188 votes
Accepted

Why is gold golden?

Yes, this is a beautiful question. As you said, in lower rows of the periodic table, there are relativistic effects for the electrons. That is, for core electrons in gold, the electrons are traveling ...
user avatar
131 votes

Why is gold golden?

See the footnotes I've included if you would like to see more of the detail behind a specific statement. The figure below compares the reflectance spectrum for silver and gold (let's forget about ...
user avatar
  • 81.9k
61 votes

Why is gold golden?

This effect comes from the band structure of the metal, not atoms. In simplest terms (see, e.g., Ashcroft & Mermin, Solid State Physics, chapter 1) the electrons in the conduction band have a ...
user avatar
  • 7,621
55 votes
Accepted

Why is chemistry unpredictable?

First of all, I'd ask: what do you admit as "chemistry"? You mentioned thermodynamics as being a field where you have "models to predict results". But thermodynamics is extremely ...
user avatar
  • 65.8k
45 votes
Accepted

Why is FORTRAN so commonly used in computational chemistry?

I don't think that's really true anymore. Some Fortran use is historical (i.e., early codes were developed in FORTRAN because that was the best programming language for number crunching in the 70s ...
user avatar
38 votes

Why is chemistry unpredictable?

Let me contribute two more reasons which make chemistry hard to analyse from a purely theoretical standpoint. The first one is that, viewed very abstractly, chemistry essentially relies on the study ...
user avatar
36 votes

Why is chemistry unpredictable?

Parts of chemistry have predictability but the combinatorial complexity of what is possible leaves a large amount of space for things that don't follow the rules Some of the ways chemistry differ from ...
user avatar
  • 32.1k
32 votes
Accepted

What's the difference between PBE and B3LYP methods?

PBE The PBE functional${}^{[1]}$ belongs to the class of generalized gradient approximation (GGA) functionals for the exchange-correlation energy $E_{\mathrm{xc}}$. Considering that the dependence $...
user avatar
  • 17.3k
32 votes

Why is FORTRAN so commonly used in computational chemistry?

I think it does make sense to provide a somewhat alternative view and to clarify the matter. FORTRAN vs. Fortran First off, one has to distinguish the old FORTRAN from the new Fortran, where, by ...
user avatar
  • 18.4k
32 votes
Accepted

Born–Oppenheimer adiabaticity

$\newcommand{\conj}[1]{\overline{#1}{}} \newcommand{\braket}[2]{\langle{#1}\,|\,{#2}\rangle} \newcommand{\bracket}[3]{\langle{#1}\,|\,{#2}\,|\,{#3}\rangle} \newcommand{\mat}[1]{\mathbf{#1}} \...
user avatar
  • 18.4k
32 votes
Accepted

What is the difference between dynamic and static electronic correlation

$\newcommand{\el}{_\mathrm{e}}$In quantum chemistry, when a nomenclature in which one distinguishes between "static" and "dynamic" correlation is used, "correlation" referrers to all the deficiencies ...
user avatar
  • 18.4k
31 votes
Accepted

How to find the second order perturbation to wave function?

Disclaimer This post is some kind of a legacy post. Find the notation used in the question in the other answer. I added this proof as I was not entirely certain I understood the notation correctly. As ...
user avatar
29 votes
Accepted

Is density functional theory an ab initio method?

First note that the acronym DFA I used in my comment originates from Axel D. Becke paper on 50 year anniversary of DFT in chemistry: Let us introduce the acronym DFA at this point for “density-...
user avatar
  • 18.4k
29 votes
Accepted

What is the exchange interaction?

In quantum chemistry, probably the easiest way to understand the "exchange interaction" is within the context of the Hartree-Fock model. $ \newcommand{\op}{\hat} \newcommand{\el}{_\mathrm{e}} \...
user avatar
  • 18.4k
28 votes

When is it true that more nodes equals higher energy?

