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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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Meaning of polarization pulse in microwave (rotational) spectroscopy

In the recent rotational spectroscopy literature, "microwave polarization pulse" is often used in conjuction with pulsed Fourier transform microwave spectroscopy (Google Scholar). Sometimes the pulse ...
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Why is the graph of fluorescence vs. wavelength continuous?

If the emission of a photon from an excited state to the ground state occurs with a precise energy, then why is it that wavelengths that differ from lambda max have values on a fluorescence vs. ...
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How to obtain spectroscopic constants Omega_e, Alpha_e, B_e, D_e and D_0 from centrifugal constants?

I need to compute a spectroscopic constants of $N_2^+$ states and I would like to do it using LEVEL16 software, due to its high interpolation flexibility. But it seems, that LEVEL16 can only compute ...
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How fast is the interconversion triplet → singlet in organic molecules?

When computational studies are performed on transition state geometries, the "thermodynamic" (that is, with no kinetic considerations) energy of the analyzed state is obtained. I was wondering: if a (...
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Molecular basis of water in denying pseudoscience of water memory [closed]

In homeopathy, it is claimed that water, when diluted with homeopathic compounds and shaken, can hold the property of original compounds despite no molecule contained in final solutions. A detailed ...
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54 views

Hartree Fock for an atom (Beryllium)?

I'm working on solution of Hartree-Fock equation for Beryllium in configuration $1s^2 2s^2$, expanding the orbital with STO-3G basis $$\phi_k=\sum_{p=0}^3 C_{kp}e^{-\alpha_{kp}r^2}$$ I know the values ...
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How can I save and read the CI vector during CASSCF calculations in Gaussian?

I have searched through the GAUSSIAN 16 manual and have not been able to solve these two related issues: (1) I want to be able to save the CI vector and load it as an initial guess for a different ...
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What is the theoretical justification for state averaging in SA-CASSCF?

According to the molpro manual, section 7.3: In order to compute excited states it is usually best to optimize the energy average for all states under consideration. This avoids root-flipping ...
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Interaction of the oscillating magnetic field of electromagnetic radiation with a molecule or atom

Whenever we read rotational, vibrational and electronic spectroscopy, we find details on the interaction of the electric dipole moment of the molecule with the oscillating electric field of ...
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What's the most complete basis set for DFT?

I'm trying to benchmark some NMR chemical shifts. The literature suggests that the pcS-4 basis set will be the most accurate. I was wondering if there were any basis sets that are more complete than ...
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Why does the exchange interaction in Hartree-Fock theory lower the total energy?

In Hartree-Fock theory, the expectation value of the total energy can be written as $$E = \langle\Psi| H |\Psi\rangle = \sum_{a} \langle a| h |a \rangle + \frac{1}{2}\sum_{ab} \big( [aa |bb] - [ab|ba]...
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Factorizing Slater determinant into product of spin-up and spin-down slater determinant

I am reading the book on MONTE CARLO METHODS IN AB INITIO QUANTUM CHEMISTRY by Hammond and Lester. (Chapter 5-Variational Trial Functions, Section 5.3-Hartree Fock and Beyond, Sub-Section 5.3.3 ...
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Computational program not using full cpu power [closed]

I started some computations on Gamess. For instance, when I compute the frequencies of an optimized structure, I see in the terminal that the job only uses 40% of the CPU core most of time (...
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Is it possible to calculate electrostatic interactions using QM methods?

I'm trying to calculate an accurate electrostatic repulsion (and possibly attraction) energy term for various small molecules, using Quantum Mechanics (QM) methods. These may include molecules such as ...
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27 views

How to give different Basis set to different atoms in molecules using GAMESS-US [closed]

I am new to GAMESS-US, I am trying to do UHF for DyC10H10 while giving different basis set to Dy, C, H to reduce the computational cast. Until I tried with following input using manual. OUTPUT *** ...
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118 views

What is the difference between a configuration state function and a Slater determinant?

I do not understand the distinction between configuration state functions and Slater determinants. Is not every Slater determinant a CSF ? Please explain the difference and the use/need of ...
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Why is nucleus charge squared in Rydbergs formula?

