# Tag Info

Accepted

### How do organic chemistry mechanisms become accepted?

Great question! When I was teaching, Anslyn and Dougherty was a decent text for this. Here are some general comments: First, please note that you cannot be sure about a mechanism. That's the real ...
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### Why is FORTRAN so commonly used in computational chemistry?

I don't think that's really true anymore. Some Fortran use is historical (i.e., early codes were developed in FORTRAN because that was the best programming language for number crunching in the 70s ...
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### DFT Functional Selection Criteria

What's up with all that magic? (A chapter formerly known as Introduction) The hunt for for the holy grail of density functional theory (DFT) has come a long way.[1] Becke states in the introduction of ...
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### How do organic chemistry mechanisms become accepted?

This doesn’t exactly concern the actual mechanism you asked for, but as part of my PhD thesis, I performed an amide alkyne coupling the mechanism of which had been researched by Arndt et al.[1] ...
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### Why not simulate every particle properties and interactions?

Forty electrons is tiny. Even if we limit ourselves to just the valence electons, cyclohexane already has 36 electrons. Anything drug-like has way more electrons that 40. For example, viagra has 178 ...
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### Why is FORTRAN so commonly used in computational chemistry?

I think it does make sense to provide a somewhat alternative view and to clarify the matter. FORTRAN vs. Fortran First off, one has to distinguish the old FORTRAN from the new Fortran, where, by ...
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### Dipole moment of cis-2-butene

Mini Research Project Time Updates Added CCSD(T) $n_i$ and dipole moments and tweaked discussion (the delay was caused by a system-wide storage upgrade on the machines which took nearly a week to ...
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### Is there an energy cost associated with flipping the spin of an electron?

I will try to describe what happens when two hydrogen atoms approach each other from infinity. At infinite separation the hydrogen atoms don't feel their mutual presence and each atom has one electron ...
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### Why is chemical accuracy defined as 1 kcal/mol?

Short answer: the goal of "thermochemical accuracy" for computational chemistry is to match or exceed experimental accuracy. Thus, ~1 kcal/mol comes from the typical error in thermochemical ...
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• 42.4k
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### Is there a formula to tell how many conformers of a molecule to generate?

I haven't seen any rigorous benchmarking or guidelines. Consider that the total number of conformers goes up like $3^n$, where $n$ is the number of rotatable bonds, and ~3 is the approximate number ...

### 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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### How are the atomic orbitals for multi electron atoms obtained?

Already in year 1928 Hartree proposed an approximate method of solving the Schrödinger equation for a many-electron atom that became known as the Hartree method (1, 2, 3). The Hartree method is based ...
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### Symmetrize nearly symmetric molecule

I've done some work in both symmetry detection and in distance matrix methods. I think it's a great idea in concept, but the devil will be in the details for large, more complex molecules. The first ...
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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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### Structure that breaks InChI

Okay, let's tackle at least one problem here. Consider the rotation of 1,2-dichloroethane (BP86/cc-pVDZ): These conformational changes can be further rationalised: C and C' are the same ...
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### When using Gaussian to calculate NMR, what's the default solvent and frequency?

While Greg's answer describes sufficiently what the program does, I would like to extend a little on his remark. (This should have been a comment, but it was too long.) The ...
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### Why is FORTRAN so commonly used in computational chemistry?

FORTRAN used to have an edge when it came to speed due to having much better optimizing compilers, in part due to its relative simplicity. Now that C and C++ compilers are nearly on par (and ...
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### Generating neat orbitals/surfaces from molden/wfn-files

Often different quantum chemical programs require different analysing tools. Unfortunately there is no universal standard (program), that can do it all. If there were, someone would come along and ...
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### PBE vs. PBEPBE functional

The designation PBEPBE is an actual terrible artefact from researchers using Gaussian. The actual publication[1, 2] only refers to it as PBE, and most other program packages (I know) implement it as ...
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