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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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How to calculate transition dipole moment from two known wavefunctions

I'm interested in calculating the transition dipole moment (TDM) from the information from two wavefunctions of different states. This is somewhat similar to calculating the molecular dipole moment ...
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What is d-orbital collapse?

When I was perusing the works of Schwarz on atomic structure, I came across the unfamiliar term of d-orbital collapse. He describes it as a variation in energetic sequence from group 1 to 3 elements ...
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Rotation around CC bond in 1,2-dioxetane: transition states and local minimum structures

TLDR: What: Potential energy surface associated with rotation around CC bond in the ring-opened version of 1,2-dioxetane. How: State average(4)-CASSCF(12,10)/VTZP constrained geometry optimizations ...
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Accuracy of quantum chemistry calculations on iodine neglecting relativistic effects

Triiodothyronine is a molecule with three iodine atoms. If we attempted to calculate its properties with ab initio methods like Hartree-Fock or more complicated ab initio methods which included ...
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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 ...
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Using symmetry and group theory arguments to explain iron(II) in a tetrahedral crystal field

I am trying to figure out how to explain $1s \rightarrow 3d$ spectroscopic transitions for $\ce{Fe^{2+}}$ in $T_\mathrm{d}$ symmetry. These transitions make up the pre-edge region in K edge X-ray ...
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Is there a reliable chemical theory that predicts pKa based on structure?

Obviously, there are general stability arguments that can be made to estimate relative pKas, such as evaluating the stability of the conjugate base of an acid, or thinking about how polarized the bond ...
sweetandtangy's user avatar
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Vanishing integral rule for two-electron integrals

I understand the vanishing integral rule for a one-electron integral to be: $$\langle i | \hat{O} | j \rangle = 0 \hspace{1cm} \mathrm{if} \hspace{0.2cm} \Gamma_O \neq \Gamma_i \otimes \Gamma_j$$ ...
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multielectron spin-orbit coupling and energy level

For multi-electron atoms for which the Hamiltonian (including the spin-orbit coupling) reads $$H=\sum_i T_i-Z\sum_i V_i+\sum_i V_i^{\text{s.o.}}+\sum_{i>j}V_{ij}$$ The $T_i$ are the kinetic ...
QuantumDot's user avatar
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Where is bifurcation situated on cyclohexane isomerisation PES?

I am wondering what does PES for cyclohexane isomerisation looks like. I have two possibilities in mind but each has some kind of vagueness. 1) Starting from boat conformation minimum, followed by ...
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Why do nuclei move considerably slower than electrons

I've been trying to learn quantum chemistry at an introductionary level. While reading I've found out that the Born-Oppenheimer approximation seems to be the reason for the basic and crucial model of ...
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Additional example determining Marcus parameters searched

Background / theory: The electron transfer in organic materials was formalized by the Marcus theory, introducing $|H_{ab}|$ to describe the how molecules (or more generally: sites) are coupled with ...
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Beer's law: UV-Vis absorbance values exceeding 2

In classical analytical chemistry, absorbance values in Beer's law can theoretically range from 0 to infinity. As general rule of thumb neither high, nor very low absorbance are used for ...
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Why SCF-MO methods fail when treating bond-breaking processes?

I'm currently studying the EVB(Empirical Valence Bond) approach. While reading on this article (https://pubs.acs.org/doi/10.1021/ja00224a011), the author mentioned about the usage of SCF-MO formalism ...
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Obtaining transition density matrix

I'm trying to assemble a transition density matrix on quantum computer using one-body excitation $\widehat{E}$ operator on 2 active spatial orbitals, i.e. on 4 spin-orbitals. The problem is, I need to ...
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Are there examples of publications using DFT instead of HF as a coupled-cluster reference?

