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Quantum chemistry is a subfield of quantum mechanics. Like its parent field, quantum chemistry focuses on physical phenomena that occur at an atomic scale, usually specifically aiming at chemical reactions.

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Help understanding the equation for Spin Energy in Zero Field

I’m working with a system that experiences spin frustration and for me this is the first time dealing with such a phenomenon. To grasp the concept, I’ve decided to go to the basics and read the ...
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1answer
30 views

Magnetic couplings using DFT in a spin frustrated system

I’m working with a trimeric copper system that have the classic triangular shape and experiences spin frustration. This is my first time dealing with such system and, so far, from what I’m reading, I ...
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2answers
32 views

Restrictions on transitions in Spectroscopy

Just for the sake of an example, let's assume a transition from one energy state to a higher energy state in atomic absorption spectrum If the energy difference between the ground state and first ...
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1answer
35 views

Eigenvalues of Hessian matrix when the molecule is not at its equilibrium configuration

Given the potential energy surface (PES) of a system, we can calculate the Hessian matrix $H$ of each point $x$ on the PES. $$H_{ij}(x) = \frac{\partial^2 E}{\partial R_i \partial R_j}|_{R = x}$$ ...
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What is the intuition of using “dimer method” for searching transition states?

I learned dimer method for searching transition state in this paper: A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. However, I still ...
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41 views

What is the difference between carbon in an excited state and with sp³ hybridization?

What is the difference between the carbon in the excited state, $\ce{[(1s)^2] (2s)^1 (2p)^3}$, and $\pu{sp^3}$ hybridization?
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27 views

Why density functional theory favors a uniform electron density?

Why density functional theory (DFT) favors a uniform electron density when we use it to calculate ground state electronic structure self-consistently? For example, when we use DFT for calculating ...
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0answers
33 views

Keeping the electronic configuration in the ground state is expensive in Ab-initio MD?

In E. Sandré & A. Pasturel's An Introduction to Ab-Initio Molecular Dynamics Schemes If using small time steps (typically in the order of a few tenth of femto-second), the changes in the ...
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1answer
54 views

Why $\psi(r_e, r_n) = \sum \psi_e^i(r_e|r_n) \chi^i(r_n)$ in Born-Huang expansion?

Given a molecular system, the time-independent Schrodinger Equation (SE) that we want to solve is $$\hat H \psi(r_e, r_n) = E \psi(r_e, r_n)$$ $\hat H$ is hermitian, so the set of solutions $\psi(r_e,...
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1answer
33 views

What exactly is a potential energy surface?

I first learned about potential energy surface (PES) when I was studying Born-Oppenheimer approximation. In the approximation, the PES is essentially electronic PES which is defined as follows: ...
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0answers
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Given 1 wavelength how would I calculate the energy change of an atom?

Ex. wavelength is $2500$nm, what is the energy change of an atom? I rearranged the equation $c=\lambda\nu$ to $\nu=c/\lambda$ and subbed it in for $\nu$ in the equation $E=h\nu$ and got $E=h(c/\...
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Calculation of average bond length from force constant and IR absorption?

I need some help beginning a problem set problem (just want hints, I'd like to figure it out myself). The problem gives me the IR absorption for a diatomic, as well as the force constant for the ...
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25 views

Energy of hydrogen like species

In a solution it is given that energy of an electron in an orbital of hydrogen like species depends only on principal quantum number.is it correct?Will it depend on Z according to the general ...
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1answer
30 views

How does the Wave-Mechanical model reconcile with the particle definition of an electron?

While just starting chemistry, I have a little pit of particle physics knowledge. I was introduced with the wave-mechanical model of an atom and was slightly confused, because from particle physics, I ...
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1answer
72 views

Is the Lowdin Orthogonalization used in diagonalizing the atomic orbitals really a basis change?

This is kind of more on the mathematics side of quantum chemistry, but I can't quite figure out why the Lowdin Orthogonalization is called a basis set change. I get how it works from the perspective ...
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42 views

Structure of atom [closed]

Consider one He+ ion in exited state(n=5).In the answer it is given that no. of spectral lines produced is 4 not 10.can't we use the general equation?
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51 views

If a single electron fills a degenerate energy level, does it fill all degenerate orbitals at once?

