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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51 views

How to plot the wave function after a Hartree-Fock calculation?

I have written a code in Mathematica and have obtained the final energy of the hydrogen molecule. I have used Gaussian basis functions for this calculation. Now I need to plot the ground state wave ...
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24 views

What is the link between the classical and quantum definition of the symmetry number?

From what I understand, the symmetry number for a molecule can be defined in 2 ways: 1. The quantum mechanical symmetry number corrects for overcounting the number of possible rotational states of a ...
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Deriving the absorption coefficient from state transitions

This source states that the absorption coefficient $\gamma$ is proportional to the difference in particle number density $\Delta n$ per unit volume between the initial and the final rotational state ...
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32 views

Uncertainity principle of a particle on a ring in $m=0$ state

When a particle is on a ring, we know that the commutator $[L_z,x]=i\hbar y$ and $[L_z,y]=-i\hbar x$. So we can't measure $L_z$ and $x$ & $L_z$ and $y$ simultaneously with infinite accuracy. When $...
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65 views

What are the rules for constructing Kohn-Sham orbitals?

I just recently began to study quantum chemistry and need some clarification for the construction of Kohn-Sham orbitals. Consider a system of several atoms. Let there be $N$ electrons in this system. ...
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31 views

Obtaining energy and wave function for excited states

Suppose I have done a HF calculation and obtain the energy and wave function of ground state. If I want to produce the spectrum of the specie (say Hydrogen molecule), should I use CI (configuration ...
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44 views

Finding vibrational frequencies and heat capacity from bond enthalpies?

So I have heat of formation data for H2, HD, and D2, and I'm being asked to find the difference in vibrational frequency and zero point energies between H-H and H-D. I know that i need to use the ...
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39 views

Probability density of a particle in a 2D box with given coordinates

Wavefunction of particle in a $2$D box with sides $L$ in the $x$ and $y$-axis: $$φ = \sqrt{\frac {4}{L^2}} \sin\frac{n_1πx}{L}\sin\frac{n_2πy}{L}$$ For the different states ($n_1=1$, $n_2=2$), (...
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Wave function characteristics

In a book, I read that the wave function of a system should not become infinite in a finite region and also should vanish at infinity. For the first part I think that if wave function is infinite in ...
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34 views

Trouble understanding equation from density functional theory

In the review by Oliviera et. al., I do not understand how the authors get equation 14: \begin{equation} E[\rho]=\sum_i^Mn_i\langle\Psi_i|-\frac{1}{2}\nabla^2+v_{ext}(\vec{r})+\frac{1}{2}\int\frac{\...
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43 views

How to graph spherical harmonics in the Mac “Grapher” application?

I was wondering if it was possible to graph the spherical harmonics in the Mac grapher app. In their manual, there is a section that talks about graphing the spherical harmonics (page 54): This is my ...
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29 views

The meaning of magnetic quantum numbers' values

The $\mathrm{m_l}$ values in quantum mechanics, $1, 0, -1$ for p orbitals for example, do they refer to specific orbitals? Like, does $\mathrm{m_l} = 1$ refer specifically to $\mathrm{p_x, m_l} = 0$ ...
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126 views

Is this approximate equivalence table between Pople's basis sets and Jensen's DFT attuned polarization consistent basis sets (pcseg-n) correct?

I have some calculations results I runned previously using Pople's basis sets, mostly 6-311+G(d), under Gaussian09. These days I read some texts on Frank Jensen's family of polarization consistent, ...
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89 views

Spin-Adapted Configuration and Slaters determinants for Open- and Closed-shell systems

Some time ago, I asked a question about spin-adapted Configuration State Functions. I read that, when we ignore the electronic repulsion in a atom or a molecule, we can write the ground state using a ...
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29 views

A general definition of bond order in molecular orbital theory?

I stumbled upon this problem upon learning organic chemistry after learning the foundations of MO theory in physical chemistry. It seems that VBT and MOT can coexist with each other in many scenarios. ...
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70 views

Reaction energies of solids from quantum chemistry?

I was reading a paper by Catlow et al. [1], and they mentioned the dissolution of copper into copper ions, which is given by this equation: $$\ce{Cu2O(s) + 2 h+ <=> Cu+(aq) + 1/2 O2(g)} \tag{...
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170 views

Hartree-Fock vs. DFT vs. Hohenberg-Kohn vs. Kohn-Sham

What is the difference between these four approaches? My current understanding is that Hartree-Fock uses only 1 slater-determine (accounting for antisymmetry) but neglects correlation between ...
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15 views

Magnitude of Coulombic integral of atomic orbitals

I'm wondering anyone knows what's the typical magnitude for Coulombic integral of atomic orbitals in large molecules (not simple diatomics). Also, is it right to say that the integral has a greater ...
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30 views

What is the justification for using the Marcus rate equation to model charge transport in organic semiconductors?

