Quantum chemistry is a subfield of quantum mechanics. Like the parent it focuses on physical phenomena that occur at an atomic scale, usually specifically aiming at chemical reactions.

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DFT Code for Atoms : Sources [duplicate]

Some time ago I implemented the restricted and unrestricted Hartree-Fock methods. I had a lot of fun and I learned a lot. It goes without saying that Szabo and Ostlund's book "Modern Quantum ...
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What should I use as the starting structure for stacking interaction energy calculation using Gaussian?

I have a crystal structure (single crystal, organic molecule- pyranone carboxamide derivative) that shows pi-pi stacking, and I want to do theoretical calculations. I am using Dispersion-correcting ...
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Raman Spectrum of carbon chains: How to identify the vibrational modes

So I am using NWChem to calculate the raman spectra of C2H2 and C4H4. For example for the first I obtained the following frequencies: ...
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Physicist notation for spatial orbital

In quantum chemistry, the two-electron integrals often denoted as physicist's and chemist's notations. For spin orbital, the physicist's and chemist's notations are $$ \langle pq | rs \rangle = \int ...
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174 views

What is the purpose of Gaussian, itself?

I have been doing some research into computational chemistry, having never looked into the area before, and have read about a popular, paid-for software "Gaussian", and free alternatives such as ...
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Conical intersection using GAMESS

I would like to know how to locate conical intersection using a GAMESS calculation. I really appreciate an example alone with this since I am a novice I do not have a clear idea among bunch of ...
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If d-electrons are such poor shielders, why do trends increase more gradually across the d-block than the s or p-block?

If I understand correctly, the shielding effect of d- (and f-) electrons seems to be much poorer than those of s- and p-electrons, due to the fact that they are less penetrating, have less electron ...
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Are electron orbitals in a helium atom degenerate?

In a hydrogen atom, the presence of only one electron allows various orbitals' energy states to be dependent only on the principal quantum number and not on angular momentum. Orbital degeneracy and ...
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118 views

Do Moore's tables really exist?

I recently have a lesson about quantum chemistry, and in one lesson we spoke about hyperfine stucture of elements. The teacher give us something which looks to be an extract of Moore's tables here ...
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Chemical bonding and atomic structure [duplicate]

In an atom, electrons keep revolving around the nucleus. So when two atoms are bonded, the bonded electrons are equally attracted by each other and I think that the bonded electron stops revolution ...
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62 views

How dependent are computed charges using the quantum theory of atoms in molecules on the used level of theory?

The quantum theory of atoms in molecules is based on the topology of the electron density. This mathematical analysis allows to find critical points and hence has a unambiguous way of separating a ...
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Basic Intro to Quantum Chemistry. Two Slit Experiment and Bohr Model

We just learned about the 2 slit experiment in Quantum Chemistry today, where electrons behave as waves when nobody is looking and behave as particles when they are being observed. So, what would ...
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501 views

The time scale of calculations in computational chemistry

Is it possible to determine or get a range of the time required for a specific calculation with knowledge of the type (MM or QM), class (semi-empirical, ab initio or DFT), the degree of theory (MP, ...
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80 views

What does Molecular Orbital Theory get wrong?

I am learning about MO theory in my advanced inorganic chemistry course and am starting to realize that it is truly the most accurate representation of how molecular orbitals look like, where they are ...
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72 views

H2 Triplet State

How does $\ce{H2}$ triplet state exist if there are electrons in both bonding and antibonding ($1\sigma$ and $1\sigma^*$) orbitals? Or am I being taught the hypothetical triplet states of the ...
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56 views

Angular Momentum Of S-subshell of an atom?

