Clayton
Hall,
Format of Workshop
Our
workshop is not a usual meeting in which experts talk about their latest
work. Instead, this meeting is about
tools. On the first day, DoD
computational chemists will describe their goals, tools, and the problems they
encounter. Then, experts in the field will speak—within the
context of the DoD presentation—about the special tools developed or
used by them. Each expert will have
about an hour – this includes discussion and questions and we will encourage
continual interaction between speakers and audience (we will keep the meeting strictly on schedule, so please be prepared). Near
the end of the workshop, we will separate into groups to develop specific
answers to questions presented in the first part of the meeting and
recommendations. The last hour of the
workshop will be the groups reporting back to everyone. As you know, the success of this workshop
depends on everyone’s participating throughout. The workshop will produce a
report, but the onsite interactions are most important.
General Questions
to Address
Specific Issues to Address
We need reliable computational
methods to predict properties of condensed phase materials of relevance to the
Army, chiefly energetic materials. In most
cases these are molecular crystals but some ionic crystals are also of
interest. The first step is a
determination of interactions between molecules, i.e., electronic structure
calculations for fixed nuclear positions.
One approach is to develop reliable potentials (force fields) including
effects of manybody forces for molecules involved. Another approach is to perform electronic structure calculations
with periodic boundary conditions for molecules of interest placed in a unit
cell. Since often one has to include a
large number of molecules in the unit cell, computational requirements of such
calculations can be large. How do the
two approaches compare?
The next step is to determine lowenergy crystal structures
of materials. If the relevant potentials
are known, one can do it using molecular packing programs. How feasible is it to go from using
empirical atomatom potentials to potentials computed using modern electronic
structure techniques? Are optimizations
of crystal structure feasible, for molecules of interest, within periodic
conditions programs calculating the interactions onthefly?
Majority of the codes developed or used by the speakers use
some flavors of the DFT method.
Comparisons of these implementations and of capabilities of various
codes will be in order. Furthermore, we
should discuss whether DFT is a proper computational method for molecular
crystals. DFT is known to often fail to
predict reasonable interaction energies, in particular for systems with a
significant dispersion component.
Interactions of large organic molecules, the main interest for the
workshop, always include a large dispersion component and for some
configurations are dispersion dominated (e.g., stacked configurations of DNA
bases).
The minimum energy structure immediately gives the density
of a material, which is one of the most important properties to know. Densities have been so far mostly predicted
with molecular packing programs based on empirical potentials. Even this approach is time consuming. One must generate a variety of crystal
structures corresponding to the various crystalline space groups from the
coordinates of a single molecule. Each
crystal structure must be optimized, i.e., the crystallographic parameters
minimized with respect to energy and all energies compared. The current criterion for selection of the
most probable structure is lattice energy; i.e., the structure that has the
lowest lattice energy is assumed to be the “correct” crystal structure. This criterion has proven to be insufficient
and additional criteria, perhaps the evaluation of DG for
each crystal structure, should be included in the selection procedure.
The molecular packing calculations
would be more predictive if empirical potentials were replaced by ab initio potentials. However, twobody (pair) potentials would
not be sufficient in most cases. How
can we compute manybody potentials?
Can polarization model of manybody forces be sufficient? Ab
initio potentials are typically more complicated than empirical potentials
and will make molecular packing calculations more time consuming.
Other important physical effects
determining the structure and properties of molecular crystals are
intramolecular degrees of freedom. Most of work in the field assumes rigid molecules.
However, partial deformations of the molecules compared to their gasphase
structures are always present in crystals.
In some cases, torsional deformations can be large. For example, molecules like RDX may be
adequately modeled as rigid structures, but allowance for molecular
deformations will likely be important for floppy molecules like PETN, as shown by Sorescu, Rice, and
Thompson. Inclusion of a large number
of intramolecular degrees of freedom is next to impossible in electronic
structure calculations of potentials.
Are onthefly approaches better suited to investigate monomer
nonrigidity effects?
In addition
to density, one needs to know several other properties of materials such as
heats of formation, burning rates, etc. Some of them can be obtained from
molecular dynamic calculations for crystals.
How reliably can we predict such properties and at what computational
cost? Can the predictions be trusted
for notional materials?
Finally,
are we able to computationally model chemical reactions in the condensed
phase? What approaches are
possible? Can periodic boundary
programs be used for this purpose taking into account that chemical reactions
locally break crystal order and therefore unit cells have to become huge?
PROGRAM
May 27: Arrive,
dinner (at 19:00 in Clayton Hall), evening orientation and introductions. Registration
starts at 18:00.
May 28:
DOD Programs and Needs for Computational Chemistry Packages

Betsy Rice, Cary Chaba lowski,
William Mattson 
ARL 
Army Needs for Crystal Structure
and other Property Predictions 

Betsy Rice 
ARL 
Parallelizing Molpak and Planewave 

Break 



David Singh 
NRL 
Pseudopotential and Planewave
Codes 
Presentations by Experts

Krzysztof Szalewicz 
U. 
Intermolecular Interaction
Potentials 

Herman Ammon 
U. 
Crystal Structure Prediction for
Energetic Materials 

Lunch 



Sally Price 
U. Coll. London 
Anisotropic AtomAtom
Intermolecular Potentials in Organic Crystal Structure and Property
Prediction 

Emilio Artacho 

Material Properties with SIESTA:
Strengths, Weaknesses, and Prospects 

Break 



Julian Gale 
Imperial Coll. 
General Utility Lattice Program 

Anne Chaka 
NIST 
Validation of Computational
Methods 

Dinner 



Discussion 

Possible Implementations of New
Approaches by Army 
May 29:
Presentations by Experts (continued)
083009:30 
Eric Bylaska 
PNNL 
Materials Properties with NWChem 

Richard Martin 
U. 
Identifying the Key Problems in
Present Density Functionals 

Break 



Gustavo Scuseria 

Condensed Phase Simulations using
Gaussian Orbitals and Periodic Boundary Conditions 

Lunch 



Emily Carter 
UCLA 
Modeling Chemistry and Physics in
Bulk 
Group discussion:

Krzysztof Szalewicz and Bob Shaw 

Summary remarks and guidelines for
discussion groups 

Discussion group I 

Chabalowski, Byrd, Mattson,
Martin, Carter, Bylaska, Scuseria, Doren, Tchoukova, Murdachaew, Sandler,
Shaw 
14:0017:00 
Discussion group II 

Rice, Kim, Ammon, Price, Gale,
Artacho, Chaka, Singh, Bukowski, AkinOjo, Wang, Rowe, Szalewicz 

Break 



Discussion 

Groups Report back with their
Conclusions 

Dinner 


May 30:
Departures
Van schedule:
Depart from Depart from
Embassy
Suites Clayton Hall
Tuesday
5/27/03: 18:00 21:00
Wednesday 5/28/03: 08:00 21:00
Thursday
5/29/03: 08:10
20:00