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-D3 calculation in Gaussian 09

Can anyone help me to run geometry optimization and frequency calculation using B2PLYPD3 functional including the DFT-D3 dispersion correction? I went through Gaussian manual, however, it was not very ...
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24 views

Supramolecules in Spartan

Maybe someone here is familiar with supramolecules calculations using Spartan? I am new in this area and trying to figure out some things: First - what's the best way to draw them? I tried to draw one ...
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34 views

Spin of electrons about it's own axis, release of energy & COLLAPSE of atom?

An electron spins about it's own axis. So it has rotational accln. Thus, according to Maxwell's Electromagnetic Theory , Rotational Kinetic energy must be released. Thus, at some pt. of time , ...
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Density Functional Definition

Is it wrong to say that Density functional means that Electron Density is a function of the wave function of all electrons in 3 dimensions, if so, why ?
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63 views

What is the reason why protons and electrons do not collide?

can someone give me an intuitive picture of why electrons don't collide with protons? I know that electrons move in a sort of cloud, which is our 'orbital', and that they mainly behave like ...
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What are the unsolved problems in quantum chemistry? [closed]

The question is not necessarily limited in quantum, but also computational and theoretical chemistry. Wikipedia listed a few https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_chemistry ...
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How do i make an molecular orbital diagram step by step?

My professor explained me, but i'm still unable to do a correct molecular orbital, for a diatomic molecule
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27 views

Why do ionisations from non-bonding orbitals result in little/no vibrational structure in a photoelectron spectrum?

I understand that ionisations from bonding and antibonding orbitals create vibrational structure from the Franck-Condon principle, resulting in a greater overlap integral between different vibrational ...
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62 views

Why Balmer series exist?

I was reading about Bohr's atomic model in our textbook when came up with this question: Since the electron in hydrogen atom is most stable in the first orbit, why should it descend from higher orbits ...
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Estimating stability and mean life of anomalous tautomers

A pair of bases in the DNA, say, A-T, have a tautomerized form A*-T* (resulting from switching the sides of both protons along the hydrogen bridges). I have studied how, by means of DFT computations, ...
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42 views

radial wave function [closed]

Well i just started to learn the basics of quantum mechanics and i came across a graph between the radial wave function plotted against the distance r from the nucleus. Could someone please explain to ...
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30 views

How long does it take for an electron to be described in a quantum state? [migrated]

Suppose we have a proton ($\ce{H^+}$). Suppose an electron comes to orbit the proton and starts to fall inwards. Suppose we know the exact position of the electron when it starts to orbit the proton. ...
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79 views

Why are antiaromatic compounds unstable?

Antiaromaticity is so destabilizing that it can cause compounds such as cyclobutadiene to elongate or manipulate their orbitals so that the pi system is no longer aromatic. I understand that aromatic ...
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50 views

Why is no dynamical correlation recovered in the CASSCF method?

Configuration interaction is the only certain way to get a calculation to converge toward the exact solution of the many-body Schrödinger equation (within the approximations used for the framework of ...
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Is HF the only method for trying to solve the many-electron Schrödinger equation?

The Schrödinger equation can only be solved analytically for the smallest of "molecular" systems. The Hartree-Fock method is a method of obtaining approximate solutions to the many-electron ...
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67 views

Is the reasoning of spin contamination correct?

The common explanation of spin contamination is like the following: The non-relativistic electronic Hamiltonian commutes with $S^2$ and $S_z$. Hence, the exact eigenfunction should be an ...
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66 views

Why is oxygen paramagnetic?

Paramagnetic molecules are molecules that have single electrons. When I draw the lewis structure of $\ce{O2}$, it appears to be a diamagnetic structure. What makes it paramagnetic?
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68 views

Why is the CASSCF method multi-configurational, while the CI method is not?

The CASSCF method is perhaps the most commonly used theoretical method for studying difficult chemical systems exhibiting multi-reference character or non-dynamical/static/strong correlation. CASSCF ...
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40 views

From a Quantum Chemistry Point of View how can I break Bonds without Heating?

Is it possible to dissolve/break bonds using strong alternating magnetic fields at high frequency? (Perhaps from tesla coils?) Could you use resonance to cause the bond to weaken over time? If so how ...
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DFT Code for Atoms : Sources

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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49 views

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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191 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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25 views

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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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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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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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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86 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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79 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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1answer
64 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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68 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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74 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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123 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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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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49 views

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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1answer
96 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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66 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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80 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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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 ...