The Periodic Table

1,3-feeds

05:33

0

Q: 2018: a year in moderation

It's New Year's Day in Stack Exchange land... A distinguishing characteristic of these sites is how they are moderated: We designed the Stack Exchange network engine to be mostly self-regulating, in that we amortize the overall moderation cost of the system across thousands of teeny-tiny sli...

A.K.

07:50

hat collected

A.K.

08:04

nvm misread criteria

Loong

09:26

19

Q: 2018: a year in closing

Continuing my sometimes-annual practice of posting these numbers, here is the information on how many questions were closed on various sites around the network in 2018: Site Asked AskedAndClosed PctAskedAndClosed AllClosed DuplicatesClosed DuplicatesReopened O...

discussion questions closed-questions statistics reopened-questions

Avnish Kabaj

12:51

@A.K. I take it you thought it was star a message intead of get a message starred?

M.A.R. ಠ_ಠ

13:08

Happy new year folks

سال نومبارک

@Loong Wow, closure rate is too high on Chem. Just . . . too high

No way we get as much relative crap as TWP

Bio is half ours

Loong

Almost half of the closed questions are closed as homework, which brings us back to our main problem.

Avnish Kabaj

Make it compulsory to have an account before you can ask

Should cut down on at least some of the zero effort he dumps

orthocresol

13:35

62

Q: "Post as guest" is a bad user experience and encourages low quality contributions. Get rid of it

Since its inception Stack Exchange has tried to make it as easy as possible to ask a question, most of the time. This was done by implementing the concept of an "unregistered" user, identified only by a cookie, who otherwise does not have to log in to a Stack Exchange site. The unregistered user...

feature-request low-quality-posts usability unregistered-users user-experience

13

Q: Why are guests not allowed to post question but allowed to post answers?

I know this isn't a big deal for me but why do guests get to post answers but not post questions?

support unregistered-users

Loong

13:49

@orthocresol hm, so Physics has that

orthocresol

14:02

Yeah, and Maths.

> Why substances show difference?

Loong

because chemistry

Avnish Kabaj

@Gaurang They come here from pages like yahoo answers and forums, etc. In general sites that do not really curate their content. What's there is there, and there might be a bit of closing and redirecting, but in principle everything is retained and the reader have to weed out the unimportant stuff themselves. We have a completely different philosophy, a different set of goals. We delete crap, and retain the good stuff. When we have a net increase of good stuff we are winning the long game. With an open philosophy like ours, you'll never discourage lazy people. — Martin - マーチン ♦ Feb 20 '18 at 13:50

I think he deleted one of his comments

orthocresol

The only deleted comment on that post is one of yours.

Avnish Kabaj

The one which said that we don't have enough traffic as compared to the larger SE sites and he doesn't mind sifting through a lot of garbage just to find one good question

@orthocresol Must have been somewhere else then

orthocresol

Yeah, you might be thinking of another post.

M.A.R. ಠ_ಠ

14:10

@orthocresol How does site traffic compare to, say, last year?

orthocresol

@M.A.R.ಠ_ಠ Well, this year we haven't had much traffic yet.

:)

Avnish Kabaj

Lulz

Loong

:-D

M.A.R. ಠ_ಠ

To be perfectly fair it's most probably us (i.e. you, not me) that's doing it right

@orthocresol OMFG THE SITEZ FAILING

Well, "last year" is a metaphor for "last year this time", which would be a metaphor for "last year around these months"

Which would be a metaphor for . . . dangit, too much procrastination

orthocresol

More seriously, 2018 activity is lower than 2017 activity.

Avnish Kabaj

14:13

Guess I'm not the boy who cried wolf

M.A.R. ಠ_ಠ

@orthocresol Well, bring out the statistician in you and analyze patterns and if there's something unusual, write a meta post

That's what mods are supposed to do. 11 suspensions in one WHOLE year? Come on, even I suspended 14 people

Avnish Kabaj

How does Military Awesome Racoon suspend other users

orthocresol

I always had a suspicion you were Shog9's sock.

M.A.R. ಠ_ಠ

@AvnishKabaj I am insulted, regular user 46306.

Avnish Kabaj

Apologies

M.A.R. ಠ_ಠ

14:17

Well get in line, I have a few more people to suspend first

orthocresol

Apology Accepted

►

M.A.R. ಠ_ಠ

Back to not procrastinating,\o

Loong

@orthocresol Users destroyed 183 0

orthocresol

Heh!

Tan Yong Boon

14:40

@orthocresol Yupz I totally agree. I remember there were more exciting discussions etc. in 2017.

chemistry.stackexchange.com/questions/91254/… By the way, anybody would like to comment on this? I don't mind criticism as long as it is constructive.

I mean that's essentially why I put a bounty on it.

orthocresol

14:54

My observation is that a lot of "regular" users have stopped visiting, or visit less. I am not sure how much we could have really done about that. I suspect the fundamental reason is that a lot of people got busy with real life.

So, even though there has been a decline in activity, I'm not inclined to kick up a big fuss about it on meta.

