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 $(\frac{d}{dr^2}+f(r)-\epsilon)P(r)=g(r) \tag1$ Usually algorithms to solve this equation assumes that the number of nodes of $P(r)$,$n_r$,is of ...
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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, ...
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Variation of intensity with temperature [closed]

My book reads that the only factor on which intensity and frequency depend is temperature intensity of radiation emitted increases with decrease of wavelength reaches maximum value and then starts ...
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Where can I find experimental IR wavenumbers of molecules? [closed]

I made a comprehensive research by using Google Scholar, but could not find anything. For example, experimental IR wavenumbers of H2, CO, HCl, etc.
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Are there Exchange integrals in Semiempirical methods?

One of the major features of semiempirical methods is that they significantly reduce the cost of evaluating 2-electron integrals. In general, they follow the formula for HF, but, introduce many ...
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How much the computation chemistry can predict the stability and properties of the (metallo-)organic molecules? [closed]

I have this vision for the pandemia preparedness platform (aka CERN/ITERA/Mars mission for preparedness) https://biology.stackexchange.com/questions/91025/can-we-prepare-for-pandemics-by-predicting-...
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State of matter, varying pressure and volume we see a drastic change in solids, as we see in gases and liquids? [closed]

Imagine a solid of a single dimension, to obtain its heat capacity at constant volume $C_p=\frac{\partial U}{\partial T}$ and the constant pressure $C_v=\frac{\partial H}{\partial T}$. It is known ...
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Does the hybridization of sp3d2 type exist?

Is the hybridization of $\ce{XeOF4}$ $\mathrm{sp^2}$ rather than $\mathrm{sp^3d^2}$, because the latter type of hybridization does not exist? Every answer in the Quora thread "How can we find the ...
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QTAIM's ZFS vs COVID-19 [closed]

Why doesn't QTAIM's zero-flux surface (ZFS) of the ACE2 membrane protein or the S(pike) protein of the COVID-19 virus suppress host infection?
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Are there any semiempirical methods that work with heavy alkaline earth metals?

I did some research. Found some semiempirical methods that work even with heavy lanthanide compounds (RM1). But could not find any references to semiempirical methods compatible with the heavier ...
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How to reproduce the Gaussian 09 SDD basis set in Gamess-US?

I'm trying to migrate away from Gaussian 09, using Gamess-US instead for molecular modelling of some heavy alkaline earth metal complexes, using density functional theory (DFT). In some of my previous ...
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1answer
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Are Hartree-Fock solutions eigenfunctions of the electronic hamiltonian?

$\newcommand{\Ket}[1]{\left|#1\right>}$ $\newcommand{\Bra}[1]{\left<#1\right|}$ I'm currently revising for my qualifiers, and I've come across a point of confusion, one that several places give ...
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238 views

Total spin for lithium atom

Why does the ground state of a Li atom have a total spin of S = 1/2? I get that the total z component of the spin has to be either Ms = 1/2 or Ms = -1/2 because of the Pauli exclusion principle, but ...
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Spin Operator algebra

I am trying to teach myself some QM. In Christopher J. Cramers textbook Essentials of Computational Chemistry: theory and models, in Appendix C, he goes over Spin algebra. I am unable to calculate ...
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141 views

What exactly are the graphs of orbitals?

In my QM class we are finding the wave functions of the hydrogen atom. In spherical coordinates, these wave functions are functions of 3 variables: $r,$ $\theta,$ and $\phi.$ My professor stated that "...
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4answers
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Software recommendations for overlaying molecular structures [closed]

I am looking for a program to overlay the structure of small molecules as an aid for my students in comparing the results of optimization geometries using different optimization methods. At this ...
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Spin-Adapted Configuration and Slaters determinants for Open- and Closed-shell systems

Some time ago, I asked a question about spin-adapted Configuration State Functions. I read that, when we ignore the electronic repulsion in a atom or a molecule, we can write the ground state using a ...
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1answer
32 views

Normal modes of vibration [closed]

Why Normal modes of vibration are important when compared to translational and rotation modes. And why the frequencies for rotational and translational modes are close to zero?
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255 views

Computing the commutator of the Fock and density matrices in AO basis (Hartree-Fock theory)

I am making a simple restricted HF code using the Python interface of Psi4. I am now evaluating convergence by tracking the change in the sum of orbital energies, but I want to do this in a better way....
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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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99 views

What is d-orbital collapse?

