Relaxed structures for the calculations reported in the manuscript are provided in "extended" XYZ format [1].

Q-TIP4P/F calculations have been carried out with the LAMMPS code [2] (interfaced via ASE [3]).

MB-pol calculations are based on an in-house ASE-calculator building on the MB-pol plugin for OpenMM (Release 1.1.2),
available from the Paesani group [4]. This implementation can only handle periodic boundary conditions for orthogonal
cells of a certain minimum size. For this reason, a 3x3x3 (3x2x7) supercell of the structures provided below for ice Ih
(ice II) has been constructed internally in our ASE MB-pol calculator.

[1] https://wiki.fysik.dtu.dk/ase/dev/_modules/ase/io/extxyz.html
[2] https://lammps.sandia.gov/doc/Howto_tip4p.html
[3] https://wiki.fysik.dtu.dk/ase/index.html
[4] https://github.com/paesanilab/mbpol_openmm_plugin


- ice Ih: 

==> ih-qtip4pf.xyz <==
288
Lattice="13.3973455228 0.000288218955933 -0.000168644185498  0.000333186810814 15.483704165 0.000699787747983  -0.000183692738196 0.000659521318457 14.5916671855"

==> ih-mbpol.xyz <==
288
Lattice="13.5401973366 0.0 0.0  0.0 15.6520245142 0.0  0.0 0.0 14.7539983072"


- ice II:

==> ii-qtip4pf.xyz <==
36
Lattice="7.69183810264 -0.00515898305999 -0.0111177281685  -3.02253873772 7.07309406065 -0.0111177281666  -3.02253873772 -4.57522765162 5.39407778325"

==> ii-mbpol.xyz <==
216
Lattice="12.9751818454 0.0 0.0  -2.99255363976e-06 22.4702673861 0.0  0.0 0.0 6.25158943718"
#
This cell given by the lattice vectors
    a^T = [12.9751818454, 0.0, 0.0]
    b^T = [-2.99255363976e-06, 22.4702673861, 0.0]
    c^T = [0.0, 0.0, 6.25158943718]
can be reduced to the primitive unit cell of ice II according to
    (a',b',c') = (a,b,c) * P = ( P_{1,1}a+P_{2,1}b+P_{3,1}c, P_{1,2}a+P_{2,2}b+P_{3,2}c, P_{1,3}a+P_{2,3}b+P_{3,3}c )
where
   ( 0.5     0.0    -0.5    )
P= (-0.16666 0.3333 -0.1666 ) 
   ( 0.33333 0.3333  0.3333 )
Note that the resulting cell (described by a',b' and c') is rotated from the conventionally chosen reference frame (a' along x-axis, b' in xy-plane).