APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012;
Boston, Massachusetts
Session D36: Focus Session: Environment I: Aerosols and Aqueous Solutions
2:30 PM–5:30 PM,
Monday, February 27, 2012
Room: 107C
Sponsoring
Unit:
DCP
Chair: Don Baer, Pacific Northwest Research Laboratory and Ilja Siepmann, University of Minnesota
Abstract ID: BAPS.2012.MAR.D36.5
Abstract: D36.00005 : 1st principle simulations of ions in water solutions: Bond structure and chemistry in the hydration shells of highly charged ions
4:06 PM–4:42 PM
Preview Abstract
Abstract
Author:
John Weare
(University of California, San Diego)
Methods of direct simulation (Monte Carlo and molecular dynamics) have
provided new insights into the structure and dynamics of electrolyte
solutions. However, these methods are limited by the difficulty of
developing reliable ion-solvent and solvent-solvent potential interactions
in the highly perturbed hydration region. To model the interactions in this
region methods of simulation that are based on the direct on the fly
solution to the electronic Schr\"{o}dinger equation (ab-initio molecular
dynamics, AIMD) are being developed. However, 1st principle methods have
their own problems because the solution to the electronic structure problem
is intractable unless rather uncontrolled approximations are made (e.g.
density functional theory, DFT) and there is high computational cost to the
solution to the Schr\"{o}dinger equation. To test the accuracy of AIMD
methods we have directly simulated the XAFS spectra for a series of
transition metal ions Ca$^{2+}$, Cr$^{3+}$, Mn$^{2+}$, Fe$^{3+}$, Co$^{2+}$,
Ni$^{2+}$, Cu$^{2+}$, and Zn$^{2+}$. Despite DFT's well know deficiencies,
the agreement between the calculated XAFS spectra and the data is almost
quantitative for these test ions. This agreement supports the extension of
the interpretation well beyond that of the usual XAFS analysis to include
higher-order multiple scattering signals in the XAFS spectra, which provide
a rigorous probe of the first shell distances and disorders. Less well
resolved features of the spectra can still be analyzed and are related to
2nd shell structure. The combination of XAFS measurements and the parameter
free AIMD method leads to new insights into the hydration structure of these
ions. While strictly local DFT +gga provides excellent agreement with data,
the addition of exact exchange seems to provide slightly better structural
agreement.
The computational complexity of these calculations requires the development
of simulation tools that scale to high processor number on massively
parallel supercomputers. Our present algorithm scales to nearly 100,000
processors. However, even with high scaling the time to solution is very
long. We are also developing and testing new methods to improve the
performance of simulation and new sampling methods that more efficiently
explore phase space and can reach longer time frames. Results of calculation
of the hydration structure and dynamics of highly charges ions and free
energy calculations of ion association will be presented.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2012.MAR.D36.5