The .castep file

Header

As well as the usual information printed at the top of the output file, the molecular dynamics data specified in the parameters file is presented.

 ************************ Molecular Dynamics Parameters ************************
  
 ensemble                                       : NPT
 variable cell method                           : fixed basis quality
 pressure                                       : see below
 temperature                                    :  293.0       K
 using                                          : Parrinello-Rahman barostat
 with characteristic cell time                  :  26.00       ps
 using                                          : Langevin thermostat
 with characteristic ionic time                 :  16.95       ps
 time step                                      : 0.2000E-02   ps
 number of MD steps                             :     500000
 using best-fit first order extrapolation for wavefunctions
 backup results every                           :          5   steps
  
 MD SCF energy / atom convergence tol.          : 0.1000E-04   eV
 MD SCF convergence tolerance window            :          3   cycles

 *******************************************************************************

This amount of information presented in this section will obviously vary depending on the type of calculation being performed, but should be self-explanatory.

Initial cell

The information specified in the .cell file is then output, this will include the ionic positions any user specified velocities if present, the usual pseudo-potential and k-point information, and details of any constraints. The total energy of the initial cell is then minimised, performing a finite basis set correction along the way if appropriate.

The message

Starting MD

is then printed, and the MD calculation proper commences. As the first time-step will require data on forces, stress (for variable cell calculations) and energies, these are evaluated and printed at this point.

Forces are output in the following format, using the force unit specified by the user, or the default of eV / Å.

 ******************************** Forces *********************************
 *                                                                       *
 *                      Cartesian components (eV/A)                      *
 * --------------------------------------------------------------------- *
 *              x                    y                    z              *
 *                                                                       *
 * Si   1     -0.02632             -0.02632             -0.02632         *
 * Si   2     -0.02653             -0.02617             -0.02617         *
 * Si   3     -0.02617             -0.02617             -0.02653         *
 * Si   4     -0.02617             -0.02653             -0.02617         *
 * Si   5      0.00075              0.05164              0.00075         *
 * Si   6      0.05204              0.05204              0.05204         *
 * Si   7      0.05164              0.00075              0.00075         *
 * Si   8      0.00075              0.00075              0.05164         *
 *                                                                       *
 *************************************************************************

The stress tensor (if calculated) is output in the specified units of pressure, along with the corresponding scalar $T=0$ pressure, i.e. one third of the trace of the stress tensor.

 ***************** Stress Tensor *****************
 *                                               *
 *          Cartesian components (GPa)           *
 * --------------------------------------------- *
 *             x             y             z     *
 *                                               *
 *  x      0.009981      1.140649      1.140649  *
 *  y      1.140649      0.009981      1.140649  *
 *  z      1.140649      1.140649      0.009981  *
 *                                               *
 *  Pressure:   -0.0100                          *
 *                                               *
 *************************************************

The current time and MD energies are then printed in the user specified energy units. Here potential energy refers to the total configurational energy of electrons plus ions obtained from the DFT calculation. Kinetic energy is the classical kinetic energy of the ions. Total energy refers to the sum of these two energies.

The enthalpy and the relevant Hamiltonian energy for the ensemble as quoted in section 2.2 is then printed.

The temperature is also output at this point. Note that this is the ionic temperature only. If performing a calculation with finite temperature electrons (i.e. EDFT) the electron temperature may not be the same. Finally total pressure, i.e. the trace of the stress tensor plus the kinetic (ideal gas) pressure is printed in the user specified pressure units.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
x                                               MD Data:     x
x                                                            x
x              time :      0.000000                   ps     x
x                                                            x
x   Potential Energy:   -865.882944                   eV     x
x   Kinetic   Energy:      0.265113                   eV     x
x   Total     Energy:   -865.617832                   eV     x
x           Enthalpy:   -865.617735                   eV     x
x   Hamilt    Energy:   -865.617735                   eV     x
x                                                            x
x        Temperature:    293.000000                    K     x
x      T/=0 Pressure:      0.172231                  GPa     x
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Iteration

The MD simulation proper now begins.

=================================================================
 Starting MD iteration    1 ...
=================================================================

Following this message the current cell information is printed and the energy is re-minimised to determine the new forces. The MD step is then performed and the updated force, stress and energy information is printed in the same manner as above.

-----------------------------------------------------------------
 ... finished MD iteration    1
-----------------------------------------------------------------

The process repeats until the calculation is killed, or the specified number of MD iterations is completed.