# [lammps-users] Tilt factor for a triclinic box

Dear Lammpser,

I am trying to use a triclinic box in Lammps. The size values for the box are:

a = 9.838049
b = 9.838049
c = 15.00000
alpha = 90.0
beta = 90.0
gamma = 60.0

The corresponding settings in the data file are:

0.000000 9.838049 xlo xhi
0.000000 8.520000 ylo yhi
0.000000 15.000000 zlo zhi
4.919020 0.000000 0.000000 xy xz yz

However, when I check the dump file, it says:

ITEM: BOX BOUNDS xy xz yz
0 14.7571 4.91902
0 8.52 0
0 15 0

I can not understand the value in for the X bounds, “14.7571”. I would expect “9.838049” here, but why Lammps does something and adds “9.838049+4.919020(which is the tile factor of xy” and uses that as the X bounds?

Could someone shot me an explanation?

Liangliang

Dear Lammpser,
I am trying to use a triclinic box in Lammps. The size values for the box
are:
a = 9.838049
b = 9.838049
c = 15.00000
alpha = 90.0
beta = 90.0
gamma = 60.0
The corresponding settings in the data file are:
0.000000 9.838049 xlo xhi
0.000000 8.520000 ylo yhi
0.000000 15.000000 zlo zhi
4.919020 0.000000 0.000000 xy xz yz
However, when I check the dump file, it says:
ITEM: BOX BOUNDS xy xz yz
0 14.7571 4.91902
0 8.52 0
0 15 0
I can not understand the value in for the X bounds, "14.7571". I would
expect "9.838049" here, but why Lammps does something and adds
"9.838049+4.919020(which is the tile factor of xy" and uses that as the X
bounds?
Could someone shot me an explanation?

the dump file has the boundaries in x, y, and z.
for x and y those are not identical to the lo-hi values.

details are in the documentation.

axel.

the dump file has the boundaries in x, y, and z.
for x and y those are not identical to the lo-hi values.

To wit, what is written to the dump file is a bounding box,
not the 6 triclinic params. See the dump doc page and
section 4.12 of the doc pages for how to convert back
and forth.

Steve

Dear Axel and Steve,

Thanks for the help. It has been welll documented at http://lammps.sandia.gov/doc/Section_howto.html#4_12, I just did not notice that.

Basically, the parameters in dump file are calculated from the following equations:

``````xlo_bound = xlo + MIN(0.0,xy,xz,xy+xz)
xhi_bound = xhi + MAX(0.0,xy,xz,xy+xz)
ylo_bound = ylo + MIN(0.0,yz)
yhi_bound = yhi + MAX(0.0,yz)
zlo_bound = zlo
zhi_bound = zhi
``````

I have not seen the advantages in doing that, but I double checked my inputs and they agree with my expectation.

Thanks again.

Liangliang

The advantage for the dump file is that various visualizers like to have
a bounding box around the entire system, so they know how to
scale the image.

Steve