1. Water graphene binding energy curve from diffusion Monte Carlo Department of Earth Sciences & Department of Physics and Astronomy, Thomas Young Centre@UCL & London Centre for Nanotechnology University College London Dario ALFÈ Angelos Michaelides, Jie Ma, Enge Wang

2. Motivation Water-carbon (such as graphene, graphite and carbon nanotubes) interactions crucial to understand phenomena such as lubrication, heterogeneous ice nucleation, water formation on interstellar dust grain, structural and phase behaviour of water at the nanoscale, etc. Adsorption energy and structure of water monomers on any carbon surface is not well established, neither theoretically nor experimentally. Minute tuning of water-carbon potentials for nanotubes immersed in water can change filled tube to empty tubes. Small variations in the water-carbon potential leads graphite surfaces to appear as hydrophobic or hydrophilic.

3. Difficulties for water monomer geometry and adsorption energy Experiments: adsorption of water quickly results in water clusters Theory: –Quantum chemistry (MP2, CCSD(T), SAPT…) only available on clusters; adsorption energy and geometry need to be extrapolated from acenes of various sizes, results in range 100 - 200 meV. –Periodic DFT is inadequate, with predictions from repulsion (BLYP, B3LYP, revPBE) to binding energy of 150 meV (LDA).

4. Water on acenes e.g. G. R. Jennes and K. D. Jordan, J. Phys. Chem. C, 113, 10242 (2009); Estimated binding energy is ~ 90 meV (SAPT)

5. Water-graphene binding curve from DFT “Two leg” ~ 10 meV more stable than “One leg” with LDA (4 meV with PBE)

6. Water-Benzene binding curve

7. QMC technical details CASINO code: R. J. Needs, M. D. Towler, N. D. Drummond, P. Lopez-Rios, CASINO user manual, version 2.1, University of Cambridge, 2007. DF pseudopotentials (Trail and Needs, J. Chem. Phys. 122, 014112 (2005); ibid 122, 174109 (2005); see also CASINO web-page) Single particle orbitals from PWSCF (plane waves), 300 Ry PW cutoff. Then expanded in B-splines. (D. Alfè and M. J. Gillan, Phys. Rev. B, 70, 161101(R), (2004))

8. Water-Benzene binding curve J. Ma, D. Alfè, A. Michaelides, E. Wang, JCP, 130, 154303 (2009)

9. Water-benzene binding energy: Nodal effects Orbitals  i from: LDA B3LYP PBE PBE0 HF Best is LDA

10. Water-graphene binding curve “Two leg” ~ 10 meV more stable than “One leg” with LDA (4 meV with PBE)

11. CASINO scaling on JaguarPF (Cray XT5, 150,000 cores at ONRL)