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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12 views

Problem with the wave intensity graph of a blackbody

I recently read about the ultraviolet catastrophe. What does the graph of lambda vs intensity represent? Is it the intensity of each light wave of each wavelength at a certain temperature or something ...
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How to calculate molecular emission spectra?

Is there free software which can calculate the fluorescence emission spectra of organic molecules? Let's say for example: rubrene There is a bunch of quantum free quantum chemistry software: https://...
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1answer
52 views

Radial Probability Distribution Curve versus ψ² versus r curve for 1s orbitals

My question is basic, but I have already referred to a couple of books. This is an excerpt from Linus Pauling's book: the most probable distance of the electron from the nucleus, which is the ...
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THe change Spin quantum no. Of Electron(of chromium) from 4s to 3d

We know that the Chromium has configuration 3d4 4s2 which then goes into 3d5 4s1 My question is that, the books usually represent the electron that goes from 4s into 3d as the one with +1/2 spin. But ...
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Additional example determining Marcus parameters searched

Background / theory: The electron transfer in organic materials was formalized by the Marcus theory, introducing $|H_{ab}|$ to describe the how molecules (or more generally: sites) are coupled with ...
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No change in energy of Hydrogen when uniform electric field is applied using gaussian software?

I have done SCF calculation for hydrogen atom and in the second job i did the SCF calculation on hydrogen atom with uniform electric field field of 0.05 a.u in a particular direction, but there is no ...
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1answer
32 views

Wave-particle duality [on hold]

Can I get an answer stating some analogies on how electrons behave like a wave, and what does it even mean for an electron to behave like a wave? Also, if going around the nucleus, is electron a real ...
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1answer
64 views

The influence of diffuse functions on electron density calculations

I am not a quantum chemist, but I am (starting to be) a user of electronic structure software. In the picture (screenshot) is the electron density of the same molecule. On the left I calculated the ...
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50 views

Wave particle duality [closed]

If the wave of an electron is not actually a physical wave, but a probability wave, (as I have learnt so far) then what does it mean for it to have a wavelength? What does wavelength in a probability ...
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How can I obtain the wave function of the dihydrogen system, by hand using Self Consistent Field DFT calculations? [duplicate]

After doing some energy calculations for the $\ce{H2}$ system in Gaussian, using ωB97X-D/STO-3G level of theory, and generating a .wfn file, I wanted to understand better how the computer does ...
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How to account for solvation of a molecule explicitly in quantum calculation?

I'm looking for a way to account for hydrogen bonds formed with my molecule in aqueous solution explicitly in DFT calculation. I'll do it in order to compare stability of two structures in water. As ...
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Developing Computational Chemistry Software

I want to learn to develop novel methods in computational chemistry (e.g. Grimme's empirical dispersion, DFTB, new DFT functionals, etc.) What is the ideal way to learn this by oneself? I have read ...
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Electronic States of the (unstable) tetrahedral CrCl4

I was probing for first order Jahn-Teller instability in the coordination complex CrCl4 (2 electrons in E set). Taking the direct product yields the states A1 + A2 + E, so I took the symmetric and ...
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What is meaning of LAV2P basis set?

I am working on energy optimization of molecular structure using DFT with Schrödinger software and I found that job is running on LAV2P basis set. I want to know what is the meaning of this basis set ...
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2answers
52 views

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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1answer
49 views

Prove that idempotency is a necessary and sufficient condition for a densiy matrix to be N-representable

As I understand, a necessary and sufficient condition for a density matrix $P$ to be represented by a wavefunction $|\Psi\rangle$ is that it is idempotent, i.e. $P^2=P$. It is easy to see that if $P=|\...
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Why are there only 6 series (Lymen, Balmer, Paschen, Bracket, Pfund and H.Davy) in the hydrogen atomic spectrum? Why can't there be more? [closed]

The Balmer series, for example, is formed when the electron jumps from $n_2 = 3, 4, 5, \ldots, ∞$ to $n_1 = 2.$ The Humphry Davy series (The last series is formed when electron jumps from $n_2 = 7, 8,...
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Is there a reliable chemical theory that predicts pKa based on structure?

