# FAQ

This is a collection of frequently asked questions about octopus

## What are these *atomic units*

Atomic units are a Gaussian system of units (by "Gaussian" I mean that the vacuum dielectric constant has no dimensions and is set to be ), in which the numerical values of the Bohr radius, the electronic charge, the electronic mass, and the reduced Planck's constant are set to one:

This simplifies formulae. (Although, in my opinion, it seriously hazards dimensionality analysis, formulae interpretation and understanding, and Physics in general. But this is just a personal taste.) This sets directly two fundamental units: the atomic units of length and of mass:

Since the squared charge must have units of energy times length, we can thus set the atomic unit of energy:

which it is called Hartree, Ha. And, since the energy has units of mass times length squared per time squared, this help us get the atomic unit of time:

Now the catch is: what about Planck's constant? Its dimensions are of energy times time, and thus we should be able to derive its value by now. But at the beginning we set it to one! The point is that from the four physics constants used () are not independent, since:

In this way, we could actually have derived the atomic unit of time in an easier way, using Planck's constant:

And combining (6) and (5) we retrieve (4).

## What are these *convenient units*?

A lot of the literature in this field is written using @AA{}ngstr@"oms and electron-volts
as the units of length and of energy, respectively. So it may be ``convenient
to define a system of units, derived from the atomic system of units, in which
we make that substitution. And so we will call it ``convenient*.*

The unit mass remains the same, and thus the unit of time must change, being now @math{\hbar /{\rm eV}}, with @math{\hbar = 6.582\,1220(20)\times 10^{-16}~\rm eV\,s}.