At this point after having minimized the electrons, and with ions displaced from their equilibrium positions, we are ready to start a CP dynamics. We need to specify 'verlet' both in ionic and electronic dynamics. The threshold in control input section will be ignored, like any parameter related to minimization strategy. The first time we perform a CP run after a minimization, it is always better to put velocities equal to zero, unless we have velocities, from a previous simulation, to specify in the input file. Restore the proper masses for the ions. In this way we will sample the microcanonical ensemble. The input section changes as follow:
&electrons
emass = 400.d0,
emass_cutoff = 2.5d0,
electron_dynamics = 'verlet',
electron_velocities = 'zero'
/
&ions
ion_dynamics = 'verlet',
ion_velocities = 'zero'
/
ATOMIC_SPECIES
C 12.0d0 c_blyp_gia.pp
H 1.00d0 h.ps
If you want to specify the initial velocities for ions, you have to set ion_velocities ='from_input', and add the IONIC_VELOCITIES card, after the ATOMIC_POSITION card, with the list of velocities in atomic units.
NOTA BENE: in restarting the dynamics after the first CP run, remember to remove or comment the velocities parameters:
&electrons
emass = 400.d0,
emass_cutoff = 2.5d0,
electron_dynamics = 'verlet'
! electron_velocities = 'zero'
/
&ions
ion_dynamics = 'verlet'
! ion_velocities = 'zero'
/
otherwise you will quench the system interrupting the sampling of the
microcanonical ensemble.
It is possible to change the temperature of the system or to sample the canonical ensemble fixing the average temperature, this is done using the Nosé thermostat. To activate this thermostat for ions you have to specify in namelist &IONS:
&ions
ion_dynamics = 'verlet',
ion_temperature = 'nose',
fnosep = 60.0,
tempw = 300.0
/
where fnosep is the frequency of the thermostat in THz, that should be
chosen to be comparable with the center of the vibrational spectrum of
the system, in order to excite as many vibrational modes as possible.
tempw is the desired average temperature in Kelvin.
Note: to avoid a strong coupling between the Nosé thermostat and the system, proceed step by step. Don't switch on the thermostat from a completely relaxed configuration: adding a random displacement is strongly recommended. Check which is the average temperature via a few steps of a microcanonical simulation. Don't increase the temperature too much. Finally switch on the thermostat. In the case of molecular system, different modes have to be thermalized: it is better to use a chain of thermostat or equivalently running different simulations with different frequencies.
It is possible to specify also the thermostat for the electrons. This is usually activated in metals or in systems where we have a transfer of energy between ionic and electronic degrees of freedom. Beware: the usage of electronic thermostats is quite delicate. The following information comes from K. Kudin:
''The main issue is that there is usually some "natural" fictitious kinetic energy that electrons gain from the ionic motion ("drag"). One could easily quantify how much of the fictitious energy comes from this drag by doing a CP run, then a couple of CG (same as BO) steps, and then going back to CP. The fictitious electronic energy at the last CP restart will be purely due to the drag effect.''
''The thermostat on electrons will either try to overexcite the otherwise "cold" electrons, or it will try to take them down to an unnaturally cold state where their fictitious kinetic energy is even below what would be just due pure drag. Neither of this is good.''
''I think the only workable regime with an electronic thermostat is a mild overexcitation of the electrons, however, to do this one will need to know rather precisely what is the fictitious kinetic energy due to the drag.''