For example, if a single electron fills the 2p orbitals. Since all orbitals are degenerate, there is no reason why there would be any preference for px, py, pz. Is the electron hence in a ...
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99 views

How to sloppy freeze a dihedral angle within a range in a Gaussian modredundant optimization?

I would like to perform an optimization in Gaussian 09 of my molecule with two dihedral angles being frozen. I would like that the values of these dihedral angles are not fixed, but can relax ...
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483 views

Visualizing atoms in XYZ file with colors representing partial charges

I have a file containing the atomic coordinates (in XYZ format) of a structure, and I also have a list of the partial atomic charges for each atom. I'd like to create an image of the structure where ...
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1answer
86 views

Wave-particle duality in electron [closed]

Electrons show wave-particle duality Particle (atom) containing a wave (electron) on Quora Atoms contain electrons. We consider atoms as particles but electrons which are inside the atom as waves (...
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0answers
33 views

Spin quantum number ($s$)?

Spin quantum number ($s$) : In atomic physics, the spin quantum number is a quantum number that parameterizes the intrinsic angular momentum of a given particle. (Definition by Wikipedia) ...
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1answer
42 views

What are the magnetic quantum numbers for specific d orbitals? [closed]

I mean I know they’re $\ce{d_{-2}}$, $\ce{d_{-1}}$, $\ce{d_{0}}$, $\ce{d_{1}}$, $\ce{d_{2}}$ but how do these numbers relate to each one of $\ce{d}$ orbitals $\ce{d_{z_2}}$, $\ce{d_{xz}}$, $\ce{d_{yz}}...
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2answers
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Why is the first affinity energy negative?

So, I've found a number of somewhat obscure responses to this question but I'm looking for something concrete: Why is the first affinity energy negative for most elements? I appreciate that this is ...
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0answers
27 views

Lewis structure of atoms and electron configuration

In chemistry textbook we usually see Lewis strucutre of carbon with 4 dots ( because it have 4 valence electrons). But my teacher say it's false representation of carbon, it's rather 2 dots( each dote ...
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1answer
48 views

Why do the 4f and 5d orbitals have similar energies?

I've come to know this phenomena when learning about the exceptions to the aufbau principle. But there is no explanation of why 4f and 5d have similar energies (as I've read in some websites).
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How are the LCAO ansatz and Slater determinant ansatz for the wavefunction connected?

Say we have a system of 2 electrons, an ansatz for the wavefunction is then a Slater determinant consisting of two orbitals. $$ \left|\Psi_{Slater}\right>= \frac{1}{\sqrt{2!}}\left(\left|\phi_1\...
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3answers
217 views

What is Nesbet's theorem?

In a talk by Frank Neese there was a slide with title "Nesbet's theorem" and an equation indicating something like "if your wavefunction has the double excitation amplitudes correct, then you can get ...
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0answers
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Is the energy of the orbital an electron “resides” in, considered to be a factor of that electrons ionization energy? [duplicate]

The answers to a Quora question mainly refer to the factors as generally being, the electrostatic forces of attraction between the electron and the positively charged nucleus and all other events, by ...
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1answer
102 views

Why do electrons in sigma bonds have zero orbital angular momentum about the internuclear axis?

As per the title, why do electrons in σ-bonds have zero orbital angular momentum about the internuclear axis? As far as I know, the outermost electrons participate in bonding. Suppose that the 3p ...
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0answers
26 views

Can I calculate the adsorption energy on the surface of an inorganic crystal in MOPAC?

I am new to quantum chemistry software and can't find if it is possible to calculate the adsorption energy on the surface of an inorganic crystal in MOPAC2016 or do I need another program like GAMESS ...
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1answer
116 views

Detail from Integral Evaluation in Szabo (Appendix)

This is a step from Appendix A in Szabo and Ostlund's Quantum Chemistry. In short, the question is how to get from \ref{A.20} to \ref{A.21}. We can factor out some constants $K$ in the expressions to ...
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67 views

Rotational barrier of biphenyl in ground and excited electronic states

I'm studying how the energy of biphenyl varies with its dihedral angle. I know how to describe the dependence quantitatively, but still, I don't know how to explain why the rotational barrier of ...
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50 views

Oscillator strength of a transition

I am trying to compute the oscillator strength in a molecule. Therefore I've got its Energy level values plus both wave functions $\Psi_i$ and $\Psi_j$ for the transition I want to calculate the ...
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0answers
55 views

Hückel method for biphenyl

I am trying to calculate the energy levels of biphenyl (see picture below) using Hückel method. Since this is my first time using Hückel I am a little confused. Biphenyl has 12 carbon atoms, ...
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What is the difference between non-adiabatic coupling and diabatic coupling?

