Temperature increase progressively with qeq method

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1

Hi, Lammps users,

I am trying to use the qeq method implemented in lammps after reading Rappa, and Rick work, i first tried the qeq/point with chi and J from Rappa’s work, a 100 ps NVT run followed by 200 ps NVE run where i dumped the trajectory. But the resulting temperature went a progressive increase. I checked the animation of the trajectory, no abnormal things happened. Then i tried qeq/dynamic and qeq/slater, again, the temperature is progressively increase. i have copy the lammps input file, and the para.qeq file, and the log file, any help about what the possible reason could be and what to track down are appreciated.

Best Regards

Jiasen Guo

1 4.168 6.974 0 0.38685 0.001
2 8.741 13.364 0 0.48725 0.001

lammps input script for Sodalite with MZHB

units metal
dimension 3
atom_style full

read_data MZHB_SOD_original.lmp

potential

bond_style harmonic
bond_coeff 1 11.650000 1.6200000

angle_style harmonic
angle_coeff 1 3.4000000 109.47000
angle_coeff 2 1.1100000 149.80000

pair_style lj/cut/coul/long 12.000
pair_coeff 1 1 0.0085997000 1.959982864 12.0000
pair_coeff 1 2 0.0052661900 1.758034219 12.0000
pair_coeff 2 2 0.0032249000 1.576892023 12.0000

special_bonds lj 1.0 1.0 1.0 coul 1.0 1.0 1.0

kspace_style ewald 1.0e-12

in.MZHB_SOD_md (1.79 KB)

para.qeq (62 Bytes)

qeq.log (309 KB)

2

Hi, Lammps users,

I am trying to use the qeq method implemented in lammps after reading Rappa,
and Rick work, i first tried the qeq/point with chi and J from Rappa's work,
a 100 ps NVT run followed by 200 ps NVE run where i dumped the trajectory.
But the resulting temperature went a progressive increase. I checked the
animation of the trajectory, no abnormal things happened. Then i tried
qeq/dynamic and qeq/slater, again, the temperature is progressively
increase. i have copy the lammps input file, and the para.qeq file, and the
log file, any help about what the possible reason could be and what to track
down are appreciated.

i would say the first thing to check out is how much energy
conservation is affected by the choice of time step.
i would expect that for fluctuating charges, you may need a shorter
time step compare to fixed charges.
i would also make a comparison of energy conservation between fixed
charges and fluctuating charges to have a reference.

axel.

3

Jiasen,

Qeq method, regardless of the qeq/point, qeq/slater, qeq/dynamic implementations, alters the equation of motion hence does not conserve total energy. Follow Axel’s advice and also dump the charges and examine them.

Ray