62
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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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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 ...
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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 ...
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35
votes
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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 ...
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33
votes
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Born–Oppenheimer adiabaticity
$\newcommand{\conj}[1]{\overline{#1}{}}
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\newcommand{\mat}[1]{\mathbf{#1}}
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31
votes
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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 ...
- 43.5k
31
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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-...
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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
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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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26
votes
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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 ...
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25
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 ...
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25
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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(...
- 7,367
24
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 ...
- 70.2k
24
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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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23
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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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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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20
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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 ...
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20
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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 ...
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19
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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 ...
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19
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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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19
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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 ...
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19
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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 ...
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19
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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, ...
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19
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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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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
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 ...
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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 ...
- 70.2k
17
votes
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Hypervalency and the octet rule
Out there in the real world, university students and school pupils alike favour strict rules that are true as often as possible (or at least have clear, easily remembered exceptions), while their ...
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17
votes
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For a given element do heavier isotopes form stronger bonds than lighter isotopes?
Isovalent isotopes will have the same force constant. However the different masses of the isotope will affect the position of the vibrational state in its potential well. You can rationalise the ...
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