title: Liquid polymorphism and density anomaly in a lattice gas model.
abstract: We present a simple model for an associating liquid in which polymorphism and density anomaly are connected. Our model combines a two dimensional lattice gas with particles interacting through a soft core potential and orientational degrees of freedom represented through thermal "ice variables." The competition between the directional attractive forces and the soft core potential leads to a phase diagram in which two liquid phases and a density anomaly are present. The coexistence line between the low density liquid and the high density liquid has a positive slope contradicting the surmise that the presence of a density anomaly implies that the high density liquid is more entropic than the low density liquid.
authors.affiliation: Instituto de Física, Universidade de Sao Paulo, Brazil. barbosa@if.ufrgs.br
authors.forename: Vera B
authors.initials: VB
authors.lastname: Henriques
authors.forename: Marcia C
authors.initials: MC
authors.lastname: Barbosa