[gmx-users] single and double

[chris.neale at utoronto.ca]

My system here is an opls-aa protein in a POPE/DMPE membrane with  
tip4p waters.

Prior to adding counterions, grompp yields a net charge of -1.999973  
and grompp_d yields -2.0. Once I have added counterions, both single  
and double precision don't give me a warning about a net charge on the  
system. Either system runs fine for many ns and they don't crash.

1. Is the net charge really as close to 0.0 in the single precision  
version as it is in the double, or just close enough that there is no  
warning issued?

2. If single is farther from 0.0, is this a problem?

3. Why does single run faster anyway? I though intel processors  
treated them the exact same way in terms of the operations that needed  
to be carried out?

Thanks.
Chris.