26 votes
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Is it right to neglect very small imaginary frequencies?

tldr: This is something of an eternal debate. IMHO very small imaginary frequencies can be okay, but it depends on your system and needs. As you might see from the various comments above, there are ...
Geoff Hutchison's user avatar
21 votes
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What's the point of doing high accuracy spectroscopy calculations?

The paper in your question currently has 159 citations and was published in one of the most prestigious journals (Physical Review Letters). It is about high-precision spectoscopy on $\ce{H2}$, which ...
Nike Dattani's user avatar
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17 votes

Is it right to neglect very small imaginary frequencies?

Just want to add a small comment upon Geoff's excellent answer. One may be tempted to think that a $<\pu{10 cm-1}$ imaginary frequency introduces a negligible error to the energy or the property of ...
wzkchem5's user avatar
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14 votes
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How to export a calculated Raman spectrum in Q-Chem, as a Gaussian profile?

I wrote the following simple Python function that allows one to use any distribution from scipy.stats for peak broadening (the ...
schneiderfelipe's user avatar
13 votes

Bond length from infrared spectra?

A partial list of bond lengths I have determined from IR spectra over my career: \begin{array}{ccc} \rm{Molecule} & \rm{Bond ~Length ~ (picometers)} & \rm{References}\\ \hline \ce{Li_2}(1^1\...
Nike Dattani's user avatar
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12 votes
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How to determine which levels correspond to each other in LS coupling and jj-coupling?

In the jj-representation, each electron from $i$ to $N$ will have: $\vec{l}_i$ (orbital angular momentum), $\vec{s}_i$ (spin angular momentum), and $\vec{j}_i=\vec{l}_i + \vec{s}_i$ (total angular ...
Nike Dattani's user avatar
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12 votes

Are there any benchmark test sets for UV-vis absorption maxima?

There is a program called PhotochemCAD by Tanuchi and Lindsey which contains a database of experimental absorption and emission spectra of more than 300 organic and organometallic compounds. The ...
wzkchem5's user avatar
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11 votes

Can you estimate the electronic excited state lifetime of a molecule?

The Einstein A coefficient (as mentioned in Cody's answer) tells you the probability per unit time of spontaneous radiative decay from an excited state to a lower state. The Fermi golden rule (as ...
Nike Dattani's user avatar
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11 votes

Bond length from infrared spectra?

This is a task suited to single crystal XRD. This does require you to be able to form a crystal of your substance which can be difficult at times but this allows for potentially high accuracy of bond ...
Tristan Maxson's user avatar
11 votes

Bond length from infrared spectra?

I agree with all answers provided so far: you cannot quantitatively deduce bond lengths from infrared spectra. However, see the answer by Nike Dattani about the inverse, predicting IR spectra from ...
ProfM's user avatar
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11 votes

How to export a calculated Raman spectrum in Q-Chem, as a Gaussian profile?

Many thanks to Felipe, the code is very useful! I know this is just a simple editing, but this is how I adjusted part of the code for Raman spectra: ...
Dmitry Eremin's user avatar
11 votes

What's the point of doing high accuracy spectroscopy calculations?

One of the eternal questions in computational modeling is whether we truly understand all the relevant contributions. I believe the combination of experiment and theory in the paper suggest that if ...
Geoff Hutchison's user avatar
10 votes
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How exactly does one compute the vibrational density of states from the output of a molecular dynamics simulation?

Ok, I figured out the answer, so I will go ahead and answer my own question. First, I will provide an overview in bullet points: [Optional] Generate the velocities at evenly spaced frames from the ...
jheindel's user avatar
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10 votes
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What is the impact of the orientation of a molecule in cartesian coordinates to molecular properties?

Actually, quantum chemistry codes tend to reorient molecules to make calculations more reproducible, since as was already mentioned in a comment above, the use of quadrature makes density functional ...
Susi Lehtola's user avatar
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10 votes
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Can you estimate the electronic excited state lifetime of a molecule?

tldr; it's related to the excitation energy The lifetime of an excited molecule assuming there are no non-radiative pathways, is related to the Einstein A coefficient: $\left(\frac{dn_2}{dt}\right)_\...
Cody Aldaz's user avatar
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9 votes
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Where did the "Computational Chemistry Comparison and Benchmark DataBase" find its scaling factors for vibrational specra?

