This example shows how to calculate the minimum energy path (MEP)
by using the nudged elastic band (NEB) method in conjunction with
the Effective Screening Medium Method (ESM) and two constant bias potential
methods (FCP & GCSCF)

Before starting this example, see also examples for the ESM method and
two constant bias potential methods (FCP & GCSCF) in pw.x
(PW/examples/ESM_example).

The example calculations are divided into the following three parts,
1) NEB example with ESM, constant-N calculation (run_example_ESM)
2) NEB example with FCP, constant-mu calculation (run_example_FCP)
3) NEB example with GCSCF, constant-mu calculation (run_example_GCSCF)

The 'run_example' file executes the above three calculations.

1) NEB example with ESM, constant-N calculation (run_example_ESM)
1-1) make an NEB calculation for H atom diffusion on Al(001) surface with
     ESM (bc3). This calculation shows the MEP of H atom (Bridge site ->
     top site -> bridge site).
     (input=Al001+H_bc3.in, output=Al001+H_bc3.out).
     Excess charge & Fermi energy at initial geometry)
     tot_charge   = 0.0
     Fermi energy = -4.5 eV
1-2) repeat #1-1 with tot_charge = 0.0215 (positively charged surface)
     (input=Al001+H_bc3_n215.in, output=Al001+H_bc3_n215.out).
     Excess charge & Fermi energy at initial geometry)
     tot_charge   = 0.0215
     Fermi energy = -5.0 eV (i.e. applied potential =-0.5 V vs PZC)


2) NEB example with FCP, constant-mu calculation (run_example_FCP)
2-1) repeat #1-2 under a constant bias potential condition using FCP.
     The target Fermi energy is -5.0 eV.
     (input=Al001+H_FCP_vm05.in, output=Al001+H_FCP_vm05.out)
     NB----------------------------------------------------------------
     | i) If first_last_opt = .FLASE. (default)                       |
     | Need to specify the TOTAL_CHARGE card in the initial and final |
     | geometries to set the Fermi energy of those systems to become  |
     | a desired value. For intermediate images, the initial charge   |
     | of the system is given by lenear interpolation between         |
     | the first and the last configurations.                         |
     | ii) If first_last_opt = .TRUE.                                 |
     | FCP can converge to the target Fermi energy without specifying |
     | the TOTAL_CHARGE card, but you can get better convergence if   |
     | you specify it.                                                |
     ------------------------------------------------------------------

3) NEB example with GCSCF, constant-mu calculation (run_example_GCSCF)
3-1) repeat #1-2 under a constant bias potential condition using GCSCF.
     The target Fermi energy is -5.0 eV.
     (input=Al001+H_GCSCF_vm05.in, output=Al001+H_GCSCF_vm05.out)
     NB------------------------------------------------------------------
     | No need to specify the TOTAL_CHARGE card. GCSCF is automatically |
     | converged to the target Fermi energy.                             |
     --------------------------------------------------------------------
