Matthew Mold1, Emma Shardlow1, Christopher Exley2. 1. The Birchall Centre, Lennard Jones Laboratories, Keele University, Keele, Staffordshire ST5 5BG, UK. 2. The Birchall Centre, Lennard Jones Laboratories, Keele University, Keele, Staffordshire ST5 5BG, UK. Electronic address: c.exley@keele.ac.uk.