1. Computer usage Notur 2007

2. Chemistry: a many-body problem
At the deepest level, molecules are simple:
charged particles in motion
governed by the laws of quantum mechanics
HΨ=EΨ
…but it is
a many-body problem…
“The underlying physical laws necessary for the mathematical treatment of a large part of physics and the whole of chemistry are thus completely known and the difficult is only that the exact application of these laws leads to equations that are too complicated to be soluble” Dirac (1927)

3. Computers came to our rescue…

4. Quantum chemistry HΨ=EΨ Simulations of chemical systems and processes
approximate solutions of the Schrödinger equation
Journal of Americal Chemical Society
40% of all articles supported by computation
most of these are quantum chemical
This is an amazing development for an experimental science
“Every attempt to to employ mathematical methods in the study of chemical questions must be considered profoundly irrational”
August Comte (1798–1857)

5. Computation: the third way
Theory, experiment and computation
interpretation and prediction of experiment
alternative to experimental measurements

6. Example: Reaction pathways

7. Example: NMR spectra
200 MHz NMR spectrum of vinyllithium

8. Methods development: DALTON
Computational models are being constantly improved
increase accuracy and predictive power
broaden the range of applicability and lower the cost
Dalton program system
Scandinavian collaboration
25 years of development
broad functionality
1300 research groups
250 computer centers

9. Towards higher accuracy…

10. Towards larger systems…
Real-world problems are typically large
computational cost typically scales cubically or high with increasing system size
however, in large systems, nearly all contributions are insignificant
these should be recognized and avoided in the computations
ideally, cost should increase in proportion to system size

11. Towards larger systems…
Energies and structure of large molecules
Quantum chemistry is typically done on 50 or less atoms
Energy and forces for this 392-atom molecule can now be done in about one hour
This is more than an order of magnitude improvement over a few years
On a parallel computer, we should be able to do this in about one minute
We should reach atoms in 2010 (currently at a few thousand)
Our bottleneck is memory

12. Computer developments
Our requirements
CPU power and memory—little or no data transfer or storage
Encouraging developments:
Powerful multicore chips
Graphical processing units (GPUs)
Improvements in bandwidths of interconnects
Discouraging developments:
Chips are not getting faster (3GHz)
Multicore chips hard to program effectively
GPU/CPU communication slow
Software and algorithm development necessary
This is our job!

13. Running on Blue Gene
Dalton scales well to over processing cores
Argonne’s Blue Gene/P
1-PFLOPS computing with PowerPC MHz processors
If you provide the hardware, we shall put it to good use…