Skip to main content
61 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 ...
orthocresol's user avatar
  • 71.5k
44 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 ...
Nicolau Saker Neto's user avatar
40 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 ...
matt_black's user avatar
  • 37.1k
36 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 ...
Wildcat's user avatar
  • 19.1k
31 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}} \...
Wildcat's user avatar
  • 19.1k
28 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 ...
matt_black's user avatar
  • 37.1k
28 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. ...
Feodoran's user avatar
  • 4,779
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(...
pentavalentcarbon's user avatar
24 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 ...
orthocresol's user avatar
  • 71.5k
23 votes

Why is chemistry unpredictable?

Predictabilty is essentially determined by the level of detail you need to accurately model the phenomenon you're trying to predict. Models that require little detail to capture the phenomenon of ...
theorist's user avatar
  • 12.2k
21 votes
Accepted

Is dipole moment a vector?

we assume for no particular reason that dipole moments must be behaving like vectors Ah, but there is a reason. Consider the interaction of a molecule with the scalar potential $$ E_{\text{int}} = \...
pentavalentcarbon's user avatar
21 votes

Why is chemistry unpredictable?

"it seems that every other STEM field has models to predict results (physics, thermodynamics, fluid mechanics, probability, etc) but chemistry is the outlier" This is only partially true, ...
Zhe's user avatar
  • 17.5k
20 votes
Accepted

Are the canonical orbitals of Hartree-Fock also the natural orbitals?

There is a bit of a terminology problem in the field that makes things very confusing and I will try to clarify some of this here. Part of the problem arises from the fact that sometimes only one kind ...
levineds's user avatar
  • 3,080
19 votes

What is charge shift bonding?

Geoff has summarized well the mathematical, VB wavefunction-based description of charge-shift bonding. This answer aims to provide further exposition, from the perspective of the distribution of the ...
hBy2Py's user avatar
  • 17.4k
19 votes
Accepted

Why is the bond order of CO+ 3.5?

For a long time it was taught in school and universities that the HOMO of carbon monoxide is anti-bonding. Without more context it was also often taught that the bond order in CO is three, since there ...
Martin - マーチン's user avatar
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 ...
Gimelist's user avatar
  • 5,452
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, ...
CoffeeIsLife's user avatar
  • 4,284
18 votes
Accepted

Group theoretical condition for an integral to be zero

By Maschke's Theorem, every direct product of representations is decomposable into a direct sum of representations, that is, the function you are integrating can be rewritten as a sum of functions ...
levineds's user avatar
  • 3,080
18 votes

Is the Self Consistent Field an actual field?

The notion of a self-consistent field comes from the orbital approximation - that a multi-electron wavefunction can be approximated as multiple one-electron wavefunctions. At that point, the question ...
Geoff Hutchison's user avatar
18 votes
Accepted

Is the notion of orbitals different in theoretical chemistry?

Unfortunately, it only gets more complicated the deeper you dig. There is some explanation here: What exactly is an orbital?, but you should bear in mind that electronic structure theory is something ...
orthocresol's user avatar
  • 71.5k
17 votes
Accepted

After a unitary transformation, is Koopmans' theorem still valid?

Disclaimer: I like Tyberius' answer, but I'd like to go a little further. $% \newcommand{\ll}{\left\langle}\newcommand{\rr}{\right\rangle} \newcommand{\lb}{\left|}\newcommand{\rb}{\right|} \newcommand{...
Martin - マーチン's user avatar
17 votes
Accepted

Can a zeroth order reaction be reversible?

A zeroth order reaction does not achieve equilibrium as a zeroth order reaction. Instead the kinetics will go away from zeroth order when we get close to the equilibrium condition. Zeroth order ...
Oscar Lanzi's user avatar
  • 59.3k
17 votes
Accepted

Why can't NH5 form?

I have pursued such questions regarding the hypervalency of particular molecules before and thus, I would like to provide some insights to the matter. A common superficial answer that one can ...
Tan Yong Boon's user avatar
17 votes

Why is chemistry unpredictable?

There are some great answers to this question already, but I'd like to provide a more practical boots-on-the-ground answer from my own perspective as an organic chemistry PhD who did computational ...
Ingolifs's user avatar
  • 291
16 votes

Uses of Fourier / Laplace transforms in chemistry (apart from spectroscopy)

For the principle «what is small, gets large; what is large, gets small» when applying Fourier transformation and inverse Fourier transformation, it is one important tool in crystallography to relate ...
Buttonwood's user avatar
  • 30.2k
15 votes
Accepted

What is it that is "symmetry-adapted" about symmetry-adapted perturbation theory?

The "symmetry" in symmetry-adapted perturbation theory refers to the anti-symmetry of the wave function with respect to electron exchange: $$\Psi(\mathbf{r}_1,\mathbf{r}_2) = -\Psi(\mathbf{r}_2,\...
Jan Jensen's user avatar
  • 4,819
15 votes
Accepted

Is it correct to talk about an empty orbital?

"The properties of an orbital are those of an electron contained in it. It is normal practice, illogical though it may sound, to talk of 'vacant orbitals'. The properties of vacant orbitals are those ...
TAR86's user avatar
  • 6,881
15 votes
Accepted

How to identify hydrogen bonds and other non-covalent interactions from structure considerations?

It is safe to say that there will always be intermolecular forces at play. At the time where you will consider these you should already have a good idea about the molecules involved in your system. ...
Martin - マーチン's user avatar
15 votes
Accepted

Why don't the MO energies sum up to Hartree-Fock SCF value?

The key misconception you have is that the sum of the orbital energies is equal to the electronic energy. Compare the formulas for the two (taken from Chapter 3 of Szabo and Ostlund's Modern Quantum ...
Tyberius's user avatar
  • 11.8k
14 votes
Accepted

Is it true that all of the physical laws and mathematics needed to describe chemistry are known?

This sounds an awful lot like something I said at a talk this week, so I feel obligated to answer. First, in terms of fundamental interactions, yes, excluding a quantum theory of gravity we have a ...
jjgoings's user avatar
  • 2,130

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