General case There is indeed a mathematical theorem that deals with the number of nodes an eigenfunction corresponding to a certain eigenvalue can possess. It was laid down by Courant$^{[1, 2]}$ and ...
user avatar
  • 17.3k
27 votes
Accepted

Why do some chemical reactions require many steps?

There is no fundamental law preventing simple chemical reactions: things are complex because of the combinatorial complexity of chemical compounds The complexity of many chemical reactions is a ...
user avatar
  • 32.1k
25 votes
Accepted

What would follow in the series sigma, pi and delta bonds?

tl;dr The next in the series is called φ bond. There is even a tiny Wikipedia article about it. Nicolau pointed me to the Wikipedia article, that had at the time a tiny section about the φ ...
user avatar
25 votes
Accepted

What are typical runtimes for CASSCF calculations?

There is nothing trivial about MCSCF calculations because it is hard to predict a priori how long a calculation will take. There are well-defined equations for calculating how many determinants $$ D(...
user avatar
23 votes

How can the dipole moment of carbon monoxide be rationalised by molecular orbital theory?

Unfortunately, nothing in the bonding situation in carbon monoxide is easily explained, especially not the dipole moment. According to the electronegativities of the elements, you would expect the ...
user avatar
22 votes

How many molecules does it take to have a phase?

This is actually an active area of research for water clusters. In principle, for $\ce{(H2O)_{n}}$ there should be a "melting" phase transition, much like for ice to liquid water. So, in ...
user avatar
22 votes
Accepted

Cyclobutadiene - Jahn–Teller effect or not?

Very interesting question, and it kept me up despite daylight saving time cheating me of one hour of sleep last night... A good reference is Albright, Burdett and Whangbo, Orbital Interactions in ...
user avatar
  • 65.8k
21 votes
Accepted

What is the pKa of the hydronium, or oxonium, ion (H3O+)?

The controversy surrounding the $\mathrm{p}K_\mathrm{a}$ of hydronium mostly arises from the definition of $K_\mathrm{a}$ or lack thereof. There is no IUPAC definition of $\mathrm{p}K_\mathrm{a}$ ...
user avatar
  • 38.9k
21 votes
Accepted

Why is there sulfur in black powder / gun powder?

Black powder is a mixture of solids. As solids, they are not particularly inclined toward fast reactions between each other. Elevated temperature generally liven things up, because it allows diffusion ...
user avatar
  • 20.6k
21 votes
Accepted

What exactly is meant by 'multi-configurational' and 'multireference'?

Your problem seems to be with the terminology used in CI methods, so let me go through the different terms you mentioned: A configuration is a certain occupation of (molecular) orbitals. ...
user avatar
  • 4,514
20 votes
Accepted

Utility of Bent's Rule - What can Bent's rule explain that other qualitative considerations cannot?

There is a reason for everything. Does Bent's rule have any utility? YES! It wouldn't be there if there was not. But I will get back to this at the end of this post. Let me go through the points ...
user avatar
20 votes
Accepted

PBP vs TBP geometry?

General Rule #1: Most elements use only s and p orbitals to form bonds, only transition elements and heavier elements use d, f, etc. orbitals in bonding. General Rule #2: The more s-character in a ...
user avatar
  • 81.9k
19 votes
Accepted

What is actually the difference between valence bond theory and molecular orbital theory?

TL;DR VB theory treats atomic orbitals (including hybridized orbitals) as providing a good mathematical/physical description of the true form of the molecular wavefunction. MO theory uses atomic ...
user avatar
  • 16.8k
19 votes

Is a compound composed out of gold, manganese, xenon, krypton, and oxalate possible?

No, this is not possible. Actually, if I would have to think of the most unlikely chemical conceivable, that would be it. Let's see why: Krypton is a noble gas that doesn't bond to anything. All of ...
user avatar
  • 4,984
19 votes

A comprehensive list of theoretical approximations that are used in computational chemistry

The goal of computational chemistry is to obtain the properties of a system. This is done by solving Dirac's Equations. Treating particles as point particles with mass In most computational software, ...
user avatar
  • 4,210

Only top scored, non community-wiki answers of a minimum length are eligible