[gmx-users] about (painful) units conversion

Mark Abraham mark.abraham at anu.edu.au
Tue Jul 11 01:17:59 CEST 2006

> Hello,
> I'm still working on the CHARMM ff convertion into the Gromacs standards
> and I wonder how exactly to convert kcals of CHARMM in to Gromacs'
> KJoules. I have found 3 different constants (at least?)
> - officially (SI) : 1 kcal = 4.1868 kJ
> - but often : 1 kcal = 4.184 kJ
> - in my graduate courses of thermodynamics = 1 kcal = 4.185 kJ
> Who's right? It's probaly a debat with no end and the three answers are
> probably correct ;-). But which conversion should be applied in our case?

There's some light shed here http://en.wikipedia.org/wiki/KCal#Versions,
so it depends on the age of your source and the way it defines a calorie.
I think the advice there is sound and it doesn't matter, but I have always
applied 4.184 in doing such conversions in chemistry.


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