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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8 views

Excited states using ORMAS flag in GAMESS [migrated]

I am trying to calculate the excited states using single reference CISD on $\ce{CH3OH},$ using ORMAS' CI-X option in GAMESS. As a test example I am using the $\ce{CH3OH}$ that has 2 core, 7 occ. ...
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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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Is the effective nuclear charge only defined for valence electrons?

I've just been taught about the effective nuclear charge and screening effect in school. I'd like to ask and confirm if the effective nuclear charge is only defined for valence electrons. If not then ...
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101 views

Oscillator Strength and Absorption Intensity in TDDFT

When I run a linear-response time-dependent DFT calculation in my favourite electronic structure program, for each excitation I'm given an oscillator strength, $f_i$, and transition dipole moment $$\...
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Using symmetry and group theory arguments to explain iron(II) in a tetrahedral crystal field

I am trying to figure out how to explain $1s \rightarrow 3d$ spectroscopic transitions for $\ce{Fe^{2+}}$ in $T_\mathrm{d}$ symmetry. These transitions make up the pre-edge region in K edge X-ray ...
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Calculating excited states using GAMESS CISD module [migrated]

I am trying to compare the energies of the excited states of $\ce{C12H9N}$ molecule using the CIS and CISD module implemented in the quantum chemistry package in GAMESS. While I can easily obtain the ...
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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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How can we explain relationship between two chemical structures using Quantum mechanics(data)?

for example, consider CH3-CH3 (eclipse) and it transformeds to CH3-CH3 (gauche) in molecular dynamics, we can explain structural transformation using Angle, bonds dihegral, and molecular energies. How ...
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Normalization of a free particle in 1 dimension

I am having difficulty normalizing the wavefunction for a free particle in 1 dimension. I have sent an attachment of my working out. My attempt at the normalisation Free electron: $$-\frac{\hbar^2}{2m}...
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Calculation of dispersion energies for organometallic complexes with Gaussian [migrated]

I've tried to calculate dispersion energies of some organometallic complexes with the Gaussian software using WB97XD/Gen (...
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Hund's rule for comparing term symbol energies in excited state

This question was asked in an exam: The lowest energy electronic state for excited state carbon atom is, a. $^1D_2$ b. $^3D_1$ c. $^3D_3$ d. $^3D_2$ Although we have been taught Hund's rule for ...
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How is kinetic energy accounted for when describing diatomic bonds with a Morse potential?

The Morse potential can be used to describe the potential energy of the a diatomic bond: This curve suggests that the lowest state $v = 0$ is the most stable, since it has the lowest potential energy....
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In half-filled atomic orbitals, why is spin +1/2?

From Hund's rule, we get that in half filled orbitals, all the electrons will have the same spin in their respective orbitals to maximize exchange energy. But, in all text books, the spin is given as +...
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Basics of wave-mechanical model of atom

Why is it that for the $\mathrm{d}$ subshell we have $\ \ \mathrm{d}_{xy}, \ \ \mathrm{d}_{yz}, \ \ \mathrm{d}_{xz}, \ \ \mathrm{d}_{x^{2}-y^{2}},$ and $\mathrm{d}_{z^{2}}$ orbitals only? Why aren't ...
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Why can't the energy of an electron exceed 0 eV?

Today we were learning about atomic structure, our teacher introduced the mathematics of the 'Niels Bohr Model of Atom', and calculated the value of total energy $\frac{-13.6z^2}{n^2} \pu{eV}$. He ...
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Splitting of atomic orbitals due to Stark effect

I am performing graduate level research on ZEKE Photoelectron Spectroscopy and central to this form of spectroscopy is Rydberg states and the Stark effect. The Stark effect is caused by an electric ...
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1answer
51 views

How to carry out basis set fitting and extrapolation

I am playing around with basis set extrapolations, and i would like to perform some simple tests by doing some parameter fitting and checking the errors. The functional form of the fitted curve is ...
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290 views

Schrödinger equation and degeneracy of atomic orbitals

How does the application of the Schrödinger equation to model a system, such as a particle in a box, help us understand the origin of the degeneracy of atomic orbitals?
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CCSD and CISD size consistency problem

I have performed a Gaussian16 calculation to explore the size-consistency problem for the CISD and CCSD methods. The system I am studying is the water dimer and the results are not what I expected at ...
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34 views

How do isotopes with different nuclear spins behave at temperatures near 0 Kelvin? [closed]

Suppose I had two isotopes of the same element, one with a spin of zero and another with a spin of 3/2. If I somehow put those isotopes at temperatures near 0K, how would those two isotopes behave ...
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1answer
105 views

How does the application of Schrödinger equation to model a particle in a box explain the origin of degeneracy of atomic orbitals?