I have been searching for a while, and only managed to find an article by M. Saitow,[1] where CAM-B3LYP orbitals are used. I was wondering if this has been done elsewhere. It was recommended to me by ...
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Hund's rule for comparing term symbol energies in excited state

This question was asked in an exam: The lowest energy electronic state for excited state carbon atom is, a. $^1D_2$ b. $^3D_1$ c. $^3D_3$ d. $^3D_2$ Although we have been taught Hund's rule for ...
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Solving the Schrödinger equation for a rotating triatomic linear molecule

This source is showing that solving the Schrödinger equation for a triatmoic linear molecule yields the same formula for the rotationaI quantum states $BJ(J+1)$ as for dipoles. For dipoles, the total ...
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How long would it take for a tank of same-spin hydrogen atoms to become a tank of H₂?

In the question Is there an energy cost associated with flipping the spin of an electron?, it is shown that it is very unlikely for two hydrogen atoms to bond if their electrons have the same spin. ...
Display Name's user avatar
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How to extract force constants for normal modes from ORCA?

I am following the computational method described in Scientific Reports.[1] Where the authors analyze the contribution of each excited state normal-modes to the shift of the transition energy. For ...
Avishai Barnoy's user avatar
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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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Vibrational Self-Consistent Field

There are a few things about VSCF that I don't understand: Which coordinates do you work in? If you work in normal mode coordinates, how do you evaluate the effect of the average of one normal mode ...
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Platt's Notation/Nomenclature for transitions

How do you read Platt's Notation / Nomenclature? Where can I learn this? Some examples where I've seen this mentioned: In Platt's notation the transitions are designated as $\ce{A -> B}$ and $\ce{...
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Program to compute multipoles of water at CCSD(T)-F12 accuracy

I want to compute traceless cartesian electrostatic multipole moment tensors up to fourth order (hexadecapole), at CCSD(T)-F12 accuracy using a large basis set. I'd like to be able to choose the ...
Jonatan Öström's user avatar
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327 views

How to define the initial guess for the electron density?

This is a technical question. If I have a one-dimensional box of length L, and an electron density existing in the box and integrating to N electrons. And if I want to determine the electron density ...
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N2 HOMO-LUMO Gap calculation

I am thinking about calculating the HOMO-LUMO energy gap for $\ce{N2}$ molecule. I thought I could model this as a particle in a box problem to find $E_\text{HOMO}$ and $E_\text{LUMO}$. The formula I ...
kevin Tah N.'s user avatar
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153 views

Combining two overlapping wavefunctions

Let's assume that we solved the many-body Schrödinger equation for two molecules and we can now write their wave functions as $$ |\Psi_1\rangle = O_1|\operatorname{vac}\rangle\,,\\ |\Psi_2\rangle = ...
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Meaning of $^2[\,1/2]$ terms in atomic spectroscopy

I was trying to analyze some atomic emission spectroscopy data and I encountered unfamiliar notation of the terms for me. Commonly, the electronic states of the atoms can be described by quantum ...
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Splitting of atomic orbitals due to Stark effect

I am performing graduate level research on ZEKE Photoelectron Spectroscopy and central to this form of spectroscopy is Rydberg states and the Stark effect. The Stark effect is caused by an electric ...
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Stress-Strain Graph Under Uniaxial and Biaxial Strain

I was going through a recently published journal paper by ACS (Applied Nano Materials) about gas separation using nanoporous Graphene. DOI No. - https://dx.doi.org/10.1021/acsanm.0c02340 Supporting ...
aritraroy24's user avatar
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On the localized screening of cerium (when compared to iron)

I was reading the paper 'A linear response approach to the calculation of the effective interaction parameters in the LDA+U method' by Cococcioni - (Paper here) . Quoting from the first paragraph on ...
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Is there a mistake in diagramatic representation of RS perturbation theory described in Szabo and Ostlund textbook?

I'm trying to understand rules of the diagrammatic representation of the many body perturbation theory that described in Szabo and Ostlund "Modern Quantum Chemistry" textbook. One of the ...
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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 ...
Max's user avatar
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How to derive a state symmetry in D2h point group from its wavefunction?

