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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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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51 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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55 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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12 views

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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37 views

What is the physical significance of the fact that spin operators do not commute?

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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Outline the steps involved in the Hartree–Fock method for the calculation of molecular electronic structure [duplicate]

I understand that the steps of the self-consistent method are as follows: guess the form of Psi(r) use psi(r) to evaluate for V1eff(r1) then solve for a new psi(r) the two wavefunctions tend to ...
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53 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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52 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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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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79 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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Can Electronegativity be explained through QM? [closed]

Even as senior in high school, I still find chemistry quite intimidating, partly due to how vast it is and partly because I don't understand when something stated in Chemistry is experimentally ...
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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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36 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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431 views

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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How to calcualte the quantum defects of S, P, and D terms for a Li atom? [closed]

I’m doing my best but still have a problem with understanding these things. I have to calculate the quantum defects of S,P, and D terms for a Li atom. I know that the binding energy of the valence ...
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21 views

Valance bond THEORY [duplicate]

The shape of a molecule was determined by Valence Shell Electron Pair Repulsion (VSEPR) theory previously before Valence Bond (VB) theory in terms of electron pair repulsions. But how does Valence ...
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1answer
51 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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Why isn't the total energy of an atom the sum of the orbital energies - what is I1 and I2?

We were told that the total energy of a helium atom is not the sum of its orbital energies and that this is Koopman's theorem. This sortof makes sense but I am struggling with contextualizing it. i....
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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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1answer
36 views

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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High temperature acids

Some of the proton production of an acid happens due to temperature and some of the proton production of an acid happens due to quantum tunnelling.But if we increase the temperature a lot the proton ...
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1answer
56 views

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
90 views

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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1answer
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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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151 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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50 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 ...
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24 views

Order of coordinates in a wavefunction - why do they matter?

For the past few weeks, I have been studying Quantum Chemistry and lately in these lectures something has bugged me: If I do a sum: $1 \times 2 \times 3 \times {...} $ This is the same as simply ...
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Is there a way to extract the diamagnetic and paramagentic components from NMR output from Gaussian16?

I'd like to be able to split the NMR value in Gaussian16 into its diamagnetic and paramagnetic terms. I'm coming over from ADF, where this can be done easily. Is there a straightforward command to ...
2
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2answers
73 views

Why does heating an atom make it emit certain frequencies? [closed]

We're going over quantum basics in chemistry right now and I'm very confused. Electrons can only accept in discreet quanta to move up an energy level, right? And they reflect other forms of light ...
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1answer
67 views

Decomposition of total Interaction energy with SAPT or EDA-NOCV

I want to perform energy decomposition analysis for some Hydrogen bonded complexes. I came to know that ORCA and Psi4 allows EDA-NOCV an SAPT(0,2,3) respectively. But I do not know which one would be ...
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1answer
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On mathematics behind molecular orbital energies

When explaining the mathematics behind calculating molecular orbital energy , namely HOMO and LUMO energies, using quantum chemistry, Chainer Chemistry documentation states that From mathematical ...
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Why $ns^2np^6$configuration so stable? [duplicate]

In my chemistry textbook it is said that noble gases have stable configuration because they have achieved the maximum outermost shell configuration, which corresponds to the $ns^2np^6$ configuration. ...
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Do I understand correctly that simulation volume influences relative permittivity calculations? If so, how is the result useful?

I am having a lot of trouble understanding the influence of volume on relative permittivity/static dielectric constant. I'm not a chemist, but am using molecular dynamics to calculate relative ...
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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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28 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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1answer
98 views

Generating molecular graph Laplacian based on Hückel theory

While studying graphs and graph Laplacians from "Laplacian eigenvectors of graphs: Perron-Frobenius and Faber-Krahn type theorems.", I encountered a type of graph Laplacians that rise from quantum ...
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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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29 views

How do these restrictions on the wavefunction logically arise?

My lecturer notes that the following restrictions on the wavefunction (in addition to it being a solution to the Schrodinger equation) are logical: i) Continuous (mathematical definition of a ...
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39 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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How long would it take for a tank of same-spin hydrogen atoms to become a tank of H₂?

In the question Is there an energy cost associated with flipping the spin of an electron?, it is shown that it is very unlikely for two hydrogen atoms to bond if their electrons have the same spin. ...
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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
55 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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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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234 views

Is the correlation energy of a system always negative?

The correlation energy of a system is defined as the difference between the exactly energy and the energy in the Hartree-Fock method: $E_\mathrm{cor} = E - E_\mathrm{HF}$. In the case of an atom or a ...
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Is there any publicly available software that can do deterministic CCSDTQP other than MRCC?

MRCC can do arbitrary order coupled cluster but much of the code is computer-generated (not hand-coded by a human) so it can be slow. There are some stochastic codes for high-order coupled cluster, ...
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1answer
112 views

On diagonal terms in the Coulomb matrix

I have seen many machine learning algorithms for prediction of quantum chemistry properties that use Coulomb matrix as their input. Coulomb matrix is defined as, $$\boldsymbol{M}_{i j}^{\mathrm{...
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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 ...