doing the timewarp

My life has regressed 20 years. I've listened to Half Man Half Biscuit non-stop for 3 days and tomorrow my friend K is coming over to help me with my chemistry. I met K at university the first time I went; unlike me he paid attention, did his homework and became Dr K and now senior academic Dr K. So I'm taking advantage of our 20 year friendship to blag some tuition, to help me catch up. I hope he can help me understand how to calculate the energies of Frenkel defects in silver halides (and also why one would even want to).