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Questions tagged [theoretical-chemistry]

For questions seeking answers deduced from or composed of theories regarding the chemical substances and about models and methods of theoretical chemistry.

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253 votes
3 answers
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Why is gold golden?

Bulk gold has a very characteristic warm yellow shine to it, whereas almost all other metals have a grey or silvery color. Where does this come from? I have heard that this property arises from ...
tschoppi's user avatar
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115 votes
1 answer
6k views

Is there a general consensus on the causes of the alpha-effect?

There have been various explanations posited for the α-effect. The α-effect refers to a phenomenon wherein nucleophiles with lone pairs on atoms adjacent (i.e., in the α- position) to the atom bearing ...
Greg E.'s user avatar
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81 votes
4 answers
42k views

Bonding in diatomic C2, a carbon-carbon quadruple bond?

Carbon is well known to form single, double, and triple $\ce{C-C}$ bonds in compounds. There is a recent report (2012) that carbon forms a quadruple bond in diatomic carbon, $\ce{C2}$. The excerpt ...
Janice DelMar's user avatar
61 votes
6 answers
13k views

Why is FORTRAN so commonly used in computational chemistry?

I've been using Ruby to write scripts for research, but I want to get into some heavier stuff that Ruby is just too slow for. I noticed there are a few things written in C and C++, but there is an ...
Melanie Shebel's user avatar
43 votes
10 answers
9k views

Why is chemistry unpredictable?

Disclaimer: I am not a chemist by any means, and I only have knowledge limited to what I learned in my university's Chemistry III course. Basic understanding of everything up to valence electron ...
Tyler M's user avatar
  • 533
42 votes
2 answers
25k views

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

Despite the fact that oxygen is much more electronegative than carbon, the bond in $\ce{CO}$ presents a weak dipole moment. This observation can easily be explained using the concept of "dative bond", ...
Isaiah G.'s user avatar
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38 votes
1 answer
5k views

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

I realise, that this question is a stretch, but I was wondering, how would a bonding orbital be called if it was formed from two $f_{x(x^2−3y^2)}$ or $f_{y(3x^2−y^2)}$ orbitals. Have there been any ...
Martin - マーチン's user avatar
37 votes
4 answers
23k views

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

Recently I have read about both of the concepts in my book (Physical Chemistry by Atkins, Paula). It was literally a reading; though I could understand the language and superposition of orbitals, ...
user avatar
36 votes
3 answers
14k views

Is density functional theory an ab initio method?

The following comment by Wildcat made me think about whether density functional theory (DFT) can be considered an ab initio method. @Martin-マーチン, this is sort of nitpicking, but DFT (where the last ...
Martin - マーチン's user avatar
36 votes
1 answer
5k views

Natural Bond Orbital analysis: Significance of stabilization energy determined by 2nd order perturbation

PREFACE: I am no expert on this topic. My questions at the bottom may be off base. I have some experience with symmetry-adapted perturbation theory (SAPT) when it comes to analyzing intermolecular ...
LordStryker's user avatar
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34 votes
1 answer
10k views

When is it true that more nodes equals higher energy?

Consider all the MOs of some isolated molecule. (It could be a single atom too; I'll use MO to refer to AOs as well.) Number them in increasing order of the number of nodes (node = surface where the ...
Silvio Levy's user avatar
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34 votes
2 answers
4k views

Hypervalency and the octet rule

I realize that the octet rule is more a suggestion than a rule, and that it applies mainly to non-transition metal compounds. Still, compounds that don't have an octet, like $\ce{BH3}$, tend to ...
ron's user avatar
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33 votes
2 answers
8k views

What is the exchange interaction?

As the wikipedia article for the exchange interaction so aptly notes, exchange "has no classical analogue." How wonderful. Exchange shows up essentially while enforcing the condition that two ...
jheindel's user avatar
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32 votes
2 answers
35k views

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

Although the wikipedia page on Hydronium indicates a $\mathrm{p}K_\text{a}$ of −1.74, I noticed in the discussion of this page that the subject seems debated (cf. http://en.wikipedia.org/wiki/Talk:...
The Quark's user avatar
  • 597
31 votes
3 answers
1k views

What's a minimal yet chemically-meaningful kinetic system for an oscillating reaction?

Oscillating reactions are a funny aspect of chemistry. I have tried to find various simplified kinetic models of oscillating reactions such as the Belouzov-Zhabotinsky, the Briggs–Rauscher or the Bray–...
F'x's user avatar
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30 votes
1 answer
14k views

What is the difference between dynamic and static electronic correlation

I am confused. In the static correlation, we use combination of Slater determinants to account for electronic correlation (known as CI or configuration interaction) and the wave function is ...
Osman Mamun's user avatar
  • 2,477
28 votes
6 answers
65k views

Overlap between computer science and chemistry?

