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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What are 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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Born–Oppenheimer adiabaticity

The Born–Oppenheimer aproximation is said to be adiabatic. What does "adiabatic" mean? What would be 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? [closed]

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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627 views

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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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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833 views

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 optimizations in quantum chemistry really in vacuum?

I know that Gaussian (or other quantum chemical packages) assumes the molecule is alone in vacuum, that is, the molecule doesn't interact with anything, when optimizing the molecule. But I've found ...
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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 temperature/...
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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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614 views

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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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 ab||ab\...
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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 3.8.0-...
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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 ...
7
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1answer
336 views

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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644 views

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: \begin{...
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1answer
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How can I determine the quantum number of a electron lost during ion formation?

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=...
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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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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 )}^3\frac{(n-\ell-...
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Abnormal termination of GAMESS in frequencies calculation [closed]

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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1answer
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Difference between radial, planar, angular and spherical nodes

What is the difference between them? I think radial nodes and spherical nodes are the same, and angular and planar nodes are the same. Reference Finally, how many spherical nodes are there in ...
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1answer
751 views

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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2answers
265 views

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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1answer
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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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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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1answer
93 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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1answer
686 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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1answer
327 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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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 ...
12
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2answers
353 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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2answers
253 views

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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1answer
839 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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1answer
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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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1answer
519 views

What is the reason for including excited states in the wave function in 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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2answers
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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 frequencies"....
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1answer
508 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 then ...
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1answer
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What is a 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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1answer
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Calculating the ionization energy for hydrogen

What energy would be needed to remove the electron from the $n = 4$ level of the hydrogen atom? $\pu{−3.49 * 10^{−17} J}$ $\pu{−1.36 * 10^{−19} J}$ $\pu{+2.18 * 10^{−18} J}$ $\pu{+1.36 * ...
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2answers
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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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1answer
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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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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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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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