Let's have an atomic carbon with the following electron configuration: $$ 1s^2 2s^2 2p^2 $$ One of it's levels is ${}^1S_0$, which is corresponding with the following state: $$ \begin{align} \left| ...
Eenoku's user avatar
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5 votes
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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 ...
obackhouse's user avatar
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5 votes
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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 ...
GingerBadger's user avatar
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5 votes
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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 ...
p_punkt's user avatar
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MCSCF Hessian transformation

I have tried doing the transformation of the Hessian in MCSCF following the notation that is used in Molecular Electronic-Structure Theory (MEST). Here I try to show what steps I have taken (to get ...
Erik Kjellgren's user avatar
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171 views

When is it necessary to consider size consistency?

For what kinds of reactions is it essential to account for size consistency? To my understanding for dissociation reactions $(\ce{AB -> A + B})$ it is important to consider. However, is it also ...
Dr.K 's user avatar
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0 answers
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Methodology and basis set for study in UV-Vis

I am currently working with a system of 30 atoms of a organic molecule. I need to make a study related to determine the UV-Vis graphic as precisely as possible. I am using DFT method. However, I am ...
ChemistBR's user avatar
5 votes
0 answers
107 views

Advanced Quantum Chemistry Reference for Reactivity Phenomena

I'm currently in organic synthesis seeking to develop quantitative "physical" (that of a physicist), rather than "chemical", intuition for areas where traditional VSEPR/ MO models cannot fully ...
Dragonsheep's user avatar
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Is there any molecule describable by three-level with mixed parity and transition dipole moments in different directions?

I would like to know if there are molecules described by a three-level system with mixed parity and transition dipole moments in different directions. By mixed parity I mean all three transitions can ...
Pu Zhang's user avatar
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0 answers
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Why isn't CASPT2 used in surface hopping simulations?

I will write what I think the answer is. I'd like someone to confirm and maybe explain in detail. I think that since surface hopping dynamics simulates movements on PES, it has to calculate gradients ...
voldermot's user avatar
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5 votes
1 answer
338 views

What are energies in computational methods relative to?

As per my understanding, in molecular mechanics (MM) the computed strain energy is based off the strain inherent in the molecule under study relative to an idealized form. For example, cyclobutane to ...
Dr.K 's user avatar
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5 votes
1 answer
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How does one calculate chemical shift in a gauge independent manner?

The literature on ab initio calculations of chemical shift in NMR experiments usually provide Lamb's and Ramsey's formulae as the solution. Yet the expressions explicitly depend on vector potential ...
Igor Traskunov's user avatar
4 votes
0 answers
61 views

The sign of the lobes in an atomic orbital

In atomic orbital diagram of an atomic orbital, the lobes are represented as either positive or negative. It is said that the wave function is 'positive' where the lobe is depicted positive and '...
Nothing's user avatar
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4 votes
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Notation for spectral density in experimental community

In an open quantum system setup where the system is coupled to infinite harmonic oscillators as bath (as in the Caldeira leggett model, for example), through the Hamiltonian, $$H = \frac{\hat{p}^2}{2m}...
Prem's user avatar
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0 answers
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How to obtain excited state energies from configuration interaction? (Helium example)

A theory of CI method For going beyond the Hartree–Fock (one determinant) approximation one introduce configuration interaction (CI). Interaction means the mixing (interaction) of different electronic ...
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Is the shortness of boron trifluoride's bonds better understood and explained now than it was "way back" in the 20th century?

Wikipedia's Boron trifluoride; Structure and bonding says: In the boron trihalides, BX3, the length of the B–X bonds (1.30 Å) is shorter than would be expected for single bonds,7 and this shortness ...
uhoh's user avatar
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4 votes
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Schrodinger operator with matrix potential and Green's function

Clearly Schrodinger operators with matrix potentials appear very naturally in molecular dynamics/quantum chemistry, particularly when considering a crude adiabatic basis or diabatic basis for an ...
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