combination of Roothaan basis set {$\phi_i$}. From my knowledge {$\psi_i$} is set ...
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Term symbols in transition metals and lanthanides

Hello I was researching a little about term symbols of lanthanides and transition metals and have got two questions. It is known that lanthanides don’t follow LS-coupling, but instead JJ-coupling. ...
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Hartree-Fock calculations of the states of Nitrogen molecule ion

I was reading the third chapter of "Modern Quantum Chemistry" by A. Szabo and N. Ostlund about HF calculations, and in the end of the chapter it shows as an example a calculation on $\ce{N2}$...
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Franck-Condon principle and shift of wavelength

I was revisiting my notes for spectroscopy, and I came across the Franck-Condon-principle. I understand it well, but want to clear some doubts regarding this. I know that the Franck-Condon-principle ...
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Does a free radical localized at one atom in a compound make sense or could it be?

In most of our cases, a dot that represents a radical is always considered affiliated to an atom in a molecule, especially when we talk about a reaction mechanism. However, Levine's Quantum Chemistry [...
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Spin-preserving excitations

I'm currently working on quantum chemistry with quantum machine learning and one of the operations considered for the calculation is the excitation (and desexcitation) of an electron from an orbital ...
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How can one obtain the same-spin and opposite-spin terms of the MP2 energy by integrating out the spin?

Just as specified on the psi4 website here: https://psicode.org/psi4manual/master/dfmp2.html, the Moller-Plesset Perturbation Theory states that the second order correction to the ground state energy, ...
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Reproducing Hydrogen Molecule Hamiltonian in OpenFermion

I am learning quantum chemistry at the moment and I'm trying to understand the Hamiltonian generated by the OpenFermion package. I'm now stuck at understanding how openfermion calculates the ...
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How to obtain reliable ligand binding energy in QM manner?

I wish to compute the binding energy of the ligand to the known cavity of the receptor. My idea was to: Perform docking to the recognized binding site using AutoDock Vina Export bound ligands and ...
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Rationalisation of stabilising dispersion interactions in HgS via QM

It is well-known that mercury(II) sulfide, HgS, is extremely inert- for example, it has one of the lowest solubilities of nearly all known compounds in water(check Wikipedia). There are two well-known ...
Kanghun Kim's user avatar
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How to calculate <ab|ab> integrals to carry out Schwarz pre-screening of molecular integrals

During writing my own two-electron integration library I faced an issue of dealing with integrals that have very small values beyond the C/C++ double data type. ...
Dmitry Govorov's user avatar
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Electronic vibrational rotational transition of electrons -- for a transition from $\Sigma$ to $\Pi$ how are P and R branches possible?

Suppose we have a diatomic molecule in a $^1\Sigma$ state, and it transitions to an excited $^1\Pi$ state. Note that the total spin of the electrons remains unchanged ($\Delta S=0$) as we can assume ...
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Computing full Electron-Nuclear Interaction Integrals using Obara-Saika Scheme?

Following my previous question Here, I had been trying to implement One and Two electron integrals in the code. I am currently stuck at the implementation of the OS Scheme, where I read the references ...
Hemanth Haridas's user avatar
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What are advantages and drawbacks of using different orders of Møller-Plesset perturbation theory?

I am about to do some research in computational chemistry, however, I do not understand one method: What is the difference between second order and third order, etc, Møller-Plesset perturbation theory?...
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Physical origin of induction and dispersion energies

I am reading this book on non-covalent interactions. Chapter 1 states that there are 3 types of non-covalent interactions: Electrostatic interactions, which are just the coulomb interactions between ...
simulation_engine's user avatar
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What are the wavefunctions for interacting electrons inside an atom?

The electron cloud for the single electron orbital $1s^1$ is radially symmetric in 3 dimensions. This is perhaps because of the probability distribution function (p.d.f.) $|\psi|^2 = ce^{-\frac{2r}{...
Awe Kumar Jha's user avatar
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Charge polarisation in acetamide molecule due to different lithium salts (varying anions)

I am trying to understand how will acetamide molecules behave in the presence of lithium salts with different anions (nitrate, bromide and perchlorate). Among the three, it is evident that bromide is ...
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In Kohn Sham DFT, what do we know theoretically about the exchange correlation energy?

