[Pw_forum] cp.x stops exceeding the maximum number of iteration

[Rolly Ng]

Dear QE users,

I am running MD with cp.x and I am trying to observe the trajectory of a 
molecule detaching from nickel-doped graphene surface.

It started with the molecule attached to the nickel atom in graphene 
sheet, electron minimization and ionic minimization in cp.x completed 
successfully. I then added a small random displacement to the system. 
When this is done. I added the followings to my input files, tempw 
increased from 100K to 200K then 300K. I have set dt=1.0d0, which is the 
smallest time step I can get.

&ELECTRONS
emass = 400.d0 ,
emass_cutoff = 2.5d0 ,
electron_dynamics = 'verlet' ,
/

&IONS
ion_dynamics = 'verlet' ,
ion_temperature = 'nose' ,
fnosep = 40.0 ,
tempw = 100.0 ,
/

Each temperature was set to run for 10,000 time steps (100K, followed by 
200K, then 300K with 'restart') which I believe equals to 0.24 fs. This 
is insufficient to obtain the trajectory, so I extended the 300K case to 
50,000 time steps to get to 1.2 ps.

But, cp.x failed at middle at about 18,000 time step, complaining 
maximum number of iteration is reached. I took a look at the .evp file 
and I saw the very last three lines produced strange numbers.

#   nfi    time(ps)        ekinc        T_cell(K) Tion(K)          
etot               enthal econs               econt          
Volume        Pressure(GPa)
   17920  4.334641E-01  4.408245E-01  0.000000E+00 2.906500E+02       
-736.06724803       -736.06724803 -735.93194407       -665.53445700  
3.183024E+04       0.82843
   17930  4.337060E-01  4.413418E-01  0.000000E+00 2.931947E+02       
-736.06864256       -736.06864256 -735.93215404       -665.53398181  
3.183024E+04       0.80824
   17940  4.339478E-01  4.415645E-01  0.000000E+00 2.960657E+02       
-736.07019548       -736.07019548 -735.93237041       -665.53381758  
3.183024E+04       0.79642
   17950  4.341897E-01  4.415359E-01  0.000000E+00 2.998132E+02       
-736.07295464       -736.07295464 -735.93338506       -665.53470772  
3.183024E+04       0.86277
   17960  4.344316E-01  4.419086E-01  0.000000E+00 3.037218E+02       
-736.07463932       -736.07463932 -735.93325017       -665.53404513  
3.183024E+04       1.19960
   17970  4.346735E-01  4.442741E-01  0.000000E+00 3.066442E+02       
-736.07570387       -736.07570387 -735.93295428       -665.53122014  
3.183024E+04       3.46370
   17980  4.349154E-01  5.721941E-01  0.000000E+00 3.111465E+02       
-736.07787056       -736.07787056 -735.93302506       -665.40319056  
3.183024E+04      22.55459

It seems the ekinc and pressure increased suddenly and the system became 
destabilized.

How can I resolved the problem?

Thanks,
Rolly

-- 
PhD. Research Fellow,
Dept. of Physics & Materials Science,
City University of Hong Kong
Tel: +852 3442 4000
Fax: +852 3442 0538