Is the particle in a box concept analogous to an electron in an orbital? If so, can we apply the equation for the allowed energies of a particle in a box $$E = \frac{h^2n^2}{8mL^2}$$ to the allowed ...
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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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1answer
95 views

Does anyone know what program was used to make this molecular structure picture? [closed]

I am a quantum chemistry beginer, and this picture color scheme is classic. I wonder how to make such pictures by myself.
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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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TD-DFT transition dipole moment vector does not align with experimental data

I have been modelling excited states of some molecules with similar stucture using TD-DFT theory, however, I have encountered an issue that sometimes transition dipole moment does not align with the ...
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1answer
125 views

Potential Energy Curve from Hartree-Fock Calculations

I am currently working on some quantum chemical calculations, in particular looking at dissociation energy curves generated from Hartree-Fock, CI, CCSD and MP2 methods for diatomic molecules. Now, I ...
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56 views

why we can use the equipartition theorem for translational motion of molecules at room temperature and above because quantization is unimportant

From the book Chemical Principles The Quest for Insight, 5th Edition by Peter Atkins, Loretta Jones The equipartition theorem is a result from classical mechanics; so we can use it for translational ...
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Quantum numbers associated with electron

I wanted to get your opinion on something. I was given the electron 5p^3 and was told to find the quantum numbers associated with it. I got: n=5 l=1 Ml=1 Ms= +1/2 However, my teacher has told me that ...
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Is it true that the better the trial function, the better approximation for the energy in variational principle?

While studying the variational principle in McQuarrie's Quantum Chemistry, I came across the following problem: to relate the difference between an approximation $\phi$ and the exact ground-state wave ...
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36 views

Fluorescence quenching by palladium(II) vs. platinum(II) complexes

Why do we observe much stronger (in most cases complete) fluorescence quenching by $\ce{Pd^2+}$ than $\ce{Pt^2+}?$ I work with complexes of both metals and there is no real 100% explanation I can find ...
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Confusion regarding $\delta$ charge compared with electronegativity

I heard on a lecture of an online site that, let us say, we have two orbitals (1 and 2). Now, it will be like a spherical cloud. Now, what they say is that Now, we can notice that for orbital number ...
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Why do electrons have to constantly be moving? [closed]

As a pretense, my understanding of chemistry extends only to a high school honors level. As I know it, electrons occupy orbitals in which they could possibly be situated, and constantly move around ...
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How to determine the number of π-electrons in a conjugated system from UV-Vis data?

Is there a way to determine the number of π-electrons in a conjugated system from UV-Vis data? I am modelling the electrons as particles in an infinite box of length $L$ equal to the length of the ...
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Confusions regarding oscillator in black body and value of one quantum [closed]

Q 1 By saying oscillator I.e atoms in the wall of black body. Do we mean electron ? Because boundary of any atom is electron right. Q2 My textbook say that quantum is the smallest value of energy that ...
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Physical interpreation of coulomb and exchange integral

When trying to solve the Schrodinger equation for the electronic hamiltonian: $$H_{el} = \sum_{i=1}^{N} \bigg( - \frac{1}{2}\nabla_i^2 - \sum_A \frac{Z}{r_{i_A}} \bigg) + \sum_{i>j=1}^{N}\frac{1}{...
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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 ...
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26 views

Why aren't all the fluorescence transitions the same wavelength?

If all the excited molecules at higher S1 vibrational states rapidly relax to the lowest S1 vibrational state, why do the fluorescence transitions from v' = 1, v' = 2, v' = 3, and other states to v&...
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2answers
64 views

Relationship between Quantum Numbers and the Wave-function

I recently started learning about quantum mechanics and its applciations in atomic structure in chemistry. In this inorganic textbook Inorganic Chemistry, it describes "Each of the wavefunctions ...
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Reaction pathway - Extra molecule used in one step only?

I am currently building a reaction mechanism through Gaussian calculations. I successfully found each TS and intermediates through the different jobs (opt, freq, irc) for my 3-step mechanism from ...
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96 views

Why does having equivalent resonance structures give more stability (Related)?

This question is based on @Heisenberg's Question with the same title. I recently offered a bounty for a more detailed answer for the same question. These are the things I would like to be in the ...
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1answer
78 views

Classically permitted position values in a quantum harmonic oscillator [closed]

In a quantum harmonic oscillator that is in the state with quantum number $n$, what range of the position $x$ is allowed classically? Does it have something to do with Bohr's correspondence principle?...
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28 views

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 ...
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1answer
86 views

Schrodinger's Equation and Wave Function

So I understand that there exists the shrodinger's equation, which on solving,gives the wave function of an electron. The wave function as I understand, gives all possible information about an ...
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36 views

E1 selection rules for transitions between two Σ states

For E1 selection rules (embodied in the electronic transition moment), to have an allowed transition the direct product of irreducible representations (IRs) must contain the totally symmetric IR. So, ...
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3answers
348 views

Can cis-trans isomers interconvert?

Can cis-trans isomers interconvert via tunneling ?
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1answer
34 views

Gaussian calculation - PCM model for IRC compared to gas phase?

Finding a reaction pathway thanks to Gaussian calculations is tricky exercise requiring a lot of resources for each step of this time-consuming process. Thus, man needs to wonder if each calculation ...
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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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1answer
66 views

How can I get analytical form of GTO primitives for a atom?

As I know, for Hydrogen atom a $1s$ Slater Type Orbital (STO) equation is (I can get it from here): \begin{equation}\label{STO} \mathrm{STO} = \sqrt{\frac{\zeta^3}{\pi}} e^{-\zeta r}. \end{...
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1answer
202 views

ORCA: How to plot an adiabatic potential in dihydrogen H2 molecule?

I stared to study ORCA, and I try to obtain classical results for dihydrogen as for example. I need to get a starting point to understand what needs to be done. So, How to plot an adiabatic potential ...
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32 views

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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