[QE-users] NVE simulation in cp.x - large fluctuation in temperature

[Jie Peng]

Dear all

I have been running MD simulations on HfS2 using cp.x code in Quantum
espresso. I start from initial configuration obtained from pwscf vc-relax,
and relax the system using cp.x by consecutive steps of: electron
relaxation->ionic relaxation->cell relaxation. Then, I just directly start
a NVE simulation starting from the equilibrium configuration. I expect the
system to almost stay stationary or the temperature should be very small
since I am allowing dynamics in a system that is already in equilibrium.
However, what I see is a huge fluctuation in the *tmpp* output of cp.x, as
I attach a figure showing variation of tmpp (Ionic temperature) with
simulation time

I did this because it is suggested in the user guide you should apply an
initial displacement to the atoms in your system after the relaxation since
otherwise there will not be any dynamics. But what I see here is a large
fluctuation of the system temperature.

The thinking or questions here are

1.Does the tmpp represents the physical temperature of the system here? I
think it should be since it is the temperature corresponding to kinetic
energy of the ions.

2.It above point is true, why is the temperature varying so fiercely? Am I
setting incorrect parameters, for instance the timestep or the fictitious
mass? But I took those from previous simulation steps where I did the
relaxation, and they all worked well since they successfully drived my
system to equilibrium, satisfying the convergence threshold on total
energy, forces acting on atoms, and the fictitious electron kinetic energy.
I am confused at this point.

The input file for NVE simulation is attached here:

*&control*
*    calculation='cp',*
*    title='Halfnium disulfide'*
*    restart_mode='restart',*
*    ndr=53,*
*    ndw=54,*
*    nstep=50000,*
*    iprint=10*
*    isave=100,*
*    tstress = .true.*
*    tprnfor = .true.*
*    dt=10,*
*    wf_collect=.true.*
*    etot_conv_thr=1e-6*
*    forc_conv_thr=1e-3*
*    ekin_conv_thr=1e-5*
*    prefix='HfS2',*
*    pseudo_dir='/home/jpeng/HfS2/potential'*
*    outdir='./tmp/',*
* /*
* &system*
*    ibrav= 4,*
*    a=3.6529*
*   c=5.6544*
*    nat=  3, ntyp= 2,*
*    ecutwfc =50*
*    vdw_corr='DFT-D',*
* !   lspinorb=.true.*
* !   noncolin=.true.*
* !   ecutrho=300*
* !   nbnd=14*
*!    occupations='smearing'*
*!    smearing='gaussian'*
*!   degauss=0.01*
* !  nspin=2*
* !   starting_magnetization(1)=0.1*
*! Hf  95.94  Hf.pbe-mt_fhi.UPF*
*! S  32.065  S.pbe-mt_fhi.UPF*
*/*
* &electrons*
*    electron_dynamics='verlet'*
*    electron_velocities='zero'*
*    emass=400*
*    emass_cutoff=1*
*/*
* &ions*
*    ion_dynamics = 'verlet'*
*    ion_damping=0.1*
*!    ion_nstepe=10*
* /*
* &cell*
*    cell_dynamics = 'none'*

*/*
*ATOMIC_SPECIES*
* Hf  95.94  Hf.pbe-mt_fhi.UPF*
* S  32.065  S.pbe-mt_fhi.UPF*
*ATOMIC_POSITIONS (crystal)*
*Hf      -0.000000000  -0.000000000  -0.000000000*
*S        0.666666667   0.333333333   0.257234636*
*S        0.333333333   0.666666667  -0.257234636*

Anyone could help me on it? Thank you very much.

Best
Jie
-- 
------------------------------------------------------------------------------------------------------------------------
Jie Peng
PhD student
2134 Glenn Martin Hall, Mechanical Engineering, University of Maryland
College Park, Maryland, USA
Phone:(+1) 240-495-9445
Email: jiepeng at umd.edu
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