As I understand Kohn Sham theory, it is proposed that the ground state energy for a molecule can be derived from the ground state of a fictitious set of non-interacting electrons moving in a ...
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How can I understand if my system has degenerate electronic configuration?

How can I understand if my system has degenerate electronic configuration? I'm running some post-HF methods and in the meanwhile try to understand which is the most accurate method for my system. ...
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Relationship between MO energies in Hartree-Fock theory and the core Hamiltonian

I am looking at Hartree-Fock (HF) theory as presented in Frank Jensen's book "Introduction to Computational Chemistry", Second edition. The total energy $E$ of the system can be expressed as ...
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Relation of partial charge with wavefunction

Suppose we have an one electron system. The probability $P$ that electron's position is in some interval $x + dx$ is given by: $$P = \int_x^{x+dx}|Ψ(x)|^2 dx$$ So if we measure $N$ times the electron'...
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Can electrons change principal quantum number upon excitation?

I wanted to know when can excitation of electron take place i.e excitation of electron only takes place between subshells having same principal quantum number or is it seen by difference in energy ...
Raghav Madan's user avatar
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Probability of finding electron in each orbital

This may include some quantum physics. Are there anyways to describe the wave function of Hydrogen's orbital (or that of a Hydrogen-like atom) in terms of the wave function of each orbital? You can ...
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Why are electrons filled in fourth shell before the third shell is filled?

I am a beginner and am learning about atomic structure and I am getting confusions regarding the aufbau principle. It is said that the electrons will be filled first in lowest available energy level. ...
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High symmetry points and x-coordinates

Is it possible to work out the x-coordinates related to high symmetry points? The software I'm using doesn't provide me with that, so I was wondering if there is a way to manually figure it out, as I ...
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Why is the energy levels in hydrogen independent of the quantum numbers l and ml?

I've been looking for an answer for this for quite a bit but I'm still quite confused. On the independence from ml, I think that it originates from the symmetry of the molecule. However, the ...
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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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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 ...
aerospace's user avatar
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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 ...
coconutmercury's user avatar
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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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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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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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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 ...
BactrianFan's user avatar
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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 ...
amilton moreira's user avatar
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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 ...
Tony Stark's user avatar
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Cross-conjugation and aromaticity in pyrene

I have read about non-huckel double bonds and clar's rule, but what made me confuse is that huckel's perception gives me the feeling that central bonds in polycyclic aromatic hydrocarbons (ex: pyrene) ...
mohamed's user avatar
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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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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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What is the link between the classical and quantum definition of the symmetry number?

From what I understand, the symmetry number for a molecule can be defined in 2 ways: 1. The quantum mechanical symmetry number corrects for overcounting the number of possible rotational states of a ...
Phy's user avatar
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Obtaining energy and wave function for excited states

Suppose I have done a HF calculation and obtain the energy and wave function of ground state. If I want to produce the spectrum of the specie (say Hydrogen molecule), should I use CI (configuration ...
Wisdom's user avatar
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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 ...
atbm's user avatar
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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 ...
Manu's user avatar
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What are the alternatives we have to the VEDA 4 vibrational energy distribution analysis software?

Said software is very useful when you need to assign the peaks in vibrational spectra to different vibrational modes found in a hessian calculation for a given molecule, as described in Jamróz, Michał ...
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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 $P(r)$, ...
amilton moreira's user avatar
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Is this approximate equivalence table between Pople's basis sets and Jensen's DFT attuned polarization consistent basis sets (pcseg-n) correct?

I have some calculations results I runned previously using Pople's basis sets, mostly 6-311+G(d), under Gaussian09. These days I read some texts on Frank Jensen's family of polarization consistent, ...
ksousa's user avatar
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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 $40^\circ\text{C}$). ...
Atom's user avatar
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A general definition of bond order in molecular orbital theory?

I stumbled upon this problem upon learning organic chemistry after learning the foundations of MO theory in physical chemistry. It seems that VBT and MOT can coexist with each other in many scenarios. ...
Macrophage's user avatar
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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 ...
Vivian Mortem's user avatar
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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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