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Subsections

5.3 CP dynamics

At this point after having minimized the electrons, and with ions dis- placed 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.

IMPORTANT: 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.

5.3.0.1 Varying the temperature

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 the ions input section:

           &ions
              ion_dynamics = 'verletâ',
              ion_temperature = 'nose',
              fnosep = 60.0,
              tempw = 300.0
           /
where fnosep is the frequency of the thermostat in THz, thatishould 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.

5.3.0.2 Nose thermostat for electrons

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 fictititious kinetic energy due to the drag.''

5.3.0.3 Self-interaction Correction

The self-interaction correction (SIC) included in the CP package is based on the Constrained Local-Spin-Density approach proposed my F. Mauri and coworkers (M. D'Avezac et al. PRB 71, 205210 (2005)). It was used for the first time in QUANTUM ESPRESSO by F. Baletto, C. Cavazzoni and S.Scandolo (PRL 95, 176801 (2005)).

This approach is a simple and nice way to treat ONE, and only one, excess charge (EC). It is moreover necessary to check a priori that the spin-up and spin-down eigenvalues are not too different, for the corresponding neutral system. working in the Local-Spin-Density Approximation (setting nspin = 2). If these two conditions are satisfied and you are interest in charged systems, you can apply the SIC. This approach is a on-the-fly method to correct the self-interaction with the excess charge with itself.

Briefly, both the Hartree and the exchange-correlation part have been corrected to avoid the interaction of the EC with tself.

For example, for the Boron atoms, where we have an even number of electrons (valence electrons = 3), the parameters for working with the SIC are:

           &system
           nbnd= 2,
           nelec= 3,
           nelup = 2,
           neldw = 1,
           sic_alpha = 1.d0,
           sic_epsilon = 1.0d0,
           sic = 'sic_mac',
           force_pairing = .true.,

           &ions
           ion_dynamics = 'none',
           ion_radius(1) = 0.8d0,
           sic_rloc = 1.0,

           ATOMIC_POSITIONS (bohr)
           B 0.00 0.00 0.00 0 0 0 1
The two main parameters are:
"force_pairing = .true.", which forces the paired electrons to be the same;
"sic='sic_mac'," which instructs the code to use Mauri's correction.
Remember to add an extra-column in ATOMIC_POSITIONS with "1" to activate SIC for those atoms.

Warning: This approach has known problems for dissociation mechanism driven by excess electrons.

Comment 1: Two parameters, ''sic_alpha'' and ''sic_epsilon'', have been introduced following the suggestion of M. Sprik (ICR(05)) to treat the radical (OH)-H2 O. In any case, a complete ab-initio approach is followed using ''sic_alpha=sic_epsilon=1''.

Comment 2: When you apply this SIC scheme to a molecule or to an atom, which are neutral, remember to add the correction to the energy level as proposed by Landau: in a neutral system, subtracting the self-interaction, the unpaired electron feels a charged system, even if using a compensating positive background. For a cubic box, the correction term due to the Madelung energy is approx. given by 1.4186/Lbox -1.047/(Lbox)3 , where Lbox is the linear dimension of your box (=celldm(1)). The Madelung coefficient is taken from I. Dabo et al. PRB 77, 115139 (2007).

(info by F. Baletto, francesca.baletto@kcl.ac.uk)


next up previous contents
Next: 5.4 Variable-cell MD Up: 5 Using CP Previous: 5.2 Relax the system Contents
Paolo Giannozzi 2009-10-01