5 Using CP

This section is intended to explain how to perform basic Car-Parrinello (CP) simulations using the CP package.

It is important to understand that a CP simulation is a sequence of different runs, some of them used to "prepare" the initial state of the system, and other performed to collect statistics, or to modify the state of the system itself, i.e. modify the temperature or the pressure.

To prepare and run a CP simulation you should:

1. Define the system:
   1. atomic positions
   2. system cell
   3. pseudopotentials
   4. number of electrons and bands
   5. cut-offs
   6. FFT grids

The first run, when starting from scratch, is always an electronic minimization, with fixed ions and cell, to bring the electronic system on the ground state (GS) relative to the starting atomic configuration.
Example of input file (Benzene Molecule):

         &control
            title = 'Benzene Molecule',
            calculation = 'cp',
            restart_mode = 'from_scratch',
            ndr = 51,
            ndw = 51,
            nstep = 100,
            iprint = 10,
            isave = 100,
            tstress = .TRUE.,
            tprnfor = .TRUE.,
            dt    = 5.0d0,
            etot_conv_thr = 1.d-9,
            ekin_conv_thr = 1.d-4,
            prefix = 'c6h6',
            pseudo_dir='/scratch/benzene/',
            outdir='/scratch/benzene/Out/'
         /
         &system
            ibrav = 14,
            celldm(1) = 16.0,
            celldm(2) = 1.0,
            celldm(3) = 0.5,
            celldm(4) = 0.0,
            celldm(5) = 0.0,
            celldm(6) = 0.0,
            nat = 12,
            ntyp = 2,
            nbnd = 15,
            nelec = 30,
            ecutwfc = 40.0,
            nr1b= 10, nr2b = 10, nr3b = 10,
            xc_type = 'BLYP'
         /
         &electrons
            emass = 400.d0,
            emass_cutoff = 2.5d0,
            electron_dynamics = 'sd'
         /
         &ions
            ion_dynamics = 'none'
         /
         &cell
            cell_dynamics = 'none',
            press = 0.0d0,
          /
          ATOMIC_SPECIES
          C 12.0d0 c_blyp_gia.pp
          H 1.00d0 h.ps
          ATOMIC_POSITIONS (bohr)
          C     2.6 0.0 0.0
          C     1.3 -1.3 0.0
          C    -1.3 -1.3 0.0
          C    -2.6 0.0 0.0
          C    -1.3 1.3 0.0
          C     1.3 1.3 0.0
          H     4.4 0.0 0.0
          H     2.2 -2.2 0.0
          H    -2.2 -2.2 0.0
          H    -4.4 0.0 0.0
          H    -2.2 2.2 0.0
          H     2.2 2.2 0.0

You can find the description of the input variables in file INPUT_CP in the Doc/ directory.