61
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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 ...
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45
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 ...
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39
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 ...
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31
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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}}
\...
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28
votes
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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 ...
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27
votes
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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. ...
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25
votes
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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 ...
- 71.9k
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 ...
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21
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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}} = \...
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21
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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 ...
- 3,090
21
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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, ...
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19
votes
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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 ...
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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 ...
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18
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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 ...
- 3,090
18
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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 ...
- 28k
18
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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 ...
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17
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After a unitary transformation, is Koopmans' theorem still valid?
Disclaimer: I like Tyberius' answer, but I'd like to go a little further.
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\newcommand{\ll}{\left\langle}\newcommand{\rr}{\right\rangle}
\newcommand{\lb}{\left|}\newcommand{\rb}{\right|}
\newcommand{...
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17
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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 ...
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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 ...
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16
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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 ...
- 10.8k
16
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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 ...
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15
votes
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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,\...
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15
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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 ...
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15
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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.
...
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15
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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 ...
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14
votes
Why do some chemical reactions require many steps?
I do not believe there is a fundamental law which prohibits complex reactions from taking place in a single step — it’s just extremely improbable.
Collision Theory
Gasses
This is especially ...
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14
votes
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Pauli exclusion principle
Wikipedia has the correct definition:
A more rigorous statement is that with respect to exchange of two identical particles the total wave function is antisymmetric for fermions, and symmetric for ...
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14
votes
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What is the difference between a configuration state function and a Slater determinant?
Slater determinants are not eigenfunctions of the $\hat S^2$ operator, but CSFs are.
The Hamiltonian commutes with the operators for total and projected spin
\begin{align}
[\hat H, \hat S^2] &= ...
- 4,759
14
votes
Is there any software that can do geometry optimization using machine learning?
There is one Python package that I know: AMP (Atomistic Machine-learning Package). It is based on ASE (Atomic Simulation Environment), a homogeneous interface to a lot of computational chemistry ...
- 2,883
13
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Is coupled cluster variational for two electrons?
No. The reason for this is not to be found in the excitations, but in the evaluation of the method, i.e. the working equations.
$$%Introducing some shortcuts
\require{cancel}
\newcommand{\op}[1]{\...
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