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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Mesomeric effect through quantum chemistry

How can you show that mesomeric effect doesnt act on the meta position in a substituted benzene ring through a quantum persepective
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Effective nuclear charge

To the best of my knowledge, the effective nuclear charge $Z_{eff}$ is usually defined such that the potential energy of an electron in an atom, $⟨V⟩$, can be expressed as $⟨V⟩= -Z_{eff}⟨ψ|\frac{1}{|...
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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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48 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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Can you explain DFT and TDDFT functioning (math aside)?

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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52 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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36 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 ...
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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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37 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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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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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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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 ...
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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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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 ...
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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 ...
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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 ...
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51 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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59 views

How to understand the (non-)degeneracy of states of singlet Σ and Δ terms for a ππ configuration of two nonequivalent electrons?

As instructed in Levine's Quantum Chemistry 7th edition, Chapter 13.8, page 377, for a $\pi \pi$ configuration of two nonequivalent electrons, four functions $\pi_{+1}(1) \pi_{-1}^{\prime}(2)$, $\pi_{+...
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Why I can't use a single Slater determinant to describe the ground state of open-shell atoms and molecules?

If we have a closed-shell atom or molecule (in the Born-Oppenheimer aproximation), and adopt the central field aproximation, we can write the ground state wave function of the system using a Slater ...
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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. ...
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Perturbational Molecular Orbital Theory - How to derive the results?

I am reading Albright's Orbital Interactions in chemistry. The qualitatively perturbational treatment of molecular orbitals seem to be a central tool used throughout the book. I've referred to the ...
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Reaction energies of solids from quantum chemistry?

I was reading a paper by Catlow et al. [1], and they mentioned the dissolution of copper into copper ions, which is given by this equation: $$\ce{Cu2O(s) + 2 h+ <=> Cu+(aq) + 1/2 O2(g)} \tag{...
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Has a concept of temperature ever been defined in the context of a single atom?

I wonder if this answer to What would happen if we supercool and then superheat an atom very abruptly? goes far enough. I almost wrote the comment: I don't think we can even talk about the ...
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How do molecules with muons and electrons bond?

Is bonding specifically for and between electrons? Why cant two atoms share muons, which are different particles of same charge, spin and different mass? Why there aren't muon-electron bonds? Why is ...
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Have IH₃ and IH₅ already been synthesized?

I've seen a paper by Sikalov [1] that says that $\ce{IH3}$ and $\ce{IH5}$ should be observable. Has anyone actually tried to synthesize such molecules? References Sikalov, A. A. Hypervalent Halogen ...
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Is there a rule of thumb to predict when coupled cluster might dip below the variational limit?

Coupled cluster is a non-variational method, meaning that it can give energies that are below the true FCI energy (the "variational limit" for variational methods). Often coupled cluster still gives ...
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Hartree-Fock vs. DFT vs. Hohenberg-Kohn vs. Kohn-Sham

What is the difference between these four approaches? My current understanding is that Hartree-Fock uses only 1 slater-determine (accounting for antisymmetry) but neglects correlation between ...
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1answer
61 views

Why don't molecular orbitals change when electrons are added or removed from a molecule?

My lecturer says that molecular orbitals don't change when electrons are added or removed from a molecule, but I don't understand why. As electrons are described by wave functions, surely more or ...
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68 views

Atomic Orbital and Wave Function

I was reading about the atomic orbital in my chemistry textbook. It says that the atomic orbital (psi) is a mathematical wave function that depends on the coordinates of the electron. It is also ...
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Tight binding model with Extended Huckel Approximation

I've been reading Albright's Orbital Interactions in Chemistry. In the chapter on solids, he provided a general approach to find the band structure of a solid state system Now if we are to model a ...
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Magnitude of Coulombic integral of atomic orbitals

I'm wondering anyone knows what's the typical magnitude for Coulombic integral of atomic orbitals in large molecules (not simple diatomics). Also, is it right to say that the integral has a greater ...
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What is the physical significance of the fact that spin operators do not commute? [closed]

So we looked at the Stern-Gerlach experiment and I understand the basic principle; when a stream of silver atoms is passed through an inhomogeneous magnetic field, and the Ag atoms are deflected ...
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1answer
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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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1answer
56 views

Rotational Energy Levels

Why does the rotational constant B decrease and transition spacings decrease as the mass of a particle increase? I understand from a purely equation perspective that since $$B = \frac{h} {8\pi ^2 ...
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1answer
48 views

What does ³D mean in energy level diagram for helium?