This isn't particularly a question about chemistry itself, but more-so the prospect of chemistry. What kind of opportunities arise when you overlap chemistry and computer science? I'm a computer ...
gator's user avatar
  • 399
28 votes
2 answers
5k views

How to find the second order perturbation to wave function?

Today, I'm looking for how to find the 2nd perturbation to the base in Rayleigh Schrödinger Perturbation Theory (RSPT). SETUP Starting from the 2nd order perturbation in Dirac's notation: \begin{...
Another.Chemist's user avatar
28 votes
4 answers
4k views

How many molecules does it take to have a phase?

A single molecule can't be solid, liquid or gas. It's just a molecule. A mole of something can be any of the three. So, how many molecules does it take for phases to be meaningful? I realize that ...
terdon's user avatar
  • 395
28 votes
1 answer
3k views

Suggest methods and basis sets for a variety of systems

Please help me with any/all of the cases below. In the following cases, the named method and basis set are not suitable for the chemical systems. Why aren't they? Could you suggest a suitable method/...
ARaf's user avatar
  • 525
28 votes
0 answers
2k views

How to calculate transition dipole moment from two known wavefunctions

I'm interested in calculating the transition dipole moment (TDM) from the information from two wavefunctions of different states. This is somewhat similar to calculating the molecular dipole moment ...
brose's user avatar
  • 322
27 votes
6 answers
3k views

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

This question comes from a rather famous quote by Paul Dirac which goes like this, The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of ...
jheindel's user avatar
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27 votes
3 answers
2k views

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

I am having trouble recognizing all the approximations that are used in computational chemistry. I would like to start an answer list (similar as to the list of resources for learning chemistry) that ...
CoffeeIsLife's user avatar
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27 votes
1 answer
40k views

What's the difference between PBE and B3LYP methods?

I can't find an answer to that question. I was told that in B3LYP, more variables implemented in the method are empirical, but I can't find anywhere if it's true, and I'm sure it's not the only ...
Basia's user avatar
  • 273
27 votes
1 answer
4k views

What is the justification for Hückel's rule?

I know that experimentally, the rule has been verified but I am having trouble understanding why it works. I see no reason why equivalent cyclic structures can undergo resonance and be just as stable. ...
CognisMantis's user avatar
  • 1,648
26 votes
2 answers
2k views

How should I go about picking a functional for DFT calculations?

I've been looking to use time-dependent density functional theory, but reading the literature I keep getting overwhelmed by how many different functionals there are. How should I go about selecting ...
Dan's user avatar
  • 1,552
25 votes
3 answers
2k views

What is charge shift bonding?

Can someone explain to me charge shift bonding? I recognize that it is proposed as another domain of bonding - one different from ionic and covalent bonding. I am also told that $\ce{C-F}$ exhibits ...
Dissenter's user avatar
  • 19k
25 votes
2 answers
8k views

For a given element do heavier isotopes form stronger bonds than lighter isotopes?

I know that $\ce{C-D}$ and $\ce{C-T}$ bonds are stronger than $\ce{C-H}$ bonds, but is this generally true for isotopes of all elements? (By isotopes I mean stable ones with large half lives.)
Hritik Narayan's user avatar
25 votes
2 answers
5k views

Cyclobutadiene - Jahn–Teller effect or not?

In transition metal chemistry the Jahn–Teller effect arises when the configuration of the metal ion and d orbital splitting set up a doubly degenerate state, which is less stable than a state without ...
EJC's user avatar
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25 votes
2 answers
8k views

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

Some topics here have touched on this before (see 1, 2, 3), but I haven't found a clear definition yet. I would like to know what exact property of the wave function these terms refer to. It would ...
snurden's user avatar
  • 1,459
25 votes
1 answer
9k views

What does "reaction coordinate" mean? What does it mean when a reaction has TWO reaction coordinates?

I'm familiar with the concept of a reaction coordinate from high school chemistry as some generic conformational parameter that all of the intermediate states of a chemical reaction lie upon. Recently,...
Dan's user avatar
  • 1,552
24 votes
1 answer
11k views

PBP vs TBP geometry?

Why are the axial bond lengths greater than those of the equatorial bonds in a trigonal bi-pyramid (TBP) geometry molecule; but the opposite is true for pentagonal bi-pyramid (PBP) geometry molecules? ...
tkhanna42's user avatar
  • 2,545
23 votes
3 answers
14k views

Is Hydrogen Bonding a Type of Dipole Dipole Interaction?

I understand that dipole dipole forces is due to the attraction of the different partials charges of atoms in different molecules due to their different electro-negativities. For hydrogen bonding, ...
Nanoputian's user avatar
  • 7,412
22 votes
5 answers
5k views

Why do some chemical reactions require many steps?