Obviously, there are general stability arguments that can be made to estimate relative pKas, such as evaluating the stability of the conjugate base of an acid, or thinking about how polarized the bond ...
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1answer
52 views

What drives vibrational cooling in an excited stated?

As we excite a molecule from its ground state, $S_{0,v=0}$, to some excited state in a higher vibrational state, i.e. $S_{1,v'=3}$, what drives vibrational cooling within that excited state manifold (...
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Ionization energy and charge localization: π-bonds/systems and ionization energy

Page 35 of my textbook, Mass Spectrometry by Jürgen H. Gross, says the following in a section on ionization energy (IE) and charge localization: Molecules with π-bonds have lower IEs than those ...
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2answers
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Why is there a z² label for d orbitals, but no x² and y² labels with corresponding shapes?

I know that the $\mathrm{d}_{z^2}$ orbitals are named as a result from the conversion of the angular component of the hydrogen-like wave function in terms of Cartesian coordinates, but how come we ...
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Spacial Wavefunction Symmetries and Identical particles

I was reading this and it mentions in the 3-electron section, that for a spacial wave function to be symmetric under fermion swapping, it must be a function of even parity. Similarly for anti-symmetry ...
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Does the radius of the atom change when an electron is excited?

The radius of the atom increases when the electron in a hydrogen atom undergoes a transition from the 1s-orbital to a 2p-orbital. How is this possible? Why does exciting an electron to a higher ...
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1answer
107 views

How to calculate S² value of a broken-symmetry wave function?

$S$ represents spin, signifies the number of unpaired electrons in the system. For example, if the number of unpaired electrons is $1$, then $S=1/2$. $S^2$ is calculated as $S(S+1)$. From what I have ...
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Stability of subshell of chlorine atom [duplicate]

Why is the valence shell electronic configuration of Chlorine atom 3s2 3p5 and not 3s1 3p6, so that it becomes more stable ?
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Why do solutions of electron in a box (and in a ring) predict coefficients for LCAO (linear combination of atomic orbitals) in 1D systems?

The solutions to the 1D particle in a box quantum mechanic system are standing waves (zero at both ends of the box) with 0,1,2... nodes for increasing energy (zero for the ground state). If I look ...
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1answer
46 views

Ab Initio and Molecular Orbitals [closed]

What is the difference between ab initio calculations and molecular orbital theory?
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1answer
44 views

What does the number of orbitals in a subshell represent? [closed]

s subshell has 1 orbital and p subshell has 3 orbitals. Does that mean that there is a high probability that the electron is in the p subshell? As orbital are the space where there is a high ...
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Explaining hypervalency in the framework of molecular orbital theory

In preparing for teaching duties in an introductory course in inorganic chemistry, I seem to have made a digression into how we understand the hypervalent molecules. Just to re-iterate what this ...
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33 views

All possible determinants forms a complete basis [closed]

How do we mathematically prove that the set of all possible excited determinants forms a complete basis for expanding the wavefunction (as in full configuration interaction)?
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26 views

Guide to DFT functionals [duplicate]

Having recently begun studying Quantum and computational chemistry in depth... I am looking for a manual, text, or guide of any form that could explain the basic formulations and potential of the ...
2
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1answer
78 views

Quantum numbers required to specify the position of an electron

In a test today, there was this question: How many quantum numbers are required to specify the position of an electron? $$(A)\ 1 \quad (B)\ 2 \quad (C)\ 3 \quad (D)\ 4$$ The answer key says ...
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Multiplicity letter codes for large amount of states

Well, let's say we have a large amount of states computed (e.g. over 50), all with the same multiplicity. So, our ground state can be $X^2\Sigma^+_g$ and we can continue with $B^2, C^2$ etc. as ...
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45 views

Getting radial Kohn-Sham orbital from the output of an electronic structure code using gaussian basis sets

As we know that in solving the Schrodinger equation (Kohn-Sham equation), we expand the wavefunction in terms of some basis sets and them optimize the energy (the expectation value of Hamiltonian) by ...
3
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1answer
107 views

Why is the dz2 orbital so different from the rest?

