Aqueous Environments

We are investigating pure water and simple aqueous environments, using first principles techniques, e.g. ab initio molecular dynamics. Our studies include:

Many of these studies are in collaboration with Eric Schwegler's group at LLNL and Francois Gygi's group at UC Davis.


Dependence of quasi-particle (GW) band gap of liquid water [1] on the number of dielectric eigenmodes (n) used in approximating the inverse dielectric matrix [2,3]. Dotted line: the expansion of the inverse dielectric matrix in terms of eigenvectors and eigenvalues is simply truncated to a finite number (n); dashed line: a model dielectric function is used in order to describe the eigenvalues not included in the truncated sum.
GW gap of liquid waters
Dielectric band structure of water and ice
The dielectric band structure of (a) liquid water and (b) ice Ih. The lowest bands in ice and water form distinctive groups, whose representative maximally localized dielectric eigenmodes are shown in I to IV [1].

We have studied the static dielectric properties of ice and liquid water from first principles, by analyzing the eigenmodes of their respective dielectric matrices in terms of maximally localized dielectric functions. These are similar, in their definition, to maximally localized Wannier orbitals obtained from Bloch eigenstates of the electronic Hamiltonian. The lowest eigenmodes of the inverse dielectric matrix are localized in real space and can be separated into groups related to the screening of lone pairs, intra-, and intermolecular bonds, respectively. The local properties of the dielectric matrix can be conveniently exploited to build approximate dielectric matrices for efficient, yet accurate calculations of quasiparticle energies [1].


Confined Water
Water in contact with several substrates and confining media has been studied using ab initio and classical simulations [4-8].

We have carried out several ab initio simulations of water in contact with hydrophilic [4] and non polar surfaces [5-8] and we have studied the interplay between the effect of confining media and the mere presence of an interface. The effect of a confining surface on the liquid structure was found to be localized within a thin interfacial layer (~ 0.3-0.5 nm) for both hydrophilic and hydrophobic substrates [4,5]. No vapor at the interface and no new phases of water under confinement were observed. Hydrogen bonds are enhanced at the interface, and subtle but important electronic polarization effects are present, even in the case of hydrophobic substrates [5-7].


Infrared Spectra for water
Comparison between power spectra (v-DOS) and infrared spectra (IR) of deuterated water confined between graphene sheets [6,7]. The case of ~ 1nm confinement is shown.

We have studied infrared spectra of water, ice [9] and water at interfaces [6,7]. Comparisons between computed power and IR spectra, and analysis of the different contributions to IR spectra reveal subtle electronic effects, i.e. how the electronic properties of water and the surface subtly change in the presence of each other and how these changes are reflected on the liquid vibrational properties.


Left: charge density differences for the benzene-water dimer. Red indicates areas where the combined system contains less charge, blue where the charge density of the combined system is higher. Ab initio simulations have been run for more than 100 ps. [10]. Right: Snapshot of an ab initio MD simulations of a solvated Chloride ion
Solvation in water

We have studied solvation of non polar species in water, e.g. benzene and hexa-fluobenzene [10,11] and we are investigating solvation of ionic species with different exchange-correlation functionals, within an ab initio Molecular dynamics framework.



References

  1. "Dielectric properties of ice and liquid water from first principle calculations", D. Lu, F. Gygi and G. Galli, Phys. Rev. Lett., 100, 147601(2008).
  2. "Efficient iterative methods for the calculation of dielectric matrices", H.Wilson, F.Gygi and G.Galli, Phys. Rev. B, 78,113303 (2008).
  3. "Iterative calculations of dielectric eigenvalue spectra", H. Wilson, D. Lu, F. Gygi and G. Galli,, Phys. Rev. B.,79, 245106 (2009).
  4. "Water at a hydrophilic solid surface probed by first principles molecular dynamics: inhomogeneous, thin layers of dense fluid", G.Cicero, J.Grossman, A.Catellani and G.Galli, J.Amer.Chem.Soc.127, 6830 (2005).
  5. "Water confined in nanotubes and between graphene sheets: a first principle study", G. Cicero, J. C. Grossman, E. Schwegler, F. Gygi and G. Galli, J. Amer. Chem. Soc., 130, 1871(2008).
  6. "Probing properties of water under confinement: Infrared spectra", M.Sharma, D.Donadio, E.Schwegler and G.Galli, Nano Lett., 8, 2959 (2008).
  7. "Electronic effects in the IR spectrum of water under confinement",D. Donadio, G. Cicero, E. Schwegler, M. Sharma and G.Galli, J. Phys. Chem. B., 113, 4170 (2009)
  8. "Water confined in carbon nanotubes: Magnetic response and proton chemical shieldings" P.Huang; E. Schwegler,and G. Galli, J.Phys. Chem. C accepted (2009)
  9. "Role of dipolar correlations in the infrared spectra of water and ice", W.Chen, M.Sharma, R.Resta, G.Galli and R.Car, Phys. Rev. B 77, 245114 (2008).
  10. "Structure of hydrophobic hydration of benzene and hexafluorobenzene from first principles", M. Allesch, E. Schwegler and G. Galli J.Phys.Chem. B 111, 1081 (2007).
  11. "First principles and classical molecular dynamics simulations of solvated benzene", M. Allesch, F. C. Lightstone, E. Schwegler, and G. Galli , J. Chem. Phys. , 128, 014501 (2008).