[gmx-users] single-precision *.tpr data after a double-precision run

[David Osguthorpe]

> 
> This means that although the minimization was run in double precision,  
> the atomic configuration is saved in single precision (the last two  
> decimal places are not used). Does this mean that each time the  
> optimized *.gro file is re-read, it needs to optimize these last two  
> decimal places anew, and this makes the difference in SEVEN orders of  
> magnitude in the maximal force?
> 

to elaborate a bit on this

simple answer - yes

truncations in coordinate precision lead to much larger errors in the forces

the reason is the .gro format has no precision in the coordinates - it is in
nm and only has 3 decimal places - this is only 1% precision in angstrom
- and remember bond length changes of 0.01 angstrom are significant
- eg. a difference between c-c bonds of 1.54 and 1.55 - which is a signifcant
energy change for a bond - hence the large change in forces

youve already  seen the solution to the problem - which is to ignore the .gro files!!

David