From: Peter Jensen Husen (
Date: Fri Dec 08 2017 - 07:16:07 CST

I'm looking at the autoionize.tcl script (v1.25 from vmd-1.9.3), and
I'm wondering about line 429 for calculating the number of ions:

> set num [expr {int(0.5 + 0.0187 * $saltConcentration * $nWater)}]

so this seems to take the molar mass of water to be 18.7 g/mol, whereas
I would expect it to be 18.015 g/mol. I also tried to measure the
density of the pre-equilibrated water box provided by the plugin and
found 55.5 mol/L as expected. The value 0.00187 in the script would
correspond to a density of rather 53.5 mol/L.

Surely, it's not a huge error, but I was wondering if there is
something I am missing? Is the value supposed to be like this (and in
that case, why?) or is it a mistake?

Peter Husen
Dept. of Physics, Chemistry and Pharmacy
University of Southern Denmark