[gmx-users] Again: Non-integer charges in tpr-files

[Erik Lindahl]

K.A. Feenstra wrote:
> Erik Lindahl wrote:
> 
>>Won't work, since we don't request that all charge groups are strictly
>>neutral. It's only with cutoffs that the neutrality is important, but
>>the concept of charge groups still speeds up neighborsearching quite a lot.
> 
> 
> Ok, then what about this (you probably thought about this also, but 
> anyway): in sum_q in topio.c, the charges are summed as follows:
> 
>     *qt  += n*atoms->atom[i].q;
> 
> qt is *double, but t_atom.q is a float. Is the increment then done in 
> double or single precision? Or is that not defined?

Double according to ANSI C. All intermediary results in C are double,
even if _all_ operands are float (although you can sometimes override 
the standard with compiler flags).

Cheers,

Erik