According to coulumbs law, the energy changes linearly with charge. So why is the nucleus charge squared in this formula? Is there a way to derive it? I was suspecting it might be because it pulls ...
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What is the ANO-RCC basis set and how does it work?

On the basis set exchange, the ANO-RCC basis looks huge, even for hydrogen: ...
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Counting basis sets in quantum chemistry

I have to use STO-nG (a "minimal basis set" - meaning that only one basis function is used for each atomic orbital in the atoms of which the molecule is made from). Let's take the example of a water ...
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GAMESS Gaussian qst2 method alternative

I've heard about the qst2 Gaussian method but I don't have any licence so I use the free GAMESS-US. Nevertheless, I didn't find any Gamess method which allows a TS finding like qst2. Have you heard ...
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Antiaromaticity can be explained by Hückel method?

Pentalene is one of the most famous antiaromatic molecules. But I obtained its energy of $\pi$ system as $8\alpha+10.46\beta$ by Hückel method (with a bit complicated but straightforward calculation). ...
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38 views

Struggling to understand wavevector K for deriving energy levels in reciprocal space [closed]

I don't really understand why, when deriving wavefunctions for arrays of atoms, the wavevector k is introduced and the general idea surrounding momentum space. please help!!
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Pauli exclusion principle

Pauli exclusion principle states that 'No two electron can exist in same state' or 'No two electron can have same set of quantum numbers'. But in reality there is no state of electron. The state is ...
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antisymmetric wavefunction [closed]

Why can't we choose any other antisymmetric function instead of a Slater determinant for a multi-electron system? Why do we choose our wavefunction for a multi-electron atom as a product of single-...
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1answer
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HCl rota-vibrational lab: ab initio simulation of rotation constants

I'm trying to set up the simulation portion of the classic undergraduate lab studying the rotavibrational spectrum of HCl and DCl. In terms of simulation (using Gaussian 16), the equilibrium ...
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Perturbation theory in Quantum/Computational Chemistry

As we know, the common methods for computation molecular properties in modern Quantum/Computational Chemistry is the Hartree–Fock method and Density Functional Theory. But from university course of ...
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Molecular orbitals used with CCSD(T) geometries

Suppose I am given some geometry data (say, of a water dimer) from CCSD(T). If I were to do a single-point energy calculation to generate orbitals for the system, wouldn't CCSD give something more ...
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1answer
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Absorbance maxima of polyenes [closed]

Can anyone tell me how to approach this question from USNCO 2013?
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2answers
122 views

Lagrange Multipliers for the derivation of Maxwell-Boltzmann distribution

When deriving the MB-Distribution while taking the number of quantum states $g_j$ into account, the final equation I'd have to solve would be: $$\ln(g_j)-\ln(n_j)+\alpha+\beta\cdot E_j=0$$ Where $n_j$ ...
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3answers
76 views

Reference states for molecular orbital energies in DFT calculations

When doing a population analysis using density functional theory (specifically as implemented in Gaussian), what are the orbital energies (i.e. alpha and beta eigenvalues) with respect to? If I have ...
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1answer
29 views

What is the velocity of de Broglie wave?

Here is my calculation: note: $ν$ – frequency, $V$ – velocity of wave, $v'$– velocity of object. $$ \begin{align} λ &= h/mv' \tag{1}\\ mc^2/h &= ν \tag{2}\\ λν &= V \quad \text{(wave ...
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281 views

Linear Combination of Atomic Orbitals

In Atkins Physical Chemistry 10th edition in chapter number 10B it is given that within the Born-Oppenheimer approximation, the Schrödinger equation can be solved for the molecular species $\ce{H2+}$ ...
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Relationship between the symmetry number of a molecule as used in rotational spectroscopy and point group

While doing a problem set I noticed that the symmetry number of a molecule turns out (usually) to be half the number of symmetry elements that the point group the molecule belongs to. When I say ...
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Canonical Restricted Open-shell Hartree–Fock equation

In Frank Jensen's Introduction to Computational Chemistry ($\mathrm{2^{nd}}$ edition), on page 99, it says: For an ROHF wave function, it is not possible to choose a unitary transformation that ...
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Why is the Maxwell–Boltzmann formula inapplicable at low temperatures?