What makes dz2 orbital so special? Although degenerate with other d orbitals, It has no nodal planes, instead it has 2 nodal "cones". Instead of having 4 lobes, it has 2 lobes and 1 ring. Also, ...
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27 views

What is the significance of antinode at r=0 in the plot of radial wave functions of s-orbitals?

Why should there be an antinode at r=0? I can comprehend the presence of node at r=0 for p,d,f orbitals; but I don't understand that of s-orbital. Is there any intuitive approach?
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What will be the form of coulomb and exchange operators for electron m in an n-electron molecule?

I know that the Fock operator for electron $m$ in $n$-electron molecule would be: $$\hat{f}\!(m)u_i(m) = \varepsilon_iu_i(m)$$ $$\hat{f}\!(m) \equiv -\frac 1 2 ∇_m^2 - \sum_α\frac{Z_α}{r_{ma}} + \...
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How To Calculate “İntermolecular Energy” as QM and MM? [closed]

In this article: http://ambermd.org/antechamber/gaff.pdf, in Table-7, authors have calculated "intermolecular energy" of a molecule set. I wonder how can I calculate this energy by using gaussian09 ...
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2answers
185 views

How do you calculate the correct charge on F after F2 dissociates?

When you ionize $\ce{F2}$ to $\ce{F2^5+}$, it quickly dissociates. Experiments show that the products of dissociation are $\ce{F^2+}$ and $\ce{F^3+}$. I have tried to model the dissociation of $\ce{...
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1answer
64 views

Reaction of HF in water [closed]

I was wondering, if we put $\ce{HF}$ into a glass of water the acid will be dissolved according to this reaction: $$\ce{HF->F- + H+}$$ Now there is a proton travelling across the water. All $\ce{...
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1answer
285 views

Why is a LCAO necessary within Hartree Fock?

As I understand it, the electronic Schrödinger equation cannot be solved for polyelectronic systems. To circumvent this problem in the Hartree-Fock method, it is assumed that the polyelectronic ...
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Can pi backdonation occur on non-metal centers?

The common example of back-donation is the interaction of a CO molecule with a metal center (d-orbitals) on a surface. Can a similar mechanism occur between CO and a non-metal center, like oxygen on ...
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1answer
35 views

Chemical reaction energy barrier width [closed]

Well chemistry before the discovery of quantum world was saying:"In order for a chemical reaction to happen , reactants must have enough energy to be converted into products."I was wondering now that ...
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57 views

Variational method: Hydrogen atom ground state in STO-3G basis expansion

I computed numerically the ground state energy of hydrogen by variational procedure, firstly expanding the wave function over the s-wave basis set STO-3G $$\psi(r)=\sum_{i=0}^3 C_{i} e^{-\alpha_ir^2}$$...
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34 views

How to Scan Aromatic Dihedrals To Obtain Force Field Parameters

I am trying to generate new force field parameters based on quantum mechanical calculation. I used amber manual "parameter development" title for this and I calculated bond-angle-non bond parameters ...
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3answers
53 views

Particle-wave duality for atoms [closed]

Hmm Hello . I was wondering if we could apply the particle-wave duality of a subatomic particle for an entire atom.I know that atom is better described as space but could we apply the de Broglie ...
6
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2answers
405 views

When does Hartree-Fock fundamentally fail?

The Hartree-Fock approximation (HFA) works by assuming each electron sees the effective electric field from all the other electrons as some self-consistent field. The HFA is known to give pretty ...
2
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0answers
57 views

Antisymmetrized Product of Strongly Orthogonal Geminal (APSG) method

To get a deeper understanding of electronic structure methods, like Hartree-Fock, MP2, and Coupled Cluster method, there are many small programs (codes) available in Python, C++ or Fortran, for ...
2
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1answer
34 views

Meaning of polarization pulse in microwave (rotational) spectroscopy

In the recent rotational spectroscopy literature, "microwave polarization pulse" is often used in conjuction with pulsed Fourier transform microwave spectroscopy (Google Scholar). Sometimes the pulse ...
6
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1answer
54 views

Why is the graph of fluorescence vs. wavelength continuous?

If the emission of a photon from an excited state to the ground state occurs with a precise energy, then why is it that wavelengths that differ from lambda max have values on a fluorescence vs. ...