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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An atom moving at its root-mean-square speed at 20 °C has a wavelength of 32.8 pm. Identify the atom

An atom moving at its root-mean-square speed at $\pu{20 °C}$ has a wavelength of $3.28×10^{-11}$ m. Identify the atom. Here's what I did-- $$u_{rms}=\sqrt{\frac{3RT}{M}}$$and $$\lambda=\frac{h}{mu_{...
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How would an aromatic current behave in a graphene quantum dot?

Aromatic current in macrocyclic compounds is always planar to the aromatic structure, but in a graphene quantum dot, the structure would no longer be one dimensional. Would this affect the aromatic ...
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How to know the wavelength if the number of spectral emission lines is given? [closed]

A sample tube consisted of atomic hydrogen in their ground state. A student illuminated the atoms with monochromatic light, i.e. light of a single wavelength. If only two spectral emission lines in ...
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Is the answer of this question wrong? [closed]

Today I stumbled upon a question which I found here. The following question given below consist of an Assertion and Reason type questions. Use the following key choose the appropriate answer. ...
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What does valence electron mean (in the context of spdf orbitals)?

For a little bit of context, my background is in physics and my understanding of chemistry doesn't go past, say, middle school level. Recently, I decided to self-study chemistry and picked up "...
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53 views

What are orbitals? Am I correct? [closed]

I was having trouble to understand what orbitals are. What I have understood I have written below. For H atom at $n=1$, the electron can be anywhere, but at an instant $t$, the most probable location ...
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How do we actually calculate attraction and repulsion while explains bond length? [closed]

I am a high school student and in my textbook there is a figure given in which two isolated hydrogen atoms approaches each other and we define bond length as the distance between their nuclei at ...
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20 views

How can I understand if my system has degenerate electronic configuration?

How can I understand if my system has degenerate electronic configuration? I'm running some post-HF methods and in the meanwhile try to understand which is the most accurate method for my system. ...
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Relationship between MO energies in Hartree-Fock theory and the core Hamiltonian

I am looking at Hartree-Fock (HF) theory as presented in Frank Jensen's book "Introduction to Computational Chemistry", Second edition. The total energy $E$ of the system can be expressed as ...
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8 views

Vibrational frequencies results with GAMESS. Is it an equilibrium geometry? [migrated]

A specific HESSIAN calculation carried out with GAMESS software presents this result: WARNING, MODE 7 HAS BEEN CHOSEN AS A VIBRATION WHILE MODE 1 IS ASSUMED TO BE A TRANSLATION/ROTATION. PLEASE ...
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What exactly is a photon gas?

I came upon this question in a JEE Advanced mock test online: The number of photons of wavelength $\lambda$ required to achieve pressure $P$ in an empty cubical box of edge length $l$ is given by $\...
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What would happen if electrons were spin-1?

We were speaking about this in class, but I can't understand it quite well. What would happen if (hypothetically of course) the allowed $m_s$ values were $-1$, $0$ and $1$? What impact does this have ...
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Calculations of the charge transfer in GAMESS US [migrated]

I'm trying to calculate the effective transfer integrals $J_{\text{eff}}$ from monomer A to monomer B within dimer AB by using GAMESS US: $$J_{\text{eff}}=\frac{J-S\cdot (e_1+e_2)/2}{1-S^2},$$ where $...
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1answer
46 views

Orbital angular momentum of an electron in s-orbital

I'm a beginner in quantum mechanics, and I'm pretty much confused with the orbital angular momentum of electron in s-orbital of hydrogen atom. I've read that average angular momentum of an electron in ...
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Quantifying location and volume of "lone pairs" for VSEPR

Of course molecular shape and the VSEPR model come up frequently. As discussed previously in a valence bond picture like VSEPR, the lone pair electrons are closer to the atom than bonding pairs, and &...
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Spin contamination and expansion of unrestrict determinants

I was studying with the book "Modern Quantum Chemistry" of Sazbo and Oustland. In the chapter 2, they talk about unrestrict (UR) determiants. It's explaneid that UR determinants are not ...
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1answer
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In the LCAO method for $H_2^+$, why do we only add or subtract orbitals but never alter one of the phases of the orbitals?

