[gmx-users] Ionization: Your solvent group size (76663) is not a multiple of 3

[Mohammed I Sorour]

Dear Gromacs users,

  I'm adding two different salts to my system in different concentrations;
first salt addition is done successfully  using the following commands:
>gmx grompp -f em.mdp -c sys_solv.gro -p topol.top -o ions.tpr
>gmx genion -s ions.tpr -o sys_solv_ions.gro -p topol.top -pname MG -pq 2
-nname CL -conc 0.1 -neutral

For the second salt, I generated a new tpr file and used the
following commands:
>gmx grompp -f em.mdp -c sys_solv_ions.gro -p topol.top -o ions2.tpr
gmx genion -s ions2.tpr -o sys_solv_ions.gro -p topol.top -pname NA -nname
CL -conc 0.006

But for the second salt addition, I get a fatal error.

Selected 4: 'SOL'

-------------------------------------------------------
Program:     gmx genion, version 2016.3
Source file: src/gromacs/gmxana/gmx_genion.cpp (line 500)



*Fatal error:Your solvent group size (76663) is not a multiple of 3*

Any hints about this issue? Also, why should the solvent group size
multiple of another number? Like, I have seen the case with other gromacs
users:
https://mailman-1.sys.kth.se/pipermail/gromacs.org_gmx-users/2014-December/094291.html



Thanks,
Mohammed Sorour