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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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 [on hold]

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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Is there any reason why we use a certain method rather than a possible one to find the needed?

I encountered a problem which used the equation E=hv to find out the frequency from a given wavelength but what if we used the simple equation c=frequencyXwavelength immediately. Hint: I tried 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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44 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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46 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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35 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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157 views

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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33 views

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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71 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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115 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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38 views

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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145 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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77 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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101 views

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 ...
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28 views

Why doesn't the atomic number change by two in the process of production of Americium?

Looking at the bomardment of uranium-238 this is what I see. First, I see a uranium-238 atom with 92 protons bombarded with a neutron. The neutron then undergoes beta decay into a proton. So we ...
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What is Barrier-less reaction in Quantum Chemistry?

After finding a reaction "Transitional State " by GAMESS and finding Hessian Eigenvalues, my project instructor said: " seems your imaginary frequency ( negative eigenvalue ) is not that big ( it is ...
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Where to find pre-optimized files for Quantum Chemistry Softwares?

Many times Geometry optimization gets very time consuming ( even with access to super-computers ). I have seen some pre-optimized libraries for biomolecules like proteins that you can easily download ...
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the particle in 1-D box momentum and total energy(P.E + K.E (x))

Employing the particle in 1-D box can momentum and total energy(P.E + K.E (x)) can be measured simultaneously?
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How do you Calculate the Work Function for the Ejection of an Electron

The question is: Below is a photoelectron spectrum of the $\ce{N2}$ molecule. The spectrum was taken from an experiment in which photons of " $= 2.4825~~nm$ were fired at gaseous $\ce{N2}$. ...
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Is probability of finding electron changing here?

We all know what is a wave function. There is a technique to convert non normalized wave function to normalized wave function. In case of non normalized wave function integration of square of wave ...
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Temperature of an atom

I read somewhere that the temperature of an atom is not defined. The definition of temperature is only for larger systems. Why is this so?
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Why neutrons are neutral?

If electrons have a negative charge and protons have a positive charge then how come neutrons have zero charge without consisting of protons and electrons ? (Excuse my bad English)
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How are nuclei stable?

We all know that the density of the nucleus is very high. Nuclei are made up of protons and neutrons, and while protons have the same charge, they are closely packed in a nucleus. How does the ...
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Slater determinant as an unperturbed atomic wave function

I've deduced following postulates from studying my chem books. 1) Slater determinants are eigenfunctions of an unperturbed atomic Hamiltonian, which contains kinetic and central potential energy ...
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Totally confused about the location of “s,p,d,f” Orbitals inside the atom

Here is the other crazy question... I know that electrons revolve around nucleus in their orbits.But my question is "how the orbitals are located in the orbits? Do they really posses Spherical, dumb ...
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Is it possible to calculate atomic radius with electron configuration?

I need to know whether is it possible to calculate the atomic radius according to the number of electrons and electron configuration. Or is there any way to calculate the atomic radius using common ...
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149 views

why do 2 atomic orbitals form 2 molecular orbitals?

According to molecular orbital theory, two atomic orbitals form two molecular orbitals analogous to waves combining constructively or destructively but how can a wave combine destructively and ...
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62 views

What properties of an element determines the maximum number of bonds it can make?

From my conjecture, I think it's a mix between valence electron and the principal energy state, but I'm not sure. For example, C, N, O and F can only make a maximum of four bonds, as they can only ...
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Why do middle school and high school chemistry classes teach students that electrons are little balls? [closed]

When I was in high school, I took two chemistry courses. The first was very elementary and didn't attempt to move beyond the picture of electrons, protons, and neutrons as little balls essentially. ...
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Is it likely that increased understanding of quantum physics will change our understanding of chemistry?

Reading that the large hadron collider will be up and running with twice as much energy in March 2015, I was curious whether our understanding of subatomic particles has changed our understanding of ...
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60 views

Why the second ionization energy of helium is greater than the first? [closed]

maybe because the first electron has a lower energy. Why does this happen?
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70 views

Unitary Transformation of the HF equations

I am working on a code to preform a HF procedure and I am a bit confused about the unitary transformation of the basis. My instructions are as follows. specify the basis and geometry (done) ...
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48 views

What are the molecular orbitals of the hypothetical linear H₃⁺ molecule?

What would be the wave function of the lowest energy molecular orbital of a hypothetical linear H3+ molecule? According to the LCAO method, I feel the lowest energy MO will be 1s(A) + 1s(B) + 1s(C). ...
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52 views

What effects do an electron in the nucleus have?

Under the understanding of quantum chemistry: There is a very miniscule chance that an electron could appear inside the nucleus of an atom. What could possibly happen if this "phenomenon" were ...
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What do short-range and long-range corrections mean in DFT methods?

Currently I am looking for the most accurate calculation method for a simple non-conjugate molecule consist of C, N, O and H. Normally I would try each method for a similar and known molecule then ...
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36 views

How is 2D spectroscopy able to show quantum coherent transport through networks?

For wave-like (quantum coherent) energy transfer in networks (eg. propagation of excitation in photosynthetic protein complexes of algae or FMO complex in plants) 2D electronic (photon echo?) ...
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How to simulate chemical reactions in GAMESS?

I am rather newbie in GAMESS field but finally could learn to make input files for most molecular simulations and even could use TD-DFT and excited states ( special thanks to Geoff Hutchison for his ...
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29 views

Non stationary effects of molecular orbitals

We have been learning a bit about molecular orbitals in class. However, we have always considered things under a quasistatic approximation. What are some effects that can only be explained by not ...
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95 views

Relativistic effects and lanthanide contraction

So I understand than the lanthanide contraction is due to poor shielding of the 4f electrons which decreases the radius. However, if Im not mistaken the relativistic effects lead to a contraction of ...
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What is the difference between Dielectric and Capacitive Polarizable Continuum Model?

I have good background knowledge on Quantum Chemistry calculations but my knowledge on Solvation Models is not that amazing ! Many times I read a paper that uses TD-DFT, they also assign some solvent ...
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Born-Oppenheimer Approximation reloaded

I'm an undergraduate Chemistry student and I'm currently writing my Final Dissertation. I've written a form of the Born-Oppenheimer approximation with the only initial assumption of parametric ...
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216 views

Why do principal energy levels in an atom get closer together as n increases?

The title says it all. Reasons that I can supply include: increased nuclear charge increasingly catches up in terms of influence to the increasing shielding and proof by contradiction in that if the ...
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Gaussian Calculations - What is the Basis?

What am I actually changing when I click 631G(d) or 631G(d,p), etc. when I choose a "basis" in restricted Hartree Fock calculations? What is the difference between using an alternative basis? Many ...
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206 views

The “rules” for LCAOs in Molecular Orbital Theory

In our course on physical chemistry, which involves MOT, we have been taught that in the LCAO approach, the wave function for a molecule... say hydrogen ion, can be approximated by a linear ...
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Dissociation of water into H+ and OH-: Does the law of mass action hold at nanoscale?

Suppose water under neutral conditions is confined in a virtual spherical nanocontainer with a radius of 25 nm. To calculate the number of hydronium ions, one uses water dissociation constant which is ...
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120 views

Is there any free GAMESS input file maker for Mac OS?

My main purpose is to simulate excited state calculations on a molecule with Time Dependent DFT. I am looking for free software that makes input files for GAMESS. I have tried Avogadro and it works ...
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340 views

Acidity of substituted phenols

Phenol has a $pK_{a}$ approximately equal to $9.9$. When one studies the acidity of chlorophenols, one notices the following: First of all, chlorophenols are more acidic than phenol, due the ...