When I was perusing the works of Schwarz on atomic structure, I came across the unfamiliar term of d-orbital collapse. He describes it as a variation in energetic sequence from group 1 to 3 elements ...
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Is Tkatchenko's MBD-NL method self-consistent?

I have a question regarding Tkatchenko's [1] new nonlocal many-body dispersion method (MBD-NL). Does anyone know if it's a self-consistent method? I read this paper many times but I'm still not sure. ...
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1answer
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How to access implicit bond angles that don't appear in Gaussian's z-matrix input?

Suppose I have a Gaussian script to calculate the energy of a relatively large molecule like hexamethyldisiloxane: ...
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1answer
47 views

Frequency region and “uncoupled oscillators” in vibrational spectroscopy

I am currently studying the textbook Infrared and Raman Spectroscopy by Larkin. In the first chapter of the textbook, the author presents the following table and accompanying explanation: The ...
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1answer
128 views

Hessian Matrix and Physical directions: Potential Energy Surface

How do the eigenvectors of the Hessian Matrix for potential energy surface correspond directly to the physical directions of translation, rotation and vibration? I see that they might correspond to ...
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2answers
63 views

Confusion in half filled or full filled electronic configuration

At the end of electronic configuration, we were taught that, electron orbitals are most stable when they are either fully filled or half filled. E.g., the final valence configuration of chromium is $\...
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1answer
140 views

How to obtain curve energy and non-bonded interaction parameters (e.g. Lennard Jones parameters) with Gaussian?

I am parameterizing a force field and would need to obtain non-bonded interaction parameters of atoms (e.g., Lennard-Jones parameters). My primary idea is to obtain a potential energy curve using the ...
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1answer
54 views

Does each proton have a uniquely associated electron? [closed]

Say you have a hydrogen atom. Would the proton of the atom and the electron of the atom, in some way, have some criteria to be uniquely associated with each other? This confusion for me occurs when I ...
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1answer
85 views

Beer's law: UV-Vis absorbance values exceeding 2

In classical analytical chemistry, absorbance values in Beer's law can theoretically range from 0 to infinity. As general rule of thumb neither high, nor very low absorbance are used for ...
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41 views

Why does Larmor precession occur in NMR?

Larmor precession is the phenomenon that in NMR the spin (and by default also the magnetic dipole moment) of protons does not line up with the applied magnetic field but rather precesses around this ...
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1answer
78 views

Estimation of ground state energy from wave function [closed]

Given the wave functions of a particle in a 1D box below determine (for each pair) if the left or right function gives a better estimation of the ground state energy if the variational method is used. ...
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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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Observing bands which I shouldn't be!

I performed the iodine spectrum experiment to observe vibronic transitions. I heated the iodine crystals to produce vapors (at a little above room temperature) and observed absorption spectrum with a ...
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44 views

Is there an anisotropy factor (g factor) for TDDFT Abs and CD calculations?

Experimentally, anisotropy factor is calculated by dividing the CD spectra by the absorbance spectra multiplying by a factor of 32980 (in order to get a nondimensional quantity) Theoretical ...
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2answers
113 views

Effective nuclear charge and electron repulsion

To the best of my knowledge, the effective nuclear charge $Z_\mathrm{eff}$ is usually defined such that the potential energy of an electron in an atom, $\langle V \rangle$, can be expressed as $$\...
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2answers
89 views

What are Degenerate Rotational Energy Levels Physically?

