Add water examples featuring fix qtpie/reaxff

examples/reaxff/water/gauss_exp.txt

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+# Gaussian orbital exponents (required for fix qtpie/reaxff) taken from Table 2.2
+# of Chen, J. (2009). Theory and applications of fluctuating-charge models.
+# The units of the exponents are 1 / (Bohr radius)^2 .
+1  0.2240  # O
+2  0.5434  # H

examples/reaxff/water/in.water.qtpie

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+# QTPIE Water
+
+boundary        p p p
+units           real
+atom_style      charge
+
+read_data       data.water
+
+variable        x index 1
+variable        y index 1
+variable        z index 1
+
+replicate       $x $y $z
+
+pair_style      reaxff NULL safezone 3.0 mincap 150
+pair_coeff      * * qeq_ff.water O H
+neighbor        0.5 bin
+neigh_modify    every 1 delay 0 check yes
+
+velocity all create 300.0 4928459 rot yes dist gaussian
+
+fix             1 all qtpie/reaxff 1 0.0 10.0 1.0e-6 reaxff gauss_exp.txt
+fix             2 all nvt temp 300 300 50.0
+
+timestep        0.5
+thermo          10
+thermo_style    custom step temp press density vol
+
+run 20

examples/reaxff/water/in.water.qtpie.field

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+# QTPIE Water
+
+boundary        p p p
+units           real
+atom_style      charge
+
+read_data       data.water
+
+variable        x index 1
+variable        y index 1
+variable        z index 1
+
+replicate       $x $y $z
+
+pair_style      reaxff NULL safezone 3.0 mincap 150
+pair_coeff      * * qeq_ff.water O H
+neighbor        0.5 bin
+neigh_modify    every 1 delay 0 check yes
+
+velocity all create 300.0 4928459 rot yes dist gaussian
+
+fix             1 all qtpie/reaxff 1 0.0 10.0 1.0e-6 reaxff gauss_exp.txt
+fix             2 all nvt temp 300 300 50.0
+fix             3 all efield 0.0 0.0 0.05
+
+timestep        0.5
+thermo          10
+thermo_style    custom step temp press density vol
+
+run 20