mirror of https://gitlab.com/QEF/q-e.git
123 lines
5.5 KiB
Plaintext
123 lines
5.5 KiB
Plaintext
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*** How to set up the parameters for minimization/dynamics ***
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Variable Meaning Typical Range
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DELTAT Time Step (au) 1 - 20
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EMASS Electron Mass (au) 100 - 1000
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EMAEC Preconditioning Mass 2 - 3
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FRICE Electronic Friction 0.2- 0.02 (minim. only)
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FRICP Ionic Friction 1/10 of FRICE (minim. only)
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QNP Ionic thermostat mass
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TEMPW Desired Ionic temperature (you set it)
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QNE Electronic thermostat mass
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EKINCW Desired Electronic kin.energy <0.001/atom ?
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*** Electronic minimization at fixed ions
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Performed if TFOR=.false. TNOSEP=.false. TNOSEE=.false. FRICE>0
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Requires: DELTAT EMASS EMAEC FRICE
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The parameter that determines the evolution of the electronic wave
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functions is Lambda=(DELTAT**2/EMASS). Its maximum value is fixed
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by the requirements that
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- iterative orthonormalization works (it will not if Lambda is too big)
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- the minimization works (as above).
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If this does not happen, DELTAT must be reduced, or EMASS increased.
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For the very first (10-20) steps, Lambda should be small, EMAEC should
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be set to a large value (large wrt ECUTW) so that preconditioning is
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effectively turned off; FRICE should be set to a large value (up to
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FRICE=1, corresponding to steepest descent).
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EMASS should be assigned a value that is good for subsequent ionic
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minimization/dynamics (see below; typical values 300-700).
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DELTAT should be given a value that produces a working minimization
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(typical values 5 or less).
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For subsequent steps, EMAEC should be set to 2 or 3 (an optimal value
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can be determined by trial and error: the best value is the one that
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allows the larger time step).
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FRICE should be set to 0.2-0.1, and reduced if the minimization
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becomes sluggish.
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DELTAT should be given the largest value that produces a working
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minimization (typical values 10-15, max. up to 20).
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*** Ionic minimization
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Performed if TFOR=.true. TNOSEP=.false. TNOSEE=.false. FRICE or FRICP>0
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Requires: DELTAT EMASS EMAEC FRICE FRICP
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As above, with FRICP about 1/10 of FRICE. If the minimization becomes
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sluggish, reducing FRICP to < 0.01 (in some cases even setting it to
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zero) usually help. Also, setting ionic masses to the same value
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usually helps. See below for bounds on DELTAT and EMASS.
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*** Dynamics with electrons and ions
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Performed if TFOR=.true. TNOSEP=.false. TNOSEE=.false. FRICE=0 FRICP=0
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Requires: DELTAT EMASS EMAEC
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In the combined electronic/ionic dynamics, one has to chose DELTAT
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as big as possible, but ensuring that
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- energy is conserved during the dynamics
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- the dynamics is adiabatic: there is no transfer of energy between
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electronic and ionic degrees of freedom. The adiabaticity is better
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conserved in systems with a large electronic energy gap.
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A typical time step for classical MD is given by 1/50 to 1/100 of the
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inverse of the maximum ionic frequency (1/10 is an upper limit). As
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an example: Si, omega_0=15 THz, 1/omega_0=60 fs, time step=1 fs or so.
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We assume here that the ionic masses are set to their physical values.
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The typical DELTAT for Car-Parrinello MD is a few times smaller than
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the time step for a classical MD (in the example above, 0.2-0.3 fs,
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or DELTAT=~10).
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Once DELTAT is fixed, a suitable value for EMASS is determined by
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the above conditions. If iterative orthonormalization gives problem
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or the energy is not conserved, reduce DELTAT or try a different value
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of EMAEC for optimal preconditioning. If there is energy transfer,
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reducing EMASS will give a more adiabatic behavior.
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*** Nose' thermostats
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If a simulation on the canonical ensemble - system in contact with
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a thermal bath at temperature T - is desired, one has to use a Nose'
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thermostat on ions, setting TNOSEP=.true., TEMPW=T desired (Kelvin).
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The kinetic energy will make wide oscillations [***how wide???]
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around the mean value gKbT/2, where g=number of ionic degrees of
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freedom, Kb=Boltzmann constant.
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The value of QNP must be set so that the frequency of such oscillations
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is of the same order of magnitude of the typical ionic frequencies of
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the systems, so as to maximize the interaction of the thermostat with
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the ionic degrees of freedom. This can be estimated by inspection or
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from the approximate formula: QNP=2*g*Kb/Omega^2, where Omega is the
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typical ionic frequency (for instance, the Debye frequency, or half
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the maximum phonon frequency). The value of QNP can also be estimated
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on a small system and extrapolated to a larger cell.
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In low-gap or zero-gap systems, it may be needed to have a Nose'
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thermostat on both ions and electrons (TNOSEE=.true.) in order
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to obtain an adiabatic dynamics.
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EKINCW must be set to approx. twice the value of the adiabatic
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contribution to the electronic kinetic energy Ekad (that is:
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the part of the electronic kinetic energy that comes from the
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adiabatic motion of electrons following the atomic motion).
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Assuming that all atoms have mass M, the following estimate hold:
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Ekad=2KbT*EMASS/M*Ekin , where Ekin=electronic kinetic energy.
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Ekad can also be estimated from the energy gained by minimizing wrt
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electronic degrees of freedom, starting from a given ionic and
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electronic configuration found during the dynamics.
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QNE must be set to a value such that the associate frequency
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Omega=sqrt(4Ekin/QNE) is larger than the highest phonon.
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*** Relevant Theory
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Adiabaticity in CPMD:
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G. Pastore, F. Buda, M. Smargiassi, PRA 44, 6334 (1991)
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Nose' Thermostats:
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P. E. Bloechl and M. Parrinello, PRB 45, 9413 (1992)
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Second-order damped dynamics, preconditioning:
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F. Tassone, F. Mauri, R. Car, PRB 50, 10561 (1994).
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