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 ...
- 71.1k
43
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 ...
- 28.3k
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 ...
- 35.4k
35
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 ...
- 18.9k
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}}
\...
- 18.9k
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 ...
- 35.4k
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
$$
D(...
- 7,428
25
votes
Accepted
What exactly is meant by 'multi-configurational' and 'multireference'?
Your problem seems to be with the terminology used in CI methods, so let me go through the different terms you mentioned:
A configuration is a certain occupation of (molecular) orbitals. ...
- 4,709
24
votes
Accepted
Cyclobutadiene - Jahn–Teller effect or not?
Very interesting question, and it kept me up despite daylight saving time cheating me of one hour of sleep last night... A good reference is Albright, Burdett and Whangbo, Orbital Interactions in ...
- 71.1k
23
votes
Why is chemistry unpredictable?
Predictabilty is essentially determined by the level of detail you need to accurately model the phenomenon you're trying to predict. Models that require little detail to capture the phenomenon of ...
- 12.1k
21
votes
Accepted
Is dipole moment a vector?
we assume for no particular reason that dipole moments must be behaving like vectors
Ah, but there is a reason. Consider the interaction of a molecule with the scalar potential
$$
E_{\text{int}} = \...
- 7,428
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, ...
- 17.3k
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 ...
- 3,050
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 ...
- 43.8k
19
votes
Accepted
How important is it that geometry be optimized at a high level of theory?
Method
Most of the time CCSD(T) would indeed be a huge overkill for geometry optimisation. DFT and MP2 have way better performance/cost ratio. Note that I said DFT and MP2 above, not or MP2; this is ...
- 18.9k
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 ...
- 5,194
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, ...
- 4,284
18
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 ...
- 17.3k
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 ...
- 27.7k
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 ...
- 71.1k
17
votes
Protons and the Heisenberg principle
But in the case of protons, we are kind of certain about their
position in the atom.
Well, yeah, kind of certain. The very notion of molecular geometry arises in the Born-Oppenheimer approximation. ...
- 18.9k
17
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 ...
- 3,050
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 ...
- 53.1k
16
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{...
- 43.8k
16
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 ...
- 281
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 ...
- 28.4k
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,\...
- 4,799
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 ...
- 6,851
15
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 ...
- 10.2k
Only top scored, non community-wiki answers of a minimum length are eligible