Dear Bedamani, FC is organized as follows (following the first title line): - the external loop over atoms which undergo finite displacements, i.e., from MD.FCfirst to MD.FClast (or all atoms in the cell by default); - for each atom, 6 displacements are applied in the order: -X,X,-Y,Y,-Z,Z ; - for each displacement, the forces induced on ALL atoms in the cell are written, line by line, divided by magnitude of displecement, three Cartesian coordinates in a line. What is actually written is -(force)/(displacement) in units of eV/(Ang^2)
Best regards Andrei Postnikov > Dear siesta users, > > We are trying to use the force-constant output from SIESTA for a > 2-atom > (2-dimensions) crystal (graphene) as input into our program > which requires a force-constant matrix in Cartesian > coordinates. The required full matrix would be 6x6 ([basicaly 3n*3n, for > a n atom system], > with elements at the head Atom1x, Atom1y, Atom1z,Atom2x, Atom2y, Atom2z. > For SIESTA output from graphene we are getting 72 > total elements which do not seem to be arranged in this order. > Can you tell us what the elements in the .FC file are, and how we could > transform to the needed Cartesian matrix?" > > The input files I haved used in siesta calculation is attached as well > as > the > output force-constant *.FC file. > We have used the executables fcbuild, siesta and vibrator included > in > the Util directory of > siesta-3.1 software package.First, we run > $ fcbuild < graphene.fdf > this builds a file FC.fdf which is a partial input for Siesta. Then, We > run > $ siesta < graphene-siesta.fdf > Siesta provides the file graphene.FC with the force constant > matrix. > > > Please help me. > best regards > bedamani >
