# Problem about Fix Heat using atom-style variable

Dear all,

My LAMMPS version is 10 Aug 2015.I try to use Fix Heat to simulate a laser pulse irradiated on a gold metal thin film. I use the following atom-style variable to define the laser pulse:

variable laser atom “((1.870348797*(exp((y-24.1088)*0.00621)))exp(-0.5((time-3150)/636.9426752)^2))”

and fix heat is used for simulating the laser irradiation:

fix 1 gold heat laser

However, I find the two systems I tested gives two very different final temperatures. The smaller thin film which sizes 1nm x 1nm x 1nm (108 atoms) gives about a double of the final temperature compared to the larger thin film which is 2nm x 1nm x 1nm in size (216 atoms).

In the lammps manual, it states that:
'If eflux is specified as an atom-style variable (see below), then the variable computes one value per atom. In this case, each value is the energy flux for a single atom, again in units of energy per unit time. In this case, each value is an “intensive” quantity, which need not be scaled with the number of atoms in the group.‘.

My question is: If the eflux is “intensive” when specified as an atom-style variable, it is supposed that it will scale up with the system size (no. of atoms)." Why does the final temperature between my two system can differ so much?

Any help would be appreciated. Thanks.

Best,
Samuel

Dear all,

My LAMMPS version is 10 Aug 2015.I try to use Fix Heat to simulate a laser
pulse irradiated on a gold metal thin film. I use the following atom-style
variable to define the laser pulse:

variable laser atom
"((1.870348797*(exp((y-24.1088)*0.00621)))*exp(-0.5*((time-3150)/636.9426752)^2))"

and fix heat is used for simulating the laser irradiation:

fix 1 gold heat laser

However, I find the two systems I tested gives two very different final
temperatures. The smaller thin film which sizes 1nm x 1nm x 1nm (108 atoms)
gives about a double of the final temperature compared to the larger thin
film which is 2nm x 1nm x 1nm in size (216 atoms).

In the lammps manual, it states that:
'If eflux is specified as an atom-style variable (see below), then the
variable computes one value per atom. In this case, each value is the energy
flux for a single atom, again in units of energy per unit time. In this
case, each value is an “intensive” quantity, which need not be scaled with
the number of atoms in the group.‘.

My question is: If the eflux is "intensive" when specified as an atom-style
variable, it is supposed that it will scale up with the system size (no. of
atoms)." Why does the final temperature between my two system can differ so
much?

you are not reading the documentation correctly. the energy
transferred is dependent on the atom style variable expression and is
*not* rescaled. the amount of energy transferred now depends on your
expression and the geometry of your system. you can retrieve the
global scaling factor from the scalar fix property and compare.

axel.