Author Archives: Andrei Derevianko

Manual for tdhf

tdhf - is a Dirac-Hartree-Fock program developed by Walter R. Johnson
Calculates energies and w.f. of valence electrons. Has Breit + QED corrections

tdhf input

read(5,1000) ident,jmax,jz,nat,nuc,ion,io, in, inf [format(a4,4i8)]
Example Fr 30 87 223 0 0 9 25 1

character*4 $ident = atomic element symbol {e.g., Fr}
$jmax = # of input fields with shell orbital description (see below)
$jz  = nuclear charge Z (atomic number for neutrals)
$nat = atomic mass number A ( atomic weight)
$nuc = (unused set to 0)
$ion = (unused set to 0)
$io = Desired relative precision = 10^(-io) - controls convergence criteria
$in = index of the 1-st valence shell in the cards with orbital description (below)
$inf = (0 or 1) 0=closed-shell only
if 1 do a solution for valence electron in the frozen core approximation

Cards with orbital description:
total number $jmax (above)
* ( n(i),kap(i),iof(i),wh(i), i = 1,jmax ) format(3i4,f12.4)
card format : n kappa iof guessed_energy_in_a.u.
* n(i): principal quantun number
* kap(i): angular quantum number kappa

if $guessed_energy >= 0.0 it is calculated internally from the hydrogenic formula
The guessed_energy for valence orbitals better be good, since the program gives some dumb answers if the hydrogenic default is used.
$iof governs some internal step, the mixing weight between previous and next iteration

the first $in cards describe core orbitals the rest $jz -$in are valence orbitals

if $inf was 1 the next input is
$xa xalpha  (some mixing parameter)

read(5,*) r0,hh,mm

These are grid parameters

read(5,*) iparm

$iparm = Type of nuclear parameters
if $iparm = 1 rnuc,cnuc,tnuc expected (all in fermis).

if $iparm != 1: cnuc,anuc,b2,b4

For example for Fr the relevant input is
1.00 ! iparm
.00000 6.83430 2.30000 !rnuc,cnuc,tnuc

We mostly use $iparm = 1. Nuclear parameters can be found here (NuclearPropsWRJ.pdf)
In this table tnuc=2.3 fm; rnuc=0.0000 and the relevant parameter is C(fm)

read(5,*) ex

$ex  affects amount of exchange in the Hartree model potential for initializing valence+frozen core potential.
Nominal value = 0

Why is the atomic many-body problem so difficult?

illustration of an atomOne could rightfully state that the atomic-structure problem has been around for a very long time. Yes, this is true - in fact quantum mechanics has been invented to explain atomic properties. Then why do we still struggle to solve it?

Should we be embarrassed by our inability to solve this basic problem? Sure we can solve it approximately, but solving it accurately is another story.

So what is holding us back? It is the very same entanglement and complexity of Hilbert space (that is where wave functions live) that makes quantum computing so powerful. To illustrate this enormous complexity, I'll take my favorite atom, cesium. It has 55 electrons. With 3 degrees of freedom per electron (x, y, and z), the Cs wave function depends on 55 \times 3 = 155 coordinates. As a result of the calculation I would need to store the wave function. If I  were to take a very poor grid of 10 points per coordinate, the storage would require $latex 10^{155}$ memory units.

$latex 10^{155}$ is of course an astronomically large number - in fact it exceed an estimated number of atoms in the Universe, 10^{80}. So even if we were able to compute the Cs wave function, there is no plausible way to store it in usable form.

Stepping stones or the value of theory

photo of stepping stones across a lake

"Way In The Lake" by Evgeni Dinev

When invited to present colloquia talks, I usually talk about atomic clocks and I am sometimes asked what a theorist is doing in this highly experimental field. I was thinking about this question for a while and then I was on a plane browsing through an in-flight catalog (you know the one that advertises stuff that you may want but really do not need) and I realized how to answer this question.

There was an ad for a beautiful wristwatch accompanied by a story on the fine craft  of clockmaking. The story was comparing making a wristwatch to building a skyscraper out of matchsticks. So I thought, by that account, experimentalists working on atomic clocks are performing miracles. And if these  experimentalists are walking on water, my role as a theorist is to show them where the stepping stones are.

Pauli's letter or the importance of technical details

This is one of my favorite illustrations of the importance of technical details. Below I am posting a figure from a paper by Peter Woit, where the Pauli's letter is displayed.

In 1958, Wolfgang Pauli sent letters to some fellow physicists, making fun of Werner Heisenberg's exaggerated claims to have developed a successful unified
theory. A copy of his letter to J. Robert Oppenheimer is shown here.

A letter of Pauli to Oppenheimer

The letter says: Comment on Heisenberg Radio advertisement.
This is to show the world that I can paint like Titian. [Empty frame with jagged sides]. Only technical details are missing.

Here is  a link to a similar letter to Gamov.