I am trying to understand the history of how Marcus theory moved from describing electron exchange reactions in aqueous ionic solutions to also being applied to describe transport in organic ...
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54 views

Theoretically predict the density of water

Is it possible to predict the density of water having just its molecular structure (basically obtain 1 g/cm3 at 3.96 °C)? Are you aware of research/textbook that dive into this topic? Is it able to ...
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25 views

Order of coordinates in a wavefunction - why do they matter?

For the past few weeks, I have been studying Quantum Chemistry and lately in these lectures something has bugged me: If I do a sum: $1 \times 2 \times 3 \times {...} $ This is the same as simply ...
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39 views

How do these restrictions on the wavefunction logically arise?

My lecturer notes that the following restrictions on the wavefunction (in addition to it being a solution to the Schrodinger equation) are logical: i) Continuous (mathematical definition of a ...
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73 views

Electron Spin, and the Stern-Gerlach Experiment

I think I understand the basics of the electron spin. However, I'm trying to figure out how to tell if, when shooting a beam of atoms (the Stern-Gerlach experiment), how to tell if an element will ...
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135 views

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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41 views

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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122 views

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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54 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 ...
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100 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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47 views

Is it possible to calculate electrostatic interactions using QM methods?

I'm trying to calculate an accurate electrostatic repulsion (and possibly attraction) energy term for various small molecules, using Quantum Mechanics (QM) methods. These may include molecules such as ...
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47 views

Photoelectric effect — Expected observations for waves

I understand why treating light as particle-like photon packets leads to the observed results. I seek to understand why one would expect different results if light were treated as a wave (and thus ...
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64 views

Significance of this unusual matrix element

Recently, I was solving some questions on Rigid Rotor. I found this question very amusing. Evaluate the integral i) <$Y_{l,m+2}$ | $L_{x}^{2}$ | $Y_{l,m}$ > I evaluated it, found that the value ...
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185 views

Why do we neglect the zero point energy in statistical mechanics?

Quantisation associated with translational modes can be modelled like a particle in a box (PiB). $$E_n = \frac{n^2h^2}{8mL^2} \qquad n \in \mathbb{Z} \, | \, n \ge 1$$ By the equation above, the ...
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252 views

Is OH + CO → CO2 + H an elementary reaction?

Is $$\rm OH + CO \longrightarrow CO_2 + H$$ an elementary reaction? How could I tell? I could break the reaction into $$\rm OH\ (+ M) \longrightarrow O + H\ (+ M)$$ $$\rm CO + O \longrightarrow ...
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813 views

Why does q/m ratio(specific charge) depend on nature of gas in anode ray experiment?

I can't understand why $q/m$ ratio changes in anode ray experiment (discharge tube experiment) (which was carried out by Goldstein) with change in nature of gas? Since $q=1.6\times10^{-19}\ \mathrm C$...
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85 views

Azimuthal Quantum Number and Magnetic Quantum Number

I know that azimuthal quantum numbers and magnetic quantum numbers are used to find the no. of and the orientation of subshells respectively, but why do they have to be called so? I reckon that ...
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2answers
61 views

Why is the first affinity energy negative?

So, I've found a number of somewhat obscure responses to this question but I'm looking for something concrete: Why is the first affinity energy negative for most elements? I appreciate that this is ...
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1answer
544 views

what is meant by stability and potential energy of molecules in cases of formation of bonds?

molecules form chemical bonds in order to gain stability. it is said that forming bonds leads to lower potential energy of molecules and makes them stable. what does stability mean? how formation of ...
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71 views

How can an atom have 2 outer shells?

For example, my textbooks says how many electrons in the outer shell in Nickel? and it says the answer is the electrons in both the d shell and s shell. Doesn't shell equal= n quantum number? And n=4 ...
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45 views

Behavior of ionic salts in solution

The electrons of an isolated sodium chloride bound pair in vacuum reside at a semi-classical level in their ground state, so that the Born-Oppenheimer approximation applies, and the 'molecule' acts ...
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601 views

Identical particles: Why is triplet oxygen more stable than singlet oxygen?

Quoting from Eisberg Resnick Quantum Physics: If we consider space variables of two electrons (identical particles) to have almost the same values, then their wavefunctions are 'almost' identical ...

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