title: Isobaric-isothermal monte carlo simulations from first principles: application to liquid water at ambient conditions.
abstract: A series of first-principles Monte Carlo simulations in the isobaric-isothermal ensemble were carried out for liquid water at ambient conditions (T=298 K and p=1 atm). The Becke-Lee-Yang-Parr (BLYP) exchange and correlation energy functionals and norm-conserving Goedecker-Teter-Hutter (GTH) pseudopotentials were employed with the CP2 K simulation package to examine systems consisting of 64 water molecules. The fluctuations in the system volume encountered in simulations in the isobaric-isothermal ensemble require a reconsideration of the suitability of the typical charge-density cutoff and the regular grid-generation method previously used for the computation of the electrostatic energy in first-principles simulations in the microcanonical or canonical ensembles. In particular, it is noted that a much higher cutoff is needed and that the most computationally efficient method of creating grids can result in poor simulations. Analysis of the simulation trajectories using a very large charge-density cutoff at 1200 Ry and four different grid-generation methods point to a significantly underestimated liquid density of about 0.8 g cm-3 resulting in a somewhat understructured liquid (with a value of about 2.7 for the height of the first peak in the oxygen-oxygen radial distribution function) for BLYP-GTH water at ambient conditions. In addition, a simulation using a charge-density cutoff at 280 Ry yields a higher density of 0.9 g cm-3, showing the sensitivity of the simulation outcome to this parameter.
authors.affiliation: Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455-0431, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: McGrath
authors.forename: J Ilja
authors.initials: JI
authors.lastname: Siepmann
authors.forename: I-Feng W
authors.initials: IF
authors.lastname: Kuo
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Mundy
authors.forename: Joost
authors.initials: J
authors.lastname: VandeVondele
authors.forename: Jürg
authors.initials: J
authors.lastname: Hutter
authors.forename: Fawzi
authors.initials: F
authors.lastname: Mohamed
authors.forename: Matthias
authors.initials: M
authors.lastname: Krack