Thank you sooooo much! I am learning a lot here! Lily
On Tue, May 17, 2011 at 2:46 PM, Emilio Artacho <[email protected]> wrote: > > On 17 May 2011, at 19:41, lily zheng wrote: > > Just got one general question about structure optimization. Usually we > need provide initial(wild) guess about coordinates of atoms in one > systems. > could the structure optimization procedure make those wildly-guessed > coordinates reasonable? > > Thanks > > Lily > > > > That would be fantastic. Unfortunately not. Or at least not necessarily. > > The relaxation techniques built in in siesta as such (conjugate gradients, > Broyden variants) are local relaxations, as opposed to global ones, > meaning that siesta will find the atomic positions given by > a local minimum of the energy, in particular, and most probably, > the one your initial position is in the basin of. > > If your global energy minimum is in a different basin of the > energy landscape (or valley or whatever way you prefer to call it) > it won't be found by those algorithms: it would need to cross energy > barriers (and the right ones) in order to get there, and there is no > easy way to do that. This happens for instance if your initial structure > is making wrong bonds among its atoms (you'll never get to the > graphite structure if starting from diamond just by relaxing). > > A traditional way of trying to address that problem is simulated annealing, > but it is extremely costly. > > More modern approaches: > - ex nihilo structure prediction: see recent papers by > Chris Pickard and Richard Needs > - genetic algorithms: recent re-discovery by Artem Oganov and co. > > Otherwise you have to guess a good initial structure, with the right > bonds, coordinations, connectivity ... > The credibility of your answer depends on how good your guess was. > > See J. Maddox, Nature 335 201 > (1988)<http://www.nature.com/nature/journal/v335/n6187/pdf/335201a0.pdf> > . > > Emilio > > -- > Emilio Artacho > > Department of Earth Sciences, University of Cambridge > Downing Street, Cambridge CB2 3EQ, UK > Tel. (+44/0) 1223 333480, Fax (+44/0) 1223 333450 > [email protected], http://www.esc.cam.ac.uk/~emilio > > > >
