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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23 views

How to generate accurate displacements along normal modes?

How can i displace my structure along a particular normal mode, without coupling to other modes ? Rather than a exact mathematical procedure, I am more interested in a software/script/package that can ...
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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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65 views

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 ...
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245 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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1answer
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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What do the boxes in an orbital diagram represent? [closed]

Now, online it says that s,p,d and f represent different orbitals. Ok, but when I searched what the boxes meant in the diagram for that orbital, I found out that each box is also an orbital with ...
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Why do both bonding and antibonding molecular orbitals exist at the same time when two waveforms could only be in-phase or out-of-phase? [closed]

I am quite confused concerning the existence of both bonding and antibonding molecular orbitals. I understand from previous posts that the number of orbitals has to be conserved, and thus when two ...
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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
86 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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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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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
105 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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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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Why is 2s more stable than 2p, if 2p experiences greater effective nuclear charge?

I'm referring specifically to this graph, which appeared in a chemistry lecture but which the lecturer could not explain. We know that 2s is lower energy than 2p. But surely those electrons in 2p ...
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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
328 views

CI without Hartree-Fock SCF

Usually textbooks on CI takes the SCF Hartree-Fock orbitals as a starting point. Can one use the configuration interaction method starting directly from the single-particle Slater determinants ...
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342 views

Can cis-trans isomers interconvert?

Can cis-trans isomers interconvert via tunneling ?
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What does Brightness/ intensity of spectral lines depend on?

My book states that it depends upon "The number of photons of same frequency or wavelength ABSORBED or EMITTED" From what I understood, if many hydrogen atoms within the discharge tube EMIT the same ...
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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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1answer
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 reasonable to use natural orbitals of an unrestricted HF calculation as a basis to start a CASSCF calculation for a radical

Let's say we want to run a CASSCF calculation of a radical, for example the cation of a neutral closed-shell molecule (therefore, we calculate an open-shell radical cation). MOLCAS, as well as ...
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How to determine occupied and closed orbitals for a Molpro CASSCF calculation? [migrated]

I am currently working with Molpro, to which I am completely new, trying to run some calculations on $\ce{PdO}$. From literature I have gathered that the ground state is given by $^3\Sigma^-$ state, ...
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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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69 views

Examples for oscillatory behaviour in the human body [closed]

I'm looking for examples in the human body which show periodicity, more precisely, I'm looking for examples of oscillating reactions that occur in the human body which can be modelled using the ...
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1answer
320 views

Mechanistic Explanation of Circular Dichroism

I'm wondering if someone can give me a physically motivated reason for why chiral molecules interact with left and right circularly polarized light (LCP) (RCP) differently. Particularly, a quantum ...
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2answers
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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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3answers
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Can hot food ever emit x-rays or gamma rays?

I was just wondering, if heating food up is the result of increasing the energy of bends and stretches in the bonds of the molecules, is it ever possible for tiny amounts of x-rays and gamma rays be ...
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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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2answers
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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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1answer
59 views

Calculating the zero-point energy for the C2N2 molecule

I shall calculate the zero-point energy for the $\ce{C2N2}$ molecule in the $^{1}\Sigma_{g}^{+}$ electron ground state with the following vibrational wavenumbers (symmetries in parentheses) which were ...
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1answer
93 views

Solution of the Roothaan Equations of H2 by Symmetry Arguments

FWIW my background is in physics and maths, but I am just starting a chemistry PhD (the last time I took a chemistry class was high school). I have only some background in representation theory, and ...
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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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95 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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35 views

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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3answers
2k views

Convergence issue in Gaussian

I have been struggling with geometry optimization of a prostaglandin molecule (20 carbon atoms). I finally got the optimization, all right. Then I did the Frequency job on the optimized structure (I ...
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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
65 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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1answer
275 views

Why does the d orbital size decreases on addition of electrons?

In Concise Inorganic Chemistry by JD Lee (4th edition; adapted by Sudarshan Guha), on page 80 under section 3.7 "The Extent of d-orbital Participation in Molecular Bonding" it is given: A second ...
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1answer
43 views

Could observer effect cause electrons in one orbital to be found in another?

The observer effect states [1] that when unobserved, absolutely small particles like electrons can simultaneously be in two different states at the same time. If we look at an atom of any element, ...
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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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1answer
30 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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59 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
154 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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