[Pw_forum] dt parameter

Paolo Giannozzi giannozz at nest.sns.it
Tue Mar 21 17:36:30 CET 2006

On Friday 17 March 2006 09:59, Dmitry Korotin wrote:

> can anyone describe me the meaning of dt parameter (and give some
> advices in choosing it's value) in detail? (I'm going to do vc-relax
> calculation) Or, may be, there are some references...

the following is in the CVS version of the users' guide:
A common mistake many new users make is to set the time step
dt inproperly to the same order of magnitude as for CP
algorithm, or not setting dt at all. This will produce
a "not evolving dynamics". Good values for the original RMW
(RM Wentzcovitch) dynamics are dt=50 to 70.

The choice of the cell mass is a delicate matter. An off-optimal mass
will make convergence slower. Too small masses, as well as too long time
steps, can make the algorithm unstable. A good cell mass will make the
oscillation times for internal degrees of freedom comparable to
cell degrees of freedom in non-damped Variable-Cell MD. Test 
calculations are advisable before extensive calculation.
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy

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