I'm coming from a Physics background and am trying to make sense of the energy level diagram for para-helium and ortho-helium. From what I've gathered each column shows the different energy levels ...
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1answer
88 views

Difference between Particle in a Box and Harmonic Oscillator approximations

Why in the particle in a box model do the values of n begin at 1 but in the harmonic oscillator they begin at 0? I understand what the wave-functions and their corresponding probabilities look like ...
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Hybrid orbitals, photoionization and time dependence

Wikipedia, in the article on Orbital Hybridisation, states the following: One misconception concerning orbital hybridisation is that it incorrectly predicts the ultraviolet photoelectron spectra of ...
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What is the difference between dynamical and non-dynamical correlation and which one is accounted for in Hartree-Fock theory? [duplicate]

It is my understanding that dynamical is related to the movement of individual electrons. I don't fully understand what this means though. Non-dynamical correlation is still very unclear from my ...
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1answer
38 views

Why can a wave-function be written as a product of 1-electron wave-functions?

Why can a wavefunction be written as a product of 1-electron wave-functions when making approximations to solve atoms other than hydrogen? (ie. helium) We used this when solving multielectron systems ...
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What is J in the rigid rotor model?

In my class notes regarding the rigid rotor model, J is described as simply the angular momentum (replacing roe or p) and is also referred to as the angular momentum quantum number. Are these two ...
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1answer
54 views

What all information can be gathered qualitatively from the radial distribution functions of a molecule?

Apart from knowing the radius of coordination shells and the probability of finding, is there any other thermodynamic information we can gather by looking at radial distribution functions of a ...
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Reason for the generation of London Dispersion Forces

While talking about gaseous state of matter we came to the topic of London Dispersion Forces while are caused by the generation of dipole in one atom which induces a dipole in another. While talking ...
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Radicals in Gaussian [closed]

I want to do a PES scan of a radical structure in Gaussian. However, I'm not completely sure how to prepare an input file. I've figured out that charge and multiplicity are 0 and 2. What I don't know ...
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2answers
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Perturbation Theory in Orbital Interactions in Chemistry

In the entire book, perturbation theory is used as a qualitative tool to rationalise some chemical phenomena. The authors write that $$\psi_i = \sum_\mu{T_{ji}\psi_j^\circ}\tag{1}$$ The proof of the ...
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1answer
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Schrödinger wave function for 3s orbital of hydrogen atom [closed]

I am unable to understand what these terms in the wave function represent and how to solve it. For 3s orbital of hydrogen atom, the normalised wave function is $$Ψ_\mathrm{3s} = \frac{1}{81\...
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1answer
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High errors in frontier orbital energy calculation using DFT

In a recent paper by Faber et . al., Mean Absolute Error and DFT (B3LYP) error (relative to the experiments) for 9 properties of QM9 dataset molecules has been reported. \begin{equation} \begin{array}...
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Chemical Anti-bonding and Pauli repulsion [closed]

We can explain chemical anti-bonding just using the Pauli repulsion correct?Let's take He2. 2 atoms of He share 4 1s electrons and since the magnetic spin for electrons has 2 values there would be 2 ...
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1answer
153 views

Energies of Virtual Molecular Orbitals (LUMO)

I just had a question about molecular orbitals. I was just wondering if the energies of the LUMO (virtual orbitals) derived from CCSD(T)/cc-pTVZ calculations could be trusted. I remember reading some ...
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What is the justification for using the Marcus rate equation to model charge transport in organic semiconductors?

I am trying to understand the history of how Marcus theory moved from describing electron exchange reactions in aqueous ionic solutions to also being applied to describe transport in organic ...
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51 views

Theoretically predict the density of water

Is it possible to predict the density of water having just its molecular structure (basically obtain 1 g/cm3 at 3.96 °C)? Are you aware of research/textbook that dive into this topic? Is it able to ...