Re: Correctness of Simulation Results

From: Axel Kohlmeyer (akohlmey_at_cmm.chem.upenn.edu)
Date: Mon Jun 09 2008 - 12:03:06 CDT

On Mon, 9 Jun 2008, Rahul wrote:

RM> Hi all,

hi rahul,

RM> I am a college student currently working on a research project aimed at
RM> porting NAMD simulations to the NVIDIA Tesla. I have basic questions
RM> regarding the importance of the accuracy of MD results, and the motivation
RM> for these questions arises from the following observations.
RM>
RM> 1. I have run NAMD for the same input coordinates and other simulation
RM> parameters on different machines. Even when NAMD is compiled without any
RM> optimizations, output coordinates are significantly different for different
RM> machines. This leads to the basic question: How important is the correctness
RM> of the trajectories as far as molecular dynamics simulation is concerned,
RM> and depending on this, how does one determine which result is correct, given
RM> that results are different on different machines?

a classical MD is practically the numerical solution to a
system of coupled partial differential equations and as such
not lyapunov stable, i.e. the tinest errors will accumulate
to exponential divergence of the trajectories. since you are
using floating point math (and particularly in single precision)
numerical errors (e.g. through summing up differently due to
different load balancing) accumulate fast.

the resulting trajectories should still be statistical mechanical
meaningful and for that you have to use parameters related to that.
the easiest property to follow is energy conservation (without
using a thermostat!). there are other properties that should be
preserved as well (average volume in variable cell, average pressure,
individual forces for a given, identical conformation, the distribution
of forces over time, radial distribution functions, velocity
auto-correlation functions etc.).

RM> 2. Running the publicly available binaries of CUDA-accelerated NAMD gives
RM> significantly different results from those obtained by running the
RM> non-accelerated versions. For reference, I carried out tests on the ApoA1
RM> benchmark, freely available on the NAMD website. Does this mean that the
RM> CUDA accelerated version of NAMD gives inaccurate results? Or alternatively,
RM> are results that important?
RM> 3. I am trying to look for opportunities for CUDAfication within the
RM> ComputeNonbondedUtil class, and just for experience, I am currently trying
RM> to port the first j loop within the function 'calc_pair' to the GPU. This
RM> loop is the one that follows the first call to 'pairlist_from_pairlist', and
RM> is defined under the NORMAL flag. So I have created a kernel function and
RM> enclosed the loop within it. I am ensuring that all required parameters are
RM> being correctly passed and any changes in variables having a bigger scope
RM> are reflected as the control comes out of the function. The odd thing is
RM> that the output is equivalent to the reference output (i.e. the output I get
RM> on the same machine using pre-compiled binaries) when this kernel function
RM> is run on the host. However, when it is run on the GPU device, the output
RM> becomes different. Analysis using gdb shows that during the first run of the
RM> kernel function itself, the values pointed by 'f_1' are different from what
RM> they are in the reference (unmodified source) compilation. The difference is
RM> very small though, of the order of 0.0000000000001 %. I am not sure whether
RM> the overall difference in output is due to this very reason, or due to other
RM> factors also. But all other variables in the scope of the kernel are
RM> absolutely acurate in their values.
RM>
RM> Does anyone have an idea about the level of accuracy required in MD
RM> simulations, and the reason for the above discrepancies?

hope this helps. you may want to have a look into a text book
on MD and its errors, too.

    axel.

RM>
RM> Thank you.
RM>
RM> Rahul
RM>

-- 
=======================================================================
Axel Kohlmeyer   akohlmey_at_cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
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