Author: Ilia Nikiforov (nikif002 |AT| umn DOT edu)
        Daniel S. Karls (karl0100 |AT| umn DOT edu)
        University of Minnesota
Date: 2019-06-02

This Test Driver computes the equilibrium lattice spacing and cohesive energy
of a 2D hexagonal monolayer crystal using Polak-Ribiere conjugate gradient
minimization in LAMMPS. The inputs consists of the choice of structure type, an
initial guess at the equilibrium lattice spacing, and the two atomic species
comprising the crystal. The currently supported structure types are:

  1: graphene-like (e.g. h-BN or graphene)
  2: 2H structure of transition metal dichalcogenides (TMDs) (e.g. MoS2, TiSe2)
  3: 1T structure of TMDs

For consistency between Tests created using this Test driver, the convention
for inputting atomic species is:

  - For structure 1, input the species in the order of the chemical formula,
    e.g.  species 1: B, species 2: N.

  - For structures 2 and 3, species 1 is the transition metal (central layer)
    and species 2 is the chalcogen (two outer layers).