I posted the following question in Physics SE and was advised to transfer it to Chemistry SE. I studied physics in college ten years ago and I recently started to learn biochemistry. I enjoy finding ...
Light Yagmi's user avatar
22 votes
3 answers
2k views

What is natural bond orbital theory used for?

My understanding is that the NBO methodology transforms the molecular orbital picture of a molecule into a collection of pairwise interactions (bond orbitals). This is obviously handy for didactic and ...
Richard Terrett's user avatar
22 votes
1 answer
50k views

Why is the bond order of CO+ 3.5?

According to my books the bond order of $\ce{CO+}$ is $3.5$. But shouldn't it be $2.5$? On googling this, I found the following answer that is on Stack Exchange but its only talks about the bond ...
Chloritone_360's user avatar
21 votes
4 answers
2k views

Group theoretical condition for an integral to be zero

In many aspects of chemistry it is of interest to find out whether a given integral $\int \psi \,\mathrm{d}\tau$ vanishes, i.e. becomes zero. Group theory provides us with a handy condition: If $\...
orthocresol's user avatar
  • 71.5k
21 votes
1 answer
584 views

What are modern approaches to describe and/or quantify aromaticity?

I know that Schleyer described an approach for evaluating aromaticity using nucleus-independent chemical shifts (NICS, see e.g. Chem. Rev. 2005, 105 (10), 3842–3888) quite some time back. I also ...
logical x 2's user avatar
  • 2,794
20 votes
1 answer
315 views

Similarities and Differences between Resonance and MCSCF treatments

Recently, there has been a question by Voldemort concerning different resonance structures of $\ce{NCO-}$, requesting an explanation why one resonance structure would be more preferred than another. ...
pH13 - Yet another Philipp's user avatar
20 votes
1 answer
615 views

Can a multi-species system oscillate around equilibrium?

In reading about chemical oscillations such as those that occur in the Belousov–Zhabotinsky reaction (BZ), it's often reported that these reactions were initially not taken seriously, because of a ...
N. Virgo's user avatar
  • 1,661
19 votes
5 answers
2k views

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

I'm just now learning about the Fourier transform, which seems like a pretty useful tool, and I know it has uses in spectroscopy (e.g. FTIR) since that is what Google shows when searching for ...
Dragonoverlord3000's user avatar
19 votes
2 answers
3k views

Is it correct to talk about an empty orbital?

Professor A. J. Kirby mentions: 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 ...
Tan Yong Boon's user avatar
18 votes
2 answers
5k views

Protons and the Heisenberg principle

The Heisenberg uncertainty principle implies that it is not possible to calculate the position and velocity of a body at the same time accurately. Electrons follow this principle since their orbitals ...
anonymous789's user avatar
18 votes
1 answer
2k views

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

My question stems from the comments on my answer to After a unitary transformation, is Koopmans' theorem still valid?. There was some confusion relating to differing terminology referring to different,...
Tyberius's user avatar
  • 11.8k
18 votes
1 answer
731 views

What is the relation between surface tension and initial molecular harmonic?

Initial harmonic is measured as Hartree/Bohr^2 ( like IHarmonic=n in Gaussian Software ). As I am from physics background I am used to look at features from their dimension point of view: Actually ...
Aug's user avatar
  • 2,141
18 votes
1 answer
789 views

Analytical solution of the Schrödinger Equation for AB^+ systems

According to Wikipedia, there are not many problems for which the Schrödinger equation can be solved analytically. [1] The $\ce{H2+}$ ion is probably the most complex molecule that can be treated this ...
pH13 - Yet another Philipp's user avatar
17 votes
1 answer
3k views

Born–Oppenheimer adiabaticity

The Born–Oppenheimer aproximation is said to be adiabatic. What does "adiabatic" mean? What would be a non-adiabatic approximation to the Schrödinger equation?
Another.Chemist's user avatar
17 votes
2 answers
1k views

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

Koopmans' theorem says that the energy of the HOMO of the Hartree-Fock orbitals is equal to the first ionization energy of whatever system is being studied. This is only approximate because it assumes ...
jheindel's user avatar
  • 13.1k
17 votes
2 answers
2k views

How to compute 2-electron integral for Hartree-Fock code?

I'm working on writing my own code for $\ce{H2}$ in an STO-3G basis set using Hartree-Fock (HF), and I am currently stuck on how to construct the two-electron integral matrix. I know how to evaluate ...
SKB0514's user avatar
  • 173
17 votes
1 answer
518 views

Are chiral quasicrystals possible?

Quasicrystals are materials that have long-range atomic order but lack the translational symmetry of conventional crystals. All quasicrystalline tilings and packings I have read about thus far, whilst ...
Richard Terrett's user avatar

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