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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Photograph of Light as Wave and Particle [on hold]

What is this? Actually its the first photo of light as wave and a particle The bottom "slice" of the image shows the particles, while the top image shows light as a wave. I have questions: How ...
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Bohr's model of atom

"the most remarkable success of the Bohr's theory is that it provides a satisfactory explanation for the line spectrum of hydrogen." But what is the problem with line spectrum of hydrogen which ...
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Is it wise to use ECPs on light atoms for electronic structure calculations?

So, I have a big organic molecule consisting of $\ce{H}$, $\ce{C}$, $\ce{O}$, and $\ce{N}$ atoms and the goal is to find its equilibrium geometry and US/Vis spectrum. The molecule is so big that I was ...
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Change of electron spin in Carbon after Hybridization?

During bond formation in Carbon, one of the electrons from the 2s orbital jumps to one the empty p orbitals creating a hybrid orbital as seen in the figure below. Now clearly one electron from the ...
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Is the virtual particle actually existing? [migrated]

I have heard virtual particles pop in and out of existence all the time, most notable being the pairs that pop out beside black holes and while one gets pulled away. But wouldn't this actually violate ...
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What do the quantum numbers actually signify?

I know how to calculate them and such stuff, but I wanted to know what they actually signify. I have a vague idea that they have something to do with an electron's position in an atom but what do all ...
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What is GAMESS and Gaussian?

I've been seeing a lot of GAMESS and Gaussian questions around here, and most of them seem very hard to answer. I extrapolated that they might be some sort of Chemistry programs, but what do they do ...
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275 views

Born-Oppenheimer adiabaticity

Born Oppenheimer aproximation is said to be adiabatic. What does adiabatic mean? What would be the case of a non adiabatic approximation to the Schrödinger equation?
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How to interpret orbital transition in TDDFT?

Consider the following TDDFT run with GAMESS: ...
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Should chemistry students worry about quarks and such?

Even chemistry textbooks less than five years old start by describing atomic structure based only on protons, neutrons and electrons. Is an understanding of chemistry fundamentals likely to become out ...
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Why Acetone does not behave like its computational values?

I am trying to simulate the excitation state of acetone. I ran TDDFT for it both in gas phase and solvated state in water (both implicit and explicit water). The experimental data say that acetone ...
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How to calculate molecular dipole moment from a known wavefunction?

Say I have a molecular wavefunction as a set of molecular orbitals and want to calculate the molecule's dipole moment, but don't know how ! I searched a lot but couldn't find any practical example. ...
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How to interpret TD-DFT results for finding λmax and amplitude

I am trying to understand how to interpret the TDDFT results ( I know there is another post for this but it does not answer my question ). Here is part of the result from GAMESS: I downloaded ...
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problem regarding hydrogen spectrum [closed]

The wavelength of the $\ce{H}_\alpha$ line of the Lyman series for the $\ce{H}$ atom is $\lambda$. What will be the transition for the $\ce{He+}$ ion which will have the same wavelength?" How do ...
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Why does Thallium form TlF and TlF3 but aluminium only forms AlF3?

Does this have something to do with the inert pair effect?
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Exchange-correlation hole

I did a google and chemistry.exchange search and found several technical definitions of the exchange correlation hole. The questions that I want to proposes are: What the exchange-correlation hole ...
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How important are those fields of chemistry in biotechnology and medicinal chemistry

At my university there is some fields of chemistry that i don't know if i should focus on them... How important are in biotechnology and medicinal chemistry those fields of chemistry? : Quantum ...
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Are molecular optimisations in Gaussian really the optimisations of molecules in vacuum?

I knew that Gaussian assumes the molecule is alone in vacuum, that is, the molecule doesn't interact with anything, when optimising the molecule. But I've found that Gaussian's optimisation(at HF or ...
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theoretical model for the Hydrogen bond

I am curious how from the perspective of quantum chemistry the Hydrogen bond is modeled. In particular I am interested in using such a model to estimate the amount of energy released per $\ce{H2O}$ ...
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Quantum mechanical explanation for Le Chatelier's principle?

We know from basic chemistry that a reaction reaches equilibrium as described by Le Châtelier's principle. What's the quantum mechanical explanation to this principle? How a change in ...
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In order for a ring system to be aromatic it must have (4n + 2) pi electrons.

Explain how the allowed energies and the wave function for a particle in a ring can be used to justify this observation.
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Charge consistency in fragment qm/mm methods

I'm calculating the multipole moments on several fragments of a molecule. What are some procedures to ensure that the charges etc are representative of the complete molecule? How do I ensure I'm not ...
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What is the relation between surface tension and initial molecular harmonic?

Initial harmonic is measured as Hartree/Bohr^2 ( like IHarmonic=n in Gaussian Software ). As I am from physics background I am used to look at features from their dimension point of view: Actually ...
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178 views

Why are 2 ways to calculate the energy of the ground state?

Setup I found in Szabo, Quantum chemistry page 88, these two ways to calculate the energy of the ground state: $$E_0= \sum_a^N \langle a|h|a\rangle + \frac{1}{2} \sum_a^N \sum_b^N \langle ...
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installation error of dalton 2015 in ubuntu 12 [closed]

what are the answer for CONFIRMATION OF REGISTRATION when register in dalton forum ? install got error, how to install dalton 2015 wonder@wonder-VirtualBox:~/dalton/DALTON-Source$ uname -r ...
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Atomic number $Z$

If electrons were spin-$\frac {3}{2}$ instead of spin-$\frac {1}{2}$ , what would be the atomic number $Z$ for the first noble gas ?
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Gaussian Convergence Issue for N2+N PES

I am trying to obtain a potential energy surface (PES) for $\ce{N2 + N}$ combination using scan feature of Gaussian. I have tried different combinations of basis ...
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How to visualize or think about spin waves (magnons)?