The diabatic couplings are the off-diagonal terms of wave functions written in a diabatic basis. There is a need to use the diabatic basis when there is a breakdown of the Born-Oppenheimer ...
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1answer
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Distinguish between two possible configurations for angular momentum in carbon atom

Consider the possible values of $S$ and $L$ for carbon configuration $1s^2 2s^2 2p^2$ and the corresponding rapresentations with arrows indicating the spins (consider only $S=0,L=0$ and $S=0,L=2$, as ...
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2answers
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Is there a finite number to the possible state of electron in an atom?

Okay, I know you may say, there are infinite numbers of possible state of an electron between its ground state and ionic state, since energy levels get continuous as electron gets farther away from ...
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0answers
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Identifying principal quantum numbers in natural population analysis

Following a Hartree-Fock calculation, I would like my program to perform some kind of Natural Bond Orbital (NBO) analysis. Preceding this I would like to output a list of atomic orbitals, along with ...
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1answer
124 views

How do restricted open shell calculations mathematically proceed in the context of the self consistent field iterations?

I'm specifically concerned with HF theory. I understand restricted closed shell calculations use the Roothaan equations. If I understand correctly these equations assume each filled molecule orbital ...
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Understanding two-electron integrals in Gaussian 09

I was comparing the two-electron integrals (electron repulsion integrals, ERI) printed by various well-established packages (Gaussian, GAMESS, Molpro etc.) and observed that Gaussian is the oddball in ...
3
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1answer
73 views

How are imaginary frequencies and force constant calculated by DFT?

Computational calculations allow us to simulate the frequencies of molecules. They can even tell us if the optimized structure is a minimum, a saddle point or a maximum according to the number of ...
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64 views

The nature of Chemical Bonds [closed]

It was nearly a year ago when I appeared for my first interview through the KVPY (a research oriented scholarship exam) when one of the questions the panel asked me was : What is a chemical bond? ...
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1answer
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Franck-Condon factors in photoelectron spectroscopy

Franck-Condon factors have been used to predict / assign the vibrational structure of photo-electron spectra. $FCF=|\langle\psi''_{vib}|\psi'_{vib}\rangle|^2 = |\langle\psi''_{vib,1}\psi''_{vib,2},\...
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How to know about radical character of a diradical molecule?

I understand that the radical character is quantified by the occupation number of the lowest unoccupied natural orbital obtained from UHF calculations or from CASSCF calculations. My issue is that I ...
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72 views

Why is spin-contamination undesirable?

Why is spin-contamination, introduced by e.g. Unrestricted Hartree Fock, undesirable, and to which problems further on can it lead? The only problem I currently see myself is that spin-contamined ...
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0answers
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Does a constrained wave function method exist similar to constrained density functional theory?

I am trying to find out if a constrained wave function method exists for running quantum chemistry calculations. I know that constrained DFT exists and can be used to constrain the charge density to ...
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Do molecules change shape when they become vibrationally excited?

In quantum mechanics we learn the the classical picture of a vibrational excitation is wrong. The atoms will 'vibrate' only while the molecule is in a coherent superposition of two vibrational states, ...
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Which natural orbitals are better for CASSCF calculations for organic diradicals?

I am trying to perform CASSCF calculations for a series of diradicals. As input orbitals I have used two types of natural orbitals. One is unrestricted natural orbitals and the other MP2-based natural ...
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Why is density fitting not used to reduce four-center integrals further down to two-center integrals?

In density fitting, why don't researchers turn all the 4-center integrals into 2-center ones instead of stopping at composing them from 3-center integrals?
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How to calculate a molecule's oscillator strength?

I was wondering how to calculate a molecule's (e.g. biphenyl) oscillator strength using a very simplified model. If I want to calculate a molecule's excited states a good start is something like a ...