Dear lammps users,
using the 'fix nvt' and 'fix npt' commands, the NVT ensembel and NPT ensemble can be simulated. In these commands, there are Tstart and Tstop options. It is difficult for me to understand these options. In my understanding, ex, the temperature must be changed from Tstart to Tstop, and after some time, the temperature must be keep around the Tstop, due to the NVT ensembele. Is my understanding right?
Best,
Chol-Jun
Both NVT and NPT require a target temperature to equilibrate to.
Tstart and Tstop simply let you change the target T over the
course of a run. If you set Tstart = Tstop, it's just like picking
a single target T.
Steve
Dear Steve,
Steve Plimpton wrote:
Both NVT and NPT require a target temperature to equilibrate to.
Tstart and Tstop simply let you change the target T over the
course of a run. If you set Tstart = Tstop, it's just like picking
a single target T.
Yes, I thought so. For example, to simulate the NVT ensemble at T=300 K, we have to set Tstart=Tstop=300. Then after enough long time step, the temperature of the system should be kept around 300 K.
However, in my solid simulation attached below, such temperature keeping at the 300 K is not observed. What do I mistake?
units metal
boundary p p p
atom_style atomic
lattice bcc 3.00
region mybox block 0 10.0 0 10.0 0 10.0
create_box 2 mybox
create_atoms 2 region mybox basis 1 1 basis 2 2
pair_style eam/alloy
pair_coeff * * NiAl_Act.eam.alloy Al Ni
neighbor 0.3 bin
neigh_modify delay 10
fix 1 all nvt 300.0 300.0 100.0 drag 0.7
compute myTemp all temp
fix 2 all ave/time 10 10 100 compute myTemp type scalar file temp.stats ave window 20
compute myS all stress/atom
compute myE all pe/atom
thermo 10
thermo_style custom step temp
timestep 0.001
dump e1 all atom 10 dump.atom
log log.B2_NiAl
run 40000
Two things: first, you should initiialize your system to 300K.
Second, your time parameter in fix nvt is 100.0, meaining
100 psec, and you're only running for 40 psec. So you
either need to run a lot longer or choose a smaller time
parameter.
Steve