According to Wikipedia: "A magnon is a quasiparticle, a collective excitation of the electrons' spin structure in a crystal lattice." I have little pictures in my mind for other quasiparticles. For ...
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Usefulness of the outdated Bohr model?

Whilst the Bohr model is incomplete and incorrect, it had limited usage in predicting spectral lines. In the same way, could it possibly with limited accuracy, be used to predict the outcome of ...
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How to find the second order perturbation to wave function?

Today, I'm looking for how to find the 2nd perturbation to the base in Rayleigh Schrödinger Perturbation Theory (RSPT). SETUP Starting from the 2nd order perturbation in Dirac's notation: ...
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Quantum Numbers and Ions

How do quantum numbers change for ions? A sample question: Give the set of four quantum numbers that could represent the electron lost to form the Rb ION from the Rb atom. The answer given is n=5; ...
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Ignoring spin, consider an electron in a hydrogen 2p orbital, what is its orbital magnetic moment?

I know that a magnetic dipole moment is given by $$\mu=\frac {-e}{2m}I$$ and that the z component of angular momentum is $$m_j\hbar.$$ However, I have also seen that angular momentum $I$ is given by ...
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353 views

Why are so many wave functions associated with hydrogen?

According to Wikipedia, there's an infinite set of possible wavefunctions (orbitals) for the hydrogen atom: $$\psi_{n\ell m}(r,\theta,\phi) = \sqrt {{\left ( \frac{2}{n a_0} \right ...
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Abnormal termination of GAMESS in frequencies calculation

I want to optimize geometry and to analyse frequencies for transition metals ($\ce{Cu}$, $\ce{Mn}$, etc.) compounds using GAMESS. I use Avogadro for preparing input files and I have a problem: one of ...
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Do core electrons in bulk material have discrete energy levels or continuous energy band?

The energy band of valence electrons gets smaller and smaller when the lattice (bulk) atoms are more and more separated from each other, until discrete energy levels are formed, due to the isolation ...
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Heisenberg's Uncertainty Principle's inequality [closed]

In the uncertainty principle , whats the significance of the inequality sign ? Why is it not equal to over there ? Why do we say greater than equal to instead of just equal to ?
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Correlation between crystal structure and underlying atomic properties

Can the crystal structure of an element/metal be explained by any underlying atomic or quantum mechanical properties of the element or is it an intrinsic property by itself (like atomic number)?
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80 views

Please explain the following graph for a quantum mechanical harmonic oscillator

Graphs such as the above keep coming up when talking about harmonic oscillators in a quantum mechanical sense. However, I simply cannot make sense of them. What does each line represent why are they ...
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69 views

What is an orbital boundary surface?

Could anybody explain the following statement: "The boundary surface is a common way to represent atomic orbitals, incorporating the volume in which there is about a 90 percent probability of finding ...
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36 views

when do we indicate the “+ or -” sign in uncertainty [closed]

how can we know when to indicate "+ or -" sign in uncertainty calculations because I noticed some problems has been solved by this sign and some has not.
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Is there a general / simple formula for Slater's rules?

I just learned how Slater's rules work on Wikipedia. These rules really are very simple. But the presentation of the rules seemed not very efficient. I would think there would be someway to set up and ...
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Show that there can be 18 electrons in a “g” subshell

I'm learning about quantum numbers right now, and I can't figure out how to calculate the number of electrons in each shell without memorizing it. Is there a formula to find the possible numbers of ...
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109 views

How can you manage SCF convergence problems?

"Failure to locate stationary point, SCF has not coverged" This is the most annoying message after Optimization run ( even with a good primary start geometry like UFF we ay see that) . The only ...
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126 views

Do multielectron atoms have nodes?

For hydrogen, other than in the 1s state, the electron wavefunctions have radial and/or angular nodes where the electron probability density is zero. In helium or further atoms with more than one ...
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What's the relation between the octet rule and quantum exclusion and how is it related to oxidation state?

What in the quantum nature of electrons gives rise to the octet rule? How is it related to the fact that an element cannot have an oxidation state higher/lower than the complementary to 8?
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Calculating the angular momentum for these orbitals

Here is the question: I got through part a (with 1, -1, 0, and 0), but I'm stuck for part b. I looked up the radial and spherical harmonic wavefunctions for $n, l, m_l$ = 211 ; and I figure since ...
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HOMO and LUMOs of Carbon based Compounds

Why are the HOMO and LUMOs often $\pi$ orbitals in carbon based compounds that contain multiple double bonds or conjugation such as ethene and butadiene?
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176 views

How to Run GAMESS and Avogadro on Command Line?

I always use GAMESS and Avogadro on my own laptop. Recently I installed them on our university supercomputer and started using them by remote logging-in. On the laptop everything was super easy but ...
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84 views

Post-Hartree Fock Methods

I'm new to computational chemistry, so perhaps this question has an obvious answer. I'm wondering what the intuitive reason for constructing Slater determinants that involve excited states is, when ...
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What is the meaning of “no Hessian Eigenvalue ”?

When running GAMESS for finding Hessian Eigenvalues and frequencies ( using Optimization with hessian calculation at the end ) , I have seen there are times that it comes out "without any ...