# String Method for Lammps

This is a Lammps [1] based implementation of the modified String method [2] for
transition path calculations. 

## Usage 

See the header of src/string_method/string_method.py
There are three types of input files:

* A configuration file `string.cfg` with parameters for the calculation
* A Lammps input file that sets up the calculation in Lammps
* A set of input images that form the first approximation of the transition path

E.g. consider the case where we want to calculate the transition path of 
a reaction using 16 images. Let's assume our initial estimates (e.g. 
linear interpolation of coordinates between initial and final state) 
are stored in `REPLICAS/data.*`, i.e. `REPLICA/data.0`, `REPLICA/data.1` ..
`REPLICA/data.15`, with data.0 and data.15 being the initial and final
state, respectively.  

The input files are described below. The calculation is started as follows:
```
mpirun -np <num_processors> string_method.py <config_file>
```

The configuration file might look as follows:

```
[setup]
# Lammps setup file
setup_file = in.lammps.string
# Number of replicas
num_replicas = 16                 
# Number of atoms
num_atoms = 311040                
# Atom types with fixed degrees of freedom
fixed_types = 3 4                 
# X, Y, Z periodicity
# Note: in case of parallelepiped-shaped
# simulations, only the orthogonal face 
# can be periodic at the moment.
periodic_boundary = 0 0 1         
[run]
# Number of Lammps iterations between reparameterizations
num_lammps_iterations = 100
# Maximum number of reparameterizations
max_string_iterations = 400
# Stop criterion 
string_displacement_threshold = 1.0e-3
# Measure run time 
measure_time = True                     
```

And the Lammps input file might look as follows:

```
#------ Create the box, the lattice, and the atoms -------#
log log.?c
units      metal
boundary   s s p
atom_style atomic
atom_modify map array
atom_modify sort 0 0.0

read_data ./REPLICAS/data.?c

pair_style eam/alloy
pair_coeff * * "MgAl-LEA.eam.alloy" Al Mg Al Mg

#------ Fix the shell ------------------------------------#
group nebatoms type 1 2
group shell type 3 4 
fix freeze shell setforce 0 0 0 

#------ Prepare minimization -----------------------------#
# The following neighborlist parameters are required during minimization.
# If they are not given, lammps will use them anyway and issue a warning.
neigh_modify delay 0 every 1 check yes
timestep 0.001
min_style fire
thermo 100
variable PE equal pe
compute  PE_atom all pe/atom
```

## Requirements

The code should be compatible with Lammps versions 2016-2018 and Python
versions 3.5 to 3.6 (and likely more recent). More recent versions of Lammps
may be incompatible if the Python interface has changed. The code uses mpi4py
for parallelization.

[1] https://github.com/lammps/lammps
[2] Weinan et al. The Journal of Chemical Physics 126, 164103 2007