The angular momentum of every S-subshell of an atom is 0 by Azimuthal Quantum No. Relation. But if angular momentum of S-subshell is zero. Then by, Angular Momentum =Mass×Velocity×Radius; Radius of ...
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Electron's negative charge, and electricity

In my understanding, electrons carry a negative charge. However, since electricity is a flow of electrons, how can this work? Wouldn't a flow of electrons mean "negative electricity"? Also, if protons ...
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67 views

Heisenberg Uncertainty Principle question

I am really confused on how to go about this problem. I know the formula, but I get confused with all the steps and how to use what I got to get to what I need. A student is examining a bacterium ...
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Why p,d and f orbitals have non spherical shapes [duplicate]

I know what orbitals are. The orbitals are solutions (also called wave functions) to the (time-independent) Schrödinger equation[ref]. What I don't understand is why p,d,f orbitals have non ...
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67 views

What are the height and width of the large and small nodes of the sp3 hybridized orbitals of carbon and silicon?

I will make an effort to interpret answers based on wave functions, but I am a senior pursuing a bachelor's degree in Biology, so I don't have the background in quantum physics for a purely ...
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Rotation around CC bond in 1,2-dioxetane: transition states and local minimum structures

TLDR: What: Potential energy surface associated with rotation around CC bond in the ring-opened version of 1,2-dioxetane. How: State average(4)-CASSCF(12,10)/VTZP constrained geometry optimizations ...
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119 views

Ab initio effective potentials

I have been reading a journal on ab-initio pseudopotentials, and I need some help understanding it. The journal is http://journals.aps.org/pra/abstract/10.1103/PhysRevA.10.1528 A summary of the ...
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30 views

Identifying the dependence of various time-independent wavefunctions

I'm tasked with finding the variables that each of the following wavefunctions depends on: (1) a free particle in 2 space (2) a free particle in 3 space (3) a hydrogen atom (4) a lithium atom. for ...
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Does a wavefunction have to be an eigenfunction of an operator in order to obtain an expectation value?

I am reviewing an intro quantum chemistry book and I came across the line "If $\hat{A}$ corresponds to a physical property of the system, whether or not $\psi$ is an eigenfunction of $\hat{A}$, the ...
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Pauli-forbidden term symbols for atomic carbon

Carbon has a $\mathrm{p^2}$ configuration, and within the Russell-Saunders coupling scheme, we have $$\begin{align} s_1 = s_2 &= \frac{1}{2} & S &= 1,0 \\ l_1 = l_2 &= 1 & L ...
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How to Calculate Transition Dipole Moment from Two Known Wavefunctions

I am interested in calculating the transition dipole moment (TDM) from the information from two wavefunctions of different states. This is somewhat similar to calculating the molecular dipole moment ...
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79 views

Brillouin's Theorem: How do other determinants mix with the ground state?

I am a bit confused about Brillouin's Theorem. I get that a singly excited determinant can not mix with the ground state wavefunction because that translates to an off-diagonal matrix element ...
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Is there an example of a closed shell molecule with a permanent ground state magnetic dipole?

It seems that molecules of point group C$_{n}$ and C$_{nh}$ can in principle support a permanent magnetic dipole moment (Barron, 2004). However, I cannot think of any closed shell molecules that ...
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Energy and Chemcial change [closed]

How is energy related to chemical change? How are they influence each other ? Does it have to do anything with elements stability? I used to know that some Chemcial reactions give out energy as the ...
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58 views

How do energy levels work in relation to HOMO, LUMO transition?

I've just started a basic quantum course and I'm facing many exercises regarding the transition from HOMO to LUMO. I've already mastered unidimensional problems, but I'm a bit confused in ...
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73 views

Nuclear zero-point energy and quantum delocalization effects of water molecules

What do we mean by "Nuclear zero-point energy" and "quantum delocalization effects" of water molecules ? It's said that without these effects "water" becomes somewhat unbounded, which is not very well ...
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25 views

Excitation Source for visible emitting Quantum dots

I read that the best excitation source for visible emitting quantum dots is UV light of wavelength $\mathrm{360~nm}$ to $\mathrm{380~nm}$. But why should the excitation source have lower frequency ...
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66 views

Why do atoms bond? Why are unfilled valence shells energetically unstable? [closed]

