Although my preference is for ise rather than ize, Chambers gives Atomize as the entry with ise as the alternative spelling. So against my principles I have entered the Z spelling.
Using a z is the American way and this is an English crossword so I have gone with S. But what about Rachmaninov-to fit in this crossword it has to be Rachmaninoff
Why does everybody keep saying -ise is the English spelling? Here is what the Oxford Dictionary has to say:
The form -ize has been in use in English since the 16th century, and despite what some people think , is not an Americanism.......For most verbs of this class either -ize or -ise is acceptable.
I was taught at school in the 40s that you used "Z" if the word came from Greek - I presume that in those days everybody studied Greek! Have to admit I usually use "Z" for the novelty value.