In last month's Practical Quantum Mechanics we showed readers how to use self-consistent LAPW calculations to diagnose problems with LED torches. That sparked a flood of letters with related queries.
Mrs Jane F from Dungeness writes:
Our local Woman's Institute is running a campaign against the extension of the operating licence of a nearby nuclear power plant. We've heard that the fuel rods at the plant are sheathed in zirconium, which is susceptible to embrittlement owing to the absorption of hydrogen at interstitial sites. We're running a series of coding bees to develop a suite of FORTRAN programs to perform LCAO band-structure calculations to model the risk of embrittlement, so that we can present persuasive evidence at a forthcoming planning enquiry. Do you think that the calculations need to be self-consistent? Also, how should we model the screening potential in the vicinity of the interstitial impurities?
Dear Mrs F,
We think it would be a picky planning inspector indeed who would insist on self-consistency for the type of calculations you have in mind, so don't bother with it. I'm sure your coding bee members will have enough on their hands as it is. A simple screened-coulomb potential should be fine for modelling the potential around the adatoms.
Lucasian Professor Stephen H from Cambridge writes:
Does it matter which way round the batteries go? I can't get the torch to work at all.
Dear Professor H,
Yes, the batteries need to go in a particular way, otherwise the electricity particles won't flow correctly. One end of each battery should have a pimply bit, and that goes where you see a little cross embossed in the battery compartment of the torch. If you get stuck, there should be people at the Cavendish Physics Laboratory in Cambridge who can help.