I'm in introductory chem, so I don't have much of the physical background to really understand what's going on yet, but we're learning about rotational energy levels, and how some rotational energy ...
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1answer
66 views

What is the effect of basis set on charge transfer?

I am calculating the energy and charge transfer of a globally neutral system, composed by two opposely charged monomers (EMIM-BF4), coming closer to each other, with DFT (B3LYP) and MP2. As a thumb ...
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79 views

Can you explain DFT and TDDFT functioning (math aside)? [closed]

I have been recently reading a lot on the quantum mechanical theory regarding Density Functional Theory, DFT, and Time Dependent Density Functional Theory, TDDFT (Oscillatory and Rotatory Strengths in ...
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1answer
73 views

How to perform Mulliken population analysis by hand with GAMESS output?

I did a simple calculation on hydrogen fluoride with GAMESS and I wanted to reproduce the Mulliken population analysis for atomic orbitals. A molecular orbital is a linear combination of atomic ...
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1answer
43 views

On the meaning of distinguishability, and wavefunctions for 3 electron atoms

In a 2-electron atom at lowest energy, the $(1s)^2$ is occupied and the electronic wave-function must satisfy anti-symmetry requirements in the particle coordinates, as the spatial wave function is ...
5
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1answer
156 views

Optimization completed on the basis of negligible forces

I am performing relaxed scan in Gaussian (B3LYP/6-31G(d,p)). There are two small organic molecules, one of which is a radical, the other has a benzene ring. I'm trying to understand the optimization ...
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1answer
44 views

exchange integral at large distances

I want to ask a question about the exchange integral $\ce{H_{AB}}$ - If I consider an exchange integral $\ce{H_{AB}}$ the following exchange integral can be evaluated: so $\ce{S_{AB}}$ is $0$ at $\...
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1answer
34 views

Photoelectric effect with 0 Kinetic Energy?

Does the photoelectric effect take place if the frequency of the photon is exactly equal to the threshold frequency of the metal? I know the electron is going to be free, but it will have 0 kinetic ...
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1answer
47 views

Do all single-electron species have the same orbital energies? [closed]

This picture from Ebbing's General Chemistry [1, p. 234] shows the electron orbital energies for a hydrogen atom, where each subshell belonging to the same principal quantum number ($n = 1, 2, \cdots$)...
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1answer
80 views

Does the Fermi level correspond to the highest occupied energy level?

I know that, in the band structure of a solid, the Fermi level is the energy level at which there is a 50% probability of finding an electron (at thermodynamic equilibrium), and that it doesn't have ...
3
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1answer
63 views

How to obtain the radial probability distribution function of a given orbital from a quantum chemical calculation?

I know that a very similar question has already been asked here and a very competent and useful answer has been provided (see here). However, that previous question was related to the total electron ...
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0answers
48 views

GAMESS(US) Polarizable Continuum Model frequency calculations are giving me what seems to be garbage

I'm trying to run Polarizable Continuum Model (PCM) calculations using the GAMESS(US) software. I'm not that well versed in what exactly these calculations are doing (beyond the whole, 'gas within a ...
2
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0answers
22 views

Carbon dioxide electronic geometry [duplicate]

It is known that allene (propadiene) $\ce{H2C=C=CH2}$ geometry is such that its hydrogen atoms are not in the same plane – do not form rectangle, but “extended tetrahedron” I wonder if structurally ...
3
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2answers
64 views

Electron Ionization and the Franck-Condon Diagram: vibrationally excited and vibrationally ground states

I am currently studying Mass Spectrometry: A Textbook, third edition, by Jürgen H. Gross. On page 41, the author says the following: As explained by the Franck-Condon diagram, hardly any molecular ...
2
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2answers
58 views

Sum of spins of electrons in a wavefunction

I wanted to ask a question about spin in an electron. We were presented earlier today with the following information: Using my synoptic knowledge, I recall that spin up $\alpha$ had a value of $+\...

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