title: Crystal structure of ammonia dihydrate II.
abstract: We have used density-functional-theory (DFT) methods together with a structure searching algorithm to make an experimentally constrained prediction of the structure of ammonia dihydrate II (ADH-II). The DFT structure is in good agreement with neutron diffraction data and verifies the prediction. The structure consists of the same basic structural elements as ADH-I, with a modest alteration to the packing, but a considerable reduction in volume. The phase diagram of the known ADH and ammonia monohydrate + water-ice structures is calculated with the Perdew-Burke-Ernzerhof density functional, and the effects of a semi-empirical dispersion corrected functional are investigated. The results of our DFT calculations of the finite-pressure elastic constants of ADH-II are compared with the available experimental data for the elastic strain coefficients.
authors.affiliation: Theory of Condensed Matter Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge CB3 0HE, United Kingdom.
authors.forename: Gareth I G
authors.initials: GI
authors.lastname: Griffiths
authors.forename: A Dominic
authors.initials: AD
authors.lastname: Fortes
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Pickard
authors.forename: R J
authors.initials: RJ
authors.lastname: Needs