Tan Yong Boon

Actually, is it just the latter half of the year?

I thought the first half was pretty lively

orthocresol

There's always a seasonal trend, probably because of the academic year.

Tan Yong Boon

Yes, who would visit this forum when they can be celebrating Christmas/welcoming the New Year...

orthocresol

More like summer holidays.

Tan Yong Boon

Actually, my activity rises during school holidays and declines during school time...

There is so much work from school that I hardly have time to think about these exciting questions or write up well-referenced answers during school time. On the other hand, you just stay at home all day during vacation...

hBy2Py

15:16

@TanYongBoon Let's discuss further here.

Tan Yong Boon

Ok

I will take a picture of the relevant page. Hang on

hBy2Py

The linear algebraic diagonalization I use to get my Eq. (11) is the same thing as Jensen's " solving 3N 1-dimensional SEs and then obtains the eigenvalues"

Tan Yong Boon

hBy2Py

Well, technically I've reduced it to 3N-6 1-dimensional SEs, by projecting out the translation and rotation.

Tan Yong Boon

Yes, I know about the very complicated thing with the rotating and translating frames, conversion of Hessian to internal coordinates.

hBy2Py

15:19

Hmmm, interesting.

Looks like (Frank) Jensen takes a different approach to get to the governing SE.

Whereas Jan Jensen went the F = ma route.

Tan Yong Boon

My principal confusion is that I don't understand what has diagonalisation of Hessian matrix got to do with solving nuclear SE

I do know that diagonalisation of Hessian is used to solve the eigenvalue equation H q = k q

hBy2Py

Right -- and that eigenvalue equation is a composite of all 3N (or 3N-6) of the 1-variable SEs.

Tan Yong Boon

H is Hessian, q is the eigenvector of vibrations, k is eigenvalues, which are also force constants

hBy2Py

And, since the SEs are linear differential eigenequations, the magic of linear algebra lets you solve all of them at once, via the diagonalization.

Tan Yong Boon

So how do we get from nuclear SE which has a completely different Hamiltonian to this simple-looking eigenvalue equation H q = k q

hBy2Py

15:23

?

Tan Yong Boon

Any thoughts on that?

hBy2Py

"H" in Hq=kq is the energy operator from the nuclear S

Tan Yong Boon

Nuclear SE has both KE operator and PE operator

hBy2Py

KE is neglected

Tan Yong Boon

Really?

hBy2Py

15:25

Assumption for this harmonic oscillator approximation is usually made that the $\frac{1}{2}m_iv_i^2$ terms are small enough relative to the PE involved in the motion that they can just be entirely dropped from the SE.

It's usually pretty reasonable for ground-state vibrations

To the best of my knowledge, anyways

You get much more dramatic deviations from the harmonic-oscillator model due to the fact that the potential isn't symmetric.

Tan Yong Boon

Ok. Could u also help me to make sense of this: When one properly solves the SE on the actual (anharmonic) PES, the SE with the help of a basis expansion is turned into a matrix eq.
H c = E S c
where c are expansion coefficients which determine psi, and S the overlap matrix. This is solved as generalized eigenvalues problem which yields both E and c.

This was sent to me by an expert I consulted

But I have no clue what it means

hBy2Py

Morse potential

The Morse potential, named after physicist Philip M. Morse, is a convenient interatomic interaction model for the potential energy of a diatomic molecule. It is a better approximation for the vibrational structure of the molecule than the QHO (quantum harmonic oscillator) because it explicitly includes the effects of bond breaking, such as the existence of unbound states. It also accounts for the anharmonicity of real bonds and the non-zero transition probability for overtone and combination bands. The Morse potential can also be used to model other interactions such as the interaction between...

Hoofh

<-- not an expert in detailed quantum solutions

Ahhmm... the harmonic approximation assumes implicitly that all of the normal modes are fully uncoupled.

I.e., a displacement along mode 1 doesn't affect the energetics of modes 2...n

Mathematically, this can be expressed by saying that the overlap matrix S between the normal modes is diagonal.

Tan Yong Boon

Yes the coupling is essentially like the cross terms in Hessian right?

hBy2Py

Mmmmmmmm......rrrggh sorta?

Tan Yong Boon

I.e. off-diagonal terms

hBy2Py

15:28

The Cartesian coordinates are not the natural coordinates for the vibrational motion

And yes, therefore you get lots of off-diagonal terms in the Cartesian H

Tan Yong Boon

The natural coordinates are the internal coordinates?

hBy2Py

No, the natural coordinates are the normal modes.

For the harmonic-oscillator approximation

The internal coordinates map much more closely to the normal modes than do the Cartesian.

But they're still not perfect.

Tan Yong Boon

I see

hBy2Py

If you look at animations of normal modes, you'll often be able to pick out a handful of internal coordinates contributing to a given single normal mode

Tan Yong Boon

Does the Wiki page on Morse potential mention KE = o assumption?

hBy2Py

15:30

Not sure, but doubt it -- the harmonic oscillator page probably does, though.

It arises there because the HO model assumes no friction in the system.

Although I'm probably mixing terminology here.