TLDR: Atkins' physical chemistry contains the quote "In general, from N atomic orbitals we can build N molecular orbitals". In the case of $H^+_2$, we use a combination of N=2 orbitals to ...
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Apparent inversion of oxidation state of sulfate ion using minimal basis set

It is well known that the sulfate ion has essentially no 3d-2p bonding; hence I used the minimal basis set(STO-3G) for the NBO calculation I did via Gaussian, just to check it. However, I got a ...
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2answers
53 views

Molecular orbitals in an ionic diatomic compound

The wavefunction of a heteronuclear diatomic molecule, after the orbital approximation and the Born-Oppenheimer approximation, is $$\psi = c_A \chi_A \pm c_B \chi_B $$ where $\chi$ are the starting AO ...
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63 views

Relation of partial charge with wavefunction

Suppose we have an one electron system. The probability $P$ that electron's position is in some interval $x + dx$ is given by: $$P = \int_x^{x+dx}|Ψ(x)|^2 dx$$ So if we measure $N$ times the electron'...
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Meaning of $^2[\,1/2]$ terms in atomic spectroscopy

I was trying to analyze some atomic emission spectroscopy data and I encountered unfamiliar notation of the terms for me. Commonly, the electronic states of the atoms can be described by quantum ...
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34 views

Spectroscopic constants weye and weze

I am using Morse potential to calculate the spectroscopic constants of a diatomic molecule. So far I was able to calculate $R_e$, $\omega_e$, $\omega_ex_e$, $B_e$ but wasn't able to calculate $\...
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1answer
43 views

Obtaining orbitals from a many-body wave function

I have seen how sometimes people talk of orbitals obtained from a CAS or DMRG calculation. I haven't found any explanation of how this is possible. Given the many body wave function (obtained from ...
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Can electrons change principal quantum number upon excitation?

I wanted to know when can excitation of electron take place i.e excitation of electron only takes place between subshells having same principal quantum number or is it seen by difference in energy ...
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1answer
72 views

Total orbital angular momentum in a filled p subshell

From "Physical Chemistry", P. Atkins, J. De Paula, 9th edition, page 358 "A closed shell has zero orbital angular momentum because all the individual orbital angular momenta sum to ...
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Direction of spins in the singlet vs. triplet state

Suppose we consider the first excited state of the helium atom. We know that the first excited state of helium can exist as a triplet or singlet. The possible functions related to the spin of the two ...
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1answer
63 views

Role of 'p' Orbitals in Graphite Carbons

In the graphite arrangement of carbons, if we model them according to hybridization theory, the carbons in graphite are sp2 hybridized. This would mean that one s and two p orbitals hybridize, making ...
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52 views

Would there be 6p½ -> π* backbonding in oganesson tetracyanide?

It is known that oganesson, although a group 18 element, is more likely to behave like a group (1)4 element; i.e. with 4 valence electrons outside a flerovium core. This contributes to the fact that ...
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154 views

Identifying radial probability distribution curve given $n$ and $l$ value

The correct radial probability distribution curve for the hydrogen atomic orbital with principal quantum number,$ n = 3$ and azimuthal quantum number, $l = 1$ is: ($4πr^2ψ^2 $= radial probability ...
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Failure Cases of Adiabatic Approximation

We may exactly write the wavefunction of a molecular system as $$ \phi(r, R, t) = \sum_{k = 0}^{\infty} \chi_k(R, t) \psi_k(r, R) $$ where $r$ is the coordinates of electrons, $R$ is the coordinates ...
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Rotating orbitals and parameters for a many-body calculation

I am confused with some results I am getting. Given a molecule (In particular, I was toying with triangulenes, some benzenoids and things like that, but this is not very important), I perform first a ...
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Comparing geometries and substrates calculated with DFT for different pathways

As DFT calculation is an excellent tool for confirming and understanding experimental results, I got deep into this field as organic chemist. I successfully isolated interesting transition states (TS) ...
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1answer
55 views

How can 2 molecular orbitals form simultaneously? [duplicate]

I understand (for example in the case of H2) that two s orbitals produce through constructive interference a bonding orbital and that the out of phase combination of two s orbitals leads to ...
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56 views

Question regarding conflicting results of checking the electron-preciseness of isoelectronic species via NBO and EHM calculations

It is well known that the tert-butyl cation's empty 2p orbital on the central carbon is stabilised by hyperconjugation from neighboring C-H bonds. This would mean that the central 2p "lone pair&...
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21 views

How to transform 'Cartesian gradient' taken from the formatted checkpoint file into the input orientation?

