concerning POSCAR file of organic crystal

[kjt0314]

I'm dealing with organic crystal
(e.g. Quarterthiophene of alpha-4T/LT phase, Z=4)

I have three questions(I've already read the workshop tutorial)

1) What does the scaling parameter mean?
How can I set the scaling parameter(2nd line) for the organic crystal?
And is "the only one scaling parameter" allowed as in the workshop examples?
When is it becomes equal to, less than, or larger than 1.0?

2) What does the lattice vector sections mean?
How can I determine the size of each lattice vector(3rd to 5th line) for the organice crystal?
In case of H2O molecule, what does it affect to change the values(15 0 0, 0 15 0, 0 0 15). In other words, why is it 15?

3) The organic crystal has no atoms on the crystal lattice site represented as 0 0 0 in position section(9th line).
How can I deal with this problem?
Is it problematic to remove the 0 0 0 line due to having no atoms

I'm a rookie to vasp and sorry for the basic questions.
But, it is hard to find solutions in google and this forum.
I hope your reply which would be precious to me.
Thank you in advance!

[forsdan]

1. The use of the scaling factor is explain in the manual, see

http://cms.mpi.univie.ac.at/vasp/vasp/node63.html

In direct coordinates it scales the supplied lattice vectors, while in carteesian coordinates it scales the supplied coordinates as well. The scaling factor is just a convinence when scaling the lattice vectors, but you can just as well set it to 1 and scale the lattice vectors (and the carteesian coodinates if you have chosen this input mode) by hand at the corresponding lines. It's entirely up to you.

2. The sizes and directions of the lattice vectors are determined by the crystal structure. However, in the case of a molecule you can put it in a cubic box of sufficient size. The length of the lattice vectors should then be choosen sufficiently large so that you avoid artifical interaction between the molecule and the molecules in the neighbouring cells due to the periodic boundary conditions. In the example you're looking at I guess 15 Ã… was an appropriate choice for that molecule.

3. You will always have atoms in your unit cell, so simply supply the coordinates for your atoms either in direct or carteesian coordinates (see the above link). You don't have to put an atom at (0,0,0) if you don't want to. Just put it somewhere else in the unit cell if you prefer that instead.

Cheers,
/Dan
<span class='smallblacktext'>[ Edited Wed Mar 04 2009, 10:51AM ]</span>