Tan Yong Boon

To properly compute the vibrational spectrum one has to solve the SE in appropriate (non-redundant) internal coordinates
(T+V(r)) psi(r)=E psi(r)
where V is the potential energy surface (PES) and T the kinetic energy operator (KEO). V you can imagine as DFT (or any ab initio) energy as a function of nuclear coordinates. For molecules it has typically the shape of a well near equilibrium geometry. It is exactly parabolic in the vicinity of the minimum and deviates from the parabola at large displacements. For example, if you pull one bond you typically get a shape like this

hBy2Py

Because the HO dynamics definitely involve a continuous exchange of PE and KE.

Tan Yong Boon

^This was the exact response I got from the consultation

hBy2Py

So, IIUC there are two ways to get to the governing equation.

1) The way Jan Jensen does it, through a forces analysis -- F = ma

Tan Yong Boon

By governing eq. you mean H q = K q?

hBy2Py

15:33

<nod>

Here, the force is provided by the displacement from equilibrium, as $-kl$, and $a={d^2l \over dt^2}$.

(roughly, those probably aren't exactly right -- see Jan's derivation for the details)

Tan Yong Boon

Yes, I have learnt this approach in my physics class on SHM

hBy2Py

2) Through a "proper" Schrodinger equation approach, as given by the consultation

Tan Yong Boon

"proper"?

hBy2Py

Where you write it down like they did, with T and V(r) and everything

There, T is the second derivative

Well, hm. How does that work, actually.

Tan Yong Boon

Can I confirm that the page I showed and this expert I consulted are both using Approach 2?

hBy2Py

15:37

Yes, that's definitely the case

But I'm afraid all I've done here is reveal my ignorance of how actually to do quantum.

Tan Yong Boon

Oh it's fine I thank you very much for the insights you have provided! Have a good year ahead!

orthocresol

16:31

Wood is mostly carbon and hydrogen. I don't need a web page to support this, it is elementary chemistry. — Waylander 1 hour ago

M.A.R. ಠ_ಠ

18:52

@Loong Well I prefer elongated torture. User nuking is so unclassy

@orthocresol My observation is after the design changes, the site doesn't feel like home to me anymore

Avnish Kabaj

19:19

I think they just got fed up with the homework trash

The suggested name “(E)-bromo-2-butene” is ambiguous and not in accordance with IUPAC nomenclature. The systematic name is (2E)-2-bromobut-2-ene. — Loong ♦ 10 mins ago

I'm pretty sure Loong was on the IUPAC committee

Loong

heh

At least it should be clear that "bromo" needs a locant in "bromo-2-butene".

orthocresol

@AvnishKabaj It's very hard to say anything with any certainty.

Homework "trash" isn't a new thing.

On the other hand, I've seen multiple people say that they've been too busy to really contribute much.

And that's been the case for me too. There have been long periods where I barely do anything on the site. And the "change" in my life (basically having to do my Master's project) was relatively small. You can imagine how it might be for somebody who has moved countries, or graduated from their PhD, etc.

Loong

19:38

such things are also not motivating:

chemistry.stackexchange.com/posts/107322/revisions

fine, have a downvote instead

orthocresol

:/

@hBy2Py spotting you in chat be like:

A surprise, to be sure, but a welcome one

►

Happy New Year!

hBy2Py

Thanks! Happy New Year!

Yeah ... I'm focused more on music and coding side projects these days.

That and work's been a much steadier drumbeat

orthocresol

Playing music, or writing music?

Loong

Hmpf, more and more snow. That will be fun tomorrow morning to find my car and dig it out.

hBy2Py

Playing/singing

We bought a proper piano; I played a clarinet duet for our church's Christmas Eve service; and I started a casual men's quartet, currently singing barbershop and Palestrina.

(@ortho)

orthocresol

20:01

Sorry, I was cooking dinner!

hBy2Py

<grin>, no worries -- sorry to pester.

orthocresol

Never knew you could do all that ;) Have you been playing the clarinet for long?

hBy2Py

Played extensively for 13yrs, 5th grade through undergrad.

Hiatus through grad school, up to this year.

orthocresol

Ooh, that's a long time.

I've been thinking about picking up either the clarinet or the oboe...

Probably won't happen for a while though. (And I'm leaning towards the oboe, haha)

hBy2Py

Lot more demand for oboes -- double-reeds are rather trickier to play, though.

My mom wanted me to play oboe when I first started in band. I was apparently quite firm in wanting to play clarinet, though.

Yeah -- in retrospect, I never should've stopped playing regularly through grad school -- all of the ensembles at MIT were crazy competitive, and I didn't want that level of commitment... but instead of just stopping, I should've tried to scrape together a more casual chamber group or something.

orthocresol

20:10

Mmhmm, I can imagine that it's tougher for woodwinds/brass players too. As a violinist, it's pretty easy to get into orchestras...

And even if you're not super serious about playing the instrument, it's ok because you can hide behind the other violinists :)

M.A.R. ಠ_ಠ

20:47

@Loong Oh but that's accidental override. The OP started editing before you finished yours

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