I would like to extract the energy gradient at a particular geometry with many digits from the formatted checkpoint file. However, Gaussian prints the gradient components related to the standard ...
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1answer
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Is the lone electron pair of an amide nitrogen part of the π system when building a Hückel matrix?

Is the lone electron pair of the nitrogen atom part of the conjugated π system in the α,β-unsaturated amide pictured below? Is there a general rule for choosing when to include a nitrogen atom in a ...
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1answer
62 views

Why don't carbon nanotubes behave like quantum dots?

If carbon nanotubes (CNTs) have the size required to have an bound electron energy state, why don't CNTs behave like quantum dots, such as having optical tunability, amongst other properties?
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245 views

Why don't the MO energies sum up to Hartree-Fock SCF value?

I would like to calculate Hartree-Fock energy of $\ce{H3^+}$ ion from orbital energies. So I performed the following calculation in Gaussian 16: ...
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Probability of finding electron in each orbital

This may include some quantum physics. Are there anyways to describe the wave function of Hydrogen's orbital (or that of a Hydrogen-like atom) in terms of the wave function of each orbital? You can ...
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1answer
85 views

Quantum chemistry Python package to solve the Coupled-Perturbed Hartree–Fock equations

Two of the most well-known Python quantum chemistry pakcages, PySCF and Psi4, can solve the Hartree–Fock equations. However, I am interested in finding analytic derivatives of the electron integrals: $...
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127 views

Can actinides form diamagnetic coordination complexes?

I am not sure how accurate I can predict the magnetic properties of actinide diamagnetic complexes, due to the fact that they share the F-block electron as Lanthanides, which are already tricky, ...
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1answer
129 views

Why doesn't the graph of radial part of wavefunction of 2p orbital change its sign? [closed]

Below I have given the graphs of radial part of the wave functions of $\mathrm{2s}$ and $\mathrm{2p}$ orbitals respectively. The graph of the $\mathrm{2s}$ orbital makes sense. It has a radial node ...
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136 views

Significance of the ground state energy in quantum chemistry?

In quantum mechanics, quantum chemistry, and condensed matter physics courses, it seems like finding the ground state energy of a Hamiltonian is incredibly important. I can understand why having the ...
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27 views

First ionization energy and electron gain enthalpy of dihydrogen / stability comparison of dihydrogen ions [duplicate]

Subset of: How can antibonding orbitals be more antibonding than bonding orbitals are bonding? Stability of $\ce{H2^{-}}<\ce{H2^{+}}<\ce{H2}$ Stability of $\ce{N2^{-}}<\ce{N2^{+}}<\ce{N2}...
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Limitations of Quantum Chemistry [closed]

I have a physics background and I am interested in the applications of quantum mechanics in predicting chemical reactions, and I’m wondering where should I start? I am particularly interested in the ...
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61 views

How are Quantum Dot molecules formed and what properties do they hold apart differently from singular quantum dots? [closed]

I am trying to understand the science and phenomena behind the function and operation of quantum dot molecules, but the only articles I run into talk about the synthesis of them. Could this be ...
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136 views

What does the wave function of an electron look like? [closed]

In quantum mechanics, in the 'particle in a box' topic, I studied that by solving the Schrödinger equation, we can actually find out what the wave function of the particle in a box looks like. ...
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1answer
67 views

Slater Matrix of Methane Molecule

I am going through Hartree-Fock calculations in detail, but the textbooks I am referring all considered homonuclear molecules as examples. So I am trying to come up with my own the Slater determinant ...
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72 views

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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1answer
62 views

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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