On Thu, 4 Jan 2007, Lawrence Lee wrote: LL> Dear sir / madam,
hi lawrence, LL> I am a new user of pwscf. I know that pw can do molecular dynamics. LL> What is the difference between that and CPMD? please be a bit more precise. do you refer to the CPMD package or the CPMD method (as implemented in cp.x )? in the first case you have two packages that implement first principles calculations using DFT and plane wave basis sets with pseudopotentials. due to differences in the interests of the developers, those two packages (quantum espresso and CPMD) have different strenghts and weaknesses. those can be mostly deduced from looking at the descriptions of the input file syntax (plus a few undocumented features, for which you have to read the source code ;-) ). in the second case you have to distinguish between the born-oppenheimer MD in pw.x, where the ground state wavefunction is fully computed in each MD step, and car-parrinello MD in cp.x where, starting from a ground state wavefunction, the wavefunction for subsequent steps is propagated along the trajectory of the atoms by means of an extenden lagrangian scheme (i.e. it is not exactly the ground state wavefunction, but 'oszillates around it'). strictly speaking, CP-dynamics are also using the born-oppenheimer approximation, but BO-MD vs CP-MD is the established nomenclature. historically, CP-MD used to be much more efficient than BO-MD, even though you have to use an about 5 times shorter time step, but more efficient diagonalization and extrapolation and the availability of more cpu power make BO-MD competitive. on top of that, CP-dynamics put a number of restrictions on the system you want to investigate (it needs a band gap, your atom dynamics is modified by the fictitious electron dynamics). LL> Besides, do you know how to restart the run in pw.x? For example, if LL> I have made a run of 400 steps with dt = 20 atomic time unit, but LL> now I find it insufficient. I need the run to have 2000 steps, say. LL> Do I have any method to proceed the run without doing it from the LL> first step again? have you tried using restart_mode='restart' instead of 'from_scratch' ?? axel. LL> LL> Lawrence. LL> LL> _______________________________________ LL> YM - ???u?T?? LL> ?N???A?S???W???A?A???B?????i?H?d?U?T?????A?A???A?W?????N?????Y?????A???????????N?????C LL> http://messenger.yahoo.com.hk -- ======================================================================= Axel Kohlmeyer akohlmey at cmm.chem.upenn.edu http://www.cmm.upenn.edu Center for Molecular Modeling -- University of Pennsylvania Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323 tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425 ======================================================================= If you make something idiot-proof, the universe creates a better idiot.
