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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25 views

How to generate accurate displacements along normal modes? [closed]

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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248 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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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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32 views

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
88 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
83 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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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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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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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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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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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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3answers
104 views

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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34 views

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
61 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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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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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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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
342 views

Can cis-trans isomers interconvert?

Can cis-trans isomers interconvert via tunneling ?
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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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23 views

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
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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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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1answer
55 views

What's the meaning of wavefunction being finite?

I know ,in quantum mechanics it's important that the wavefunction has to be finite everywhere , including infinity. It's a criteria for acceptable wavefunction . What exactly does this statement mean ?...
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1answer
34 views

Radial wave-functions and spherical harmonics of p-orbitals in a certain hypothesis

I know that the s-orbitals of H atom are spherically symmetrical. But the p,d or f orbitals aren't. So, the H-atom needn't necessarily be spherically symmetrical, as the wave-function would be a ...
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65 views

Why is the radial distribution at the nucleus 0 but not the radial wave function?

The modulus squared of the radial wave function gives the probability of finding an electron in an infinitesimal volume dv. On the other hand, the radial distribution gives the probability of finding ...
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What common observations in chemistry were unexplainable by an early twentieth century chemistry who had no knowledge of relativity

I was reading quite a well-known paper titled 'Relativistic Effects in Structural Chemistry' by PEKKA PYYKKO, and in it, he states that 'it was not until the 1970s that the full relevance of ...
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1answer
61 views

Why are s orbitals more “penetrating” than p orbitals?

In polyelectronic atoms, the reduction in the net central force due to electron-electron repulsion is accounted for through an effective nuclear charge that depends on a “shielding effect” of inner ...
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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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1answer
94 views

Is the pz orbital wave function three- or four-dimensional?

The wave function of the $2\mathrm{p}_z$ orbital is $$Ψ = \frac{1}{4\sqrt{2π}}\left(\frac Z a\right)^{5/2} r \mathrm e^{-Zr/a}\cos θ.$$ I'm confused if this function will be a three-dimensional ...
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1answer
40 views

Quantum Espresso, phonon band dispersion

I am having a trouble calculating the phonon band dispersion of graphene at $K$-point using Quantum-Espresso, using normconserving+LDA and normconserving+PBE, ecutwfs=60, Ecutrho=400.0, forc_conv_thr=...
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1answer
59 views

The quantum mechanics behind periodicity of elements

Especially in high school/first-year undergraduate chemistry courses, we learn with great dedication the periodicities along groups and periods. There are various useful and interesting trends. I ...
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32 views

Potential energy expectation value and virial theorem

I have been trying to solve this problem Part a) can be proved using the formula . For part b), the expectation value of the potential energy can be calculated from the integral , where Therefore, ...

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