I have a different page on the NIST website (https://cccbdb.nist.gov/vibscale.asp) that gives uncertainties in the scaling factors. I believe many of these were actually established by NIST themselves ...
Geoff Hutchison's user avatar
9 votes

Bond length from infrared spectra?

Addendum: While the method described below is commonly used by experimentalists, I was mistaken that IR spectrum could not be used to obtain bond lengths. Nike's answer does a great job explaining how ...
Tyberius's user avatar
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9 votes
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Problem obtaining a Raman spectra of a graphene structure

Since you include the command cphf=RdFreq, Gaussian expects you to have entered a perturbation frequency after the rest of the input. You need to specify a ...
Tyberius's user avatar
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9 votes
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Different types of transition dipole moment in ORCA output

You have stumbled across a problem that was described in 1983 in the following way (note that the "40 years" refers to 1943-1983): "Over the past 40 years, many authors have tried to ...
Nike Dattani's user avatar
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8 votes
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Regarding oscillatory strength theoretical units to experimental ones

The expression you are describing is equation (6) from your first link: $$R_j=\frac{3\hbar c\ln(10)1000}{16\pi^2N_A}\int_\text{band j}\frac{\Delta\epsilon}{\omega}d\omega\tag{1}$$ which defines the ...
Tyberius's user avatar
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8 votes

How can I simulate the work function given by Ultraviolet Photoelectron Spectrum (UPS)?

You can use Galore package. Galore is a package which applies Gaussian and Lorentzian broadening to data from ab initio calculations, such as VASP. The two main intended applications are: Gaussian ...
Jack's user avatar
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8 votes

Can you estimate the electronic excited state lifetime of a molecule?

Fermi golden rule should give you the transition probability (per time), from which you should be able to estimate the lifetime. You should know of be able to estimate the density of states in the ...
sleepy's user avatar
  • 291
8 votes

Understanding emission spectra using TDDFT calculation

In general, you should use the $S_1$ state by default, unless you have reasons to suspect that your molecule is anti-Kasha. IMHO satisfying one of the following criteria guarantees that your molecule ...
wzkchem5's user avatar
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8 votes

Emission and absorption UV-Visible spectrum using GAUSSIAN-09

Roughly speaking, absorption spectra are obtained by TDDFT calculations on optimized ground-state geometries, while emission spectra are obtained by TDDFT calculations on optimized excited state ...
wzkchem5's user avatar
  • 8,946
8 votes

What's the point of doing high accuracy spectroscopy calculations?

Your arguments can be said about numerous other articles (if not all but a very few ones). What seems negligible in scientific impact at the publication time might be heavily influenced by other ...
nickpapior's user avatar
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7 votes
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What phosphorescence material emits light for a 1 hour period?

Strontium aluminate can be doped with europium to give a material with properties like you are expecting. This can be optimized to give maximum fluorescent yield as well, but keep in mind this will ...
Tristan Maxson's user avatar
7 votes
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How to create a differential spectrum from two simulated spectra, calculated by two different basis sets?

If the peaks shift inconsistently, you will never fix this with scaling factors etc. Consider if you can make a table of peak assignments with relative shifts + maximum peak heights instead. However,...
Tristan Maxson's user avatar
7 votes
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How are molecular rotational states modeled within the Born Oppenheimer Approximation for polyatomic molecules?

The Hamiltonian Just as for vibrations we have the harmonic oscillator approximation, for rotations we often use the rigid rotor approximation, where bond lengths are fixed. Recall the rigid-rotor ...
Nike Dattani's user avatar
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7 votes

Transformation between two different sets of molecular vibrational normal coordinate systems

The paradox discovered by @HansWurst comes by the assumption that it is possible to treat both minima on the 1D PES separately and define a separate set of normal coordinates for them. 3N normal ...
Emil Zak's user avatar
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7 votes
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Transformation between two different sets of molecular vibrational normal coordinate systems

I agree with Emil Zak's answer that normal mode coordinates from different minima should, in general, be put together with special care. However, relating the two different sets of normal mode origin ...
Y. Zhai's user avatar
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