This Wikipedia article states that at sufficiently low temperatures (or high pressure), quantum effects lead to a breakdown of Maxwell–Boltzmann statistics. I know that the spacing of energy levels ...
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1answer
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Mixing of molecular orbitals in polyatomic molecules

I am trying to solve the following Hamiltonian,$$H=-\frac{1}{2}\nabla^{2}-\sum_{i=1}^{10}\frac{1}{|\vec{r}-\vec{R_{i}}|}$$ Here $Z=1$ . To get all ther molecular orbitals of this system. I took the ...
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Good software/module in python to determine nuclear attraction integral in Slater type orbital basis sets

I am trying to solve a Hamiltonian of one electron and 10 stationary nuclear centers. here, the electron is treated quantum mechanically and nuclear centers are treated classically objects. The ...
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Why is the ROHF spin density of the propenyl radical not symmetric?

I was preparing some material for my students and in order to illustrate the concept of spin density and how the ROHF method gives a purer description of this property than the UHF method. I prepared ...
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51 views

Why is the volume related to the probability of finding an electron? [closed]

thank you for your time. I was wondering about the correlation between volume and probability of finding an electron. we know that if we move away from the nucleus, we are going to get less ...
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3answers
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Heisenberg uncertainty principle and lifetime of the excited state

A key relation in optical spectroscopy also used to derive natural bandwidth is: $\Delta E \Delta t=ℏ$ $\Delta E$ is the energy uncertainty of a state, $\Delta t$ should be the uncertainty of the ...
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Why doesn't the particle in a ring model have a potential energy term?

I was thinking of the "particle on a ring" problem as a subdivision of a bigger problem, i.e. the hydrogen atom. While in the hydrogen atom problem, the Hamiltonian contains a potential energy term, ...
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1answer
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Finding the color of crystal violet theoretically

I am trying to find the color of an organic molecule, crystal violet, theoretically. I suspect that the color is due to the large conjugated system. My question is, how would I go about finding the ...
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How to evaluate the expectation value of the Hamiltonian for a three electron Slater determinant?

I'm having trouble with a few concepts dealing with slater determinants and many electron systems. In particular, when dealing with 3 electron systems, I know that $$ \psi_0 = \frac{1}{\sqrt{6}} \det ...
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1answer
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Radial pair distribution function (VMD). How to define it for a water NaCl system with multiple Na and Cl?

I'm struggling to understand a paper in which someone has presented radial distribution functions (RDFs) for a system that contains multiple Na, Cl, and water atoms/molecules. I don't know exactly ...
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1answer
52 views

How does increasing temperature reduce the efficiency of solar cells?

I've seen several answers and trying to confirm. 1) Does temperature increase the ground state energy of electrons once they reach the positively doped semiconductor, thus reducing the voltage ...
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Why do identical protons not couple with each other in 1H NMR? [duplicate]

So basically I'm wondering why the different protons in ...let's say trans-stilbene do not couple with each other. I'm aware of the fact that they are chemically and magnetically equivalent. They ...
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1answer
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Quantitative MO theory- how is the overlap integral and the weighting coefficients of the molecular orbital actually evaluated?

Title. In my inorganic chemistry course, we learn about SALCs and qualitative MO treatment, with only a fleeting reference to the S integral and actual molecular wave functions (only hydrogen, $\ce{H2}...
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42 views

Molecular Orbital Mixing to get the Molecular Orbitals of a large molecule

I am trying to get the molecular orbital for arbitrarily placed nuclear centres (identical). In other words , I am trying to solve the following one electron hamiltonian. $$h=-\dfrac{1}{2}\nabla^{2}-\...
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1answer
52 views

Chemical bonding based on hybridisation model

A snippet from a textbook: Therefore, the hybridization model predicts that an $\mathrm{sp}$-hybridized carbon atom is more electronegative than an $\mathrm{sp}^3$-hybridized carbon atom. Evidence ...
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161 views

Dominant contribution of v = 0 → v = 1 transition in vibrational spectroscopy

Recently I had studied vibrational spectroscopy using the quantum harmonic oscillator model. After stating the gross selection rule [that vibrational oscillation demands a change in dipole moment for ...