# lattice constant

Dear users
I want simulate a paper about MD.the author wrote"molecules arranged as a FCC lattice" and lattice constant is equal 2.774 Angstrom
I use next script in lammps:
lattice fcc 2.774

when i create atom,the number of atoms i created is more than number of atoms of paper therefore the magnitude of density that i calculated is not equal with density of paper
why?

Dear users
I want simulate a paper about MD.the author wrote"molecules arranged as
a FCC lattice" and lattice constant is equal 2.774 Angstrom
I use next script in lammps:
lattice fcc 2.774

when i create atom,the number of atoms i created is more than number of
atoms of paper therefore the magnitude of density that i calculated is not
equal with density of paper
why?

impossible to say with this limited information. perhaps you made a
mistake in the other commands in your input, e.g. how you defined the
size of the simulation box.

axel.

Note that the 2.774 LAMMPS uses for
the lattice constant is the cubic lattice
constant, and for fcc there are 4 atoms
per cubic cell. I.e. it is not the primitive
unit cell for fcc (which is not cubic).

Steve

There are many problems with what you are doing. Mainly, your script is far to complicated. In computer programming, and each LAMMPS script is a program, you should proceed one step at a time. So, first write a script that constructs a block of platinum with the correct structure. One you can do that, you will be in a much better position to solve the other problems. I will also point out a specific error. You mention 3 different lattice constants, none of which is commensurate with your box dimensions, so you are guaranteed to get extra layers of atoms.

[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin

b = [58.3,38.5,72.2]
for l in b:
… print l/2.774

21.0165825523
13.8788752704
26.0273972603
for l in b:
… print l/5.95

9.79831932773
6.47058823529
12.1344537815
for l in b:
… print l/2.776

21.0014409222
13.8688760807
26.0086455331