Added a simple neural network potential

examples/mliap/README

@@ -72,3 +72,11 @@ or
 
 The resultant log.lammps output should be identical to that generated
 by in.mliap.snap.Ta06A and in.mliap.pytorch.Ta06A.
+
+in.mliap.pytorch.relu1hidden
+----------------------------
+This example demonstrates a simple neural network potential
+using PyTorch and SNAP descriptors. It uses a ReLU activation
+function with just 1 hidden layer.
+  
+`lmp -in in.mliap.pytorch.relu1hidden -echo both`

examples/mliap/in.mliap.pytorch.relu1hidden

@@ -0,0 +1,53 @@
+# Demonstrate MLIAP interface to linear SNAP potential
+
+# Initialize simulation
+
+variable nsteps index 100
+variable nrep equal 4
+variable a equal 3.316
+units           metal
+
+# generate the box and atom positions using a BCC lattice
+
+variable nx equal ${nrep}
+variable ny equal ${nrep}
+variable nz equal ${nrep}
+
+boundary        p p p
+
+lattice         bcc $a
+region          box block 0 ${nx} 0 ${ny} 0 ${nz}
+create_box      1 box
+create_atoms    1 box
+
+mass 1 180.88
+
+# choose potential
+
+include relu1hidden.mliap.pytorch
+
+# Setup output
+
+compute  eatom all pe/atom
+compute  energy all reduce sum c_eatom
+
+compute  satom all stress/atom NULL
+compute  str all reduce sum c_satom[1] c_satom[2] c_satom[3]
+variable press equal (c_str[1]+c_str[2]+c_str[3])/(3*vol)
+
+thermo_style    custom step temp epair c_energy etotal press v_press
+thermo          10
+thermo_modify norm yes
+
+# Set up NVE run
+
+timestep 0.5e-3
+neighbor 1.0 bin
+neigh_modify once no every 1 delay 0 check yes
+
+# Run MD
+
+velocity all create 300.0 4928459 loop geom
+fix 1 all nve
+run             ${nsteps}
+

examples/mliap/relu1hidden.mliap.pytorch

@@ -0,0 +1,18 @@
+# DATE: 2014-09-05 UNITS: metal CONTRIBUTOR: Aidan Thompson athomps@sandia.gov CITATION: Thompson, Swiler, Trott, Foiles and Tucker, arxiv.org, 1409.3880 (2014) 
+
+# Definition of SNAP potential Ta_Cand06A
+# Assumes 1 LAMMPS atom type
+ 
+variable zblcutinner equal 4
+variable zblcutouter equal 4.8
+variable zblz equal 73
+
+# Specify hybrid with SNAP, ZBL
+
+pair_style hybrid/overlay &
+zbl ${zblcutinner} ${zblcutouter} &
+mliap model mliappy relu1hidden.mliap.pytorch.model.pkl &
+descriptor sna Ta06A.mliap.descriptor
+pair_coeff 1 1 zbl ${zblz} ${zblz}
+pair_coeff * * mliap Ta
+