I teach school Physics. And am trying to get a reasonable explanation of the root cause of the energetic instability of unfilled outermost electron shells that leads to atoms bonding. For most of what ...
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Programs for chemical structure visualisation and drawing 2d and 3d [duplicate]

I want to create list of simple and useful software to draw molecules 2d and 3d with small description. Not necessary only freeware software. Also web-based services are interesting. Your help in ...
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84 views

Confusion about atomic/molecular orbital terminology

I have come across several different orbital terms: atomic orbitals, natural orbitals, split-localized orbitals, molecule-intrinsic orbitals, quasi-atomic orbitals. I don't know what the difference ...
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Why ground-state configuration is consistent (“additive”) from one element to the next?

Ground-state configuration ("electron shells") is consistent throughout the periodic table, for example: (source) Why is it this way if the Hamiltonian of each atom is different? In other words, why ...
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Px and Py orbital and eigenfunction of the hydrogen atom

By solving the Shcrodinger equation of the hydrogen atom, we can get a series of wave functions, as listed in the following link. ...
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Wavefunction convergence error and keywords. (Spartan)

I am currently working with Spartan software and am calculating MOs. My problem is that when I try to do calculations SCF error occures. And every time I try to put some keywords (like CONVERGE, ...
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Particle in an Infinite Well and Particle in a finite Well

a) What conditions give rise to quantisation of energy in the the particle in a finite well model? b) State the symmetry condition or conditions on the resulting stationary wavefunctions ...
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Explaining the Relative Energies of Various Vibrational Modes

Specifically, I'm wondering why it is that the asymmetric stretch in most (although there are exceptions to this I believe) molecules is higher in energy than the symmetric stretch. Similarly, I ...
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Newtonian physics scale [closed]

So as I am starting to understand there is a scale at which Newtonian physics ceases and the quantum world takes hold. My question is what is the smallest molecular structure that still exhibits ...
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How to explain the discrepancy between the calculated absorption wavelength within the particle in the box theory and the observed wavelength?

The experimental observed wavelength for this molecule is 425 nm. We are supposed to estimate this wavelength theoretically using this equation (energy change in a 1D box), where $N$ is the number of ...
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How to identify the number of pi electrons in a conjugated system to calculate the HOMO-LUMO gap with the particle in the box approach?

It's my understanding that for a particle in a 1-D box, the change in energy from the HOMO to LUMO can be approximated with the following equation: $$\Delta E=\frac{h^2 ( N + 1 )}{8 m_\mathrm{e} ...
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240 views

Is there any software package for quantum chemistry that includes CAMB3LYP?

I want to do calculations on systems with photoinduced electron transfer, and I've read that the Coloumb Attenuating Method is a modification of the functionals that makes calculations including ...
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Use of statistics in this field

I am currently pursuing a statistics course and I an interested in knowing how can statistics help this field in future. (I am interested because I love chemistry very much) Specifically, are there ...
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What is the difference between Fukui FMO approach and Woodward-Hoffman rules?

If I understand correctly Frontier MO-theory is based on the perturbation theory of reactivity and the Woodward-Hoffman rules consider conservation of orbital symmetry. Does FMO use symmetry ...
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62 views

How the Azimuthal quantum number describes 3d shape of the orbital?

I read yesterday that Azimuthal Quantum number describes the shape of orbital....
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142 views

Jahn-Teller Distortions in Square Planar Complexes?

A Jahn-Teller distortion is predicted whenever a non-linear symmetric molecule has degenerate orbitals and has unequal electron occupation in those degenerate orbitals. Of course this most often is ...
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Constructing a Walsh diagram of AH2

According to Wikipedia, for the $\ce{AH2}$ Walsh diagram: and the Walsh diagram in Shriver and Atkins Inorganic Chemistry, the $a_1$ orbital sinks in energy going from 90 to 180°. Furthermore, the ...
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How to determine the best computational method for energy calculation of a molecule?

In computational or quantum chemistry text books comparing between various methods for energy calculation of a molecule (e.g geometry optimization) such as Semi-empirical, Ab-initio or DFT and also ...