Interplay between molecule-molecule and molecule-substrate interactions: first-principles study of fluoroform aggregates on a hexagonal ice (0001) surface.

The adsorption of fluoroform molecules on a hexagonal ice (0001) surface was studied using static density functional theory (DFT) calculations and Car-Parrinello molecular dynamics (CP-MD) simulations. Extending our previous work on isolated molecules we focus in the present study on the interplay between molecule-molecule and molecule-substrate interactions. Coverages of up to a full monolayer were modeled by introducing two, three and four fluoroform molecules per unit cell of the ice (0001) substrate. Lowest-energy structures of fluoroform aggregates on the ice surface were determined in a systematic search by performing geometry optimizations from a large set of initial configurations chosen by chemical intuition and from snapshots taken from CP-MD simulations. In the vibrational analysis of the optimized geometries both conventional red- and unusual blue-shifting hydrogen bonds were found. The finite temperature stability of the lowest-energy configurations was probed by CP-MD simulations and conformational changes were analyzed in terms of transformations between the global and local minima structures.