[CP2K-user] [CP2K:17137] COM Drift in Droplet Thermolysis

[Daniel Depew]

Hi,

I'm new to CP2K and am hoping to simulate droplet evaporation/thermolysis 
of ionic liquids. I had previously set up these droplets with classical MD 
and plenty of surrounding vacuum (~15-20 Angstroms on each side, sized for 
~1 MPa of pressure for ideal gaseous products).

Based on a literature review, I had set up a simulation with a revPBE-D3 
functional and standard DZVP-MOLOPT-SR-GTH basis set. I'll post the whole 
&DFT section below, but to summarize I used a grid cutoff of 400 Ry and SCF 
convergence of 1e-6. I also used XC smoothing (NN10) for the grid.

I heated the droplet up to 600K from an initial 300K using 0.5 ps 
simulations with steps of 50 K). The time step was 0.5 fs. At this point, I 
was hoping to do production runs, but I noticed that when I restarted at 
600 K, the initial temperature spiked to over 1800 K despite being ~600 K 
on the previous step. I traced this down to a nonzero center of mass 
velocity and the way that initial temperature is computed in md_vel_utils.F 
-- the COM velocity is subtracted on the initial time step when computing 
temperature, but it is not on subsequent steps.

This post <https://groups.google.com/g/cp2k/c/VmDctR1A1Jg> suggests that 
I'm not computing the forces accurately enough, so I went back to check 
grid convergence for the first step. The total charge density 
<https://www.cp2k.org/faq:cutoff> is on the order 10^-8 and I'm unable to converge 
the cutoff <https://www.cp2k.org/howto:converging_cutoff> even at 1000 Ry. 
Because of the system size, I'm hesitant to increase the computational cost 
much more. That post suggests either trying GAPW or simply subtracting the 
COM.

Before going any further, I wanted to consult your expertise. Is it 
primarily the lack of energy convergence driving the COM drift? Would 
higher temperatures (i.e. larger atomic displacements) tend to exacerbate 
this effect? Is this something encountered in bulk-phase simulations but 
gets washed out due to the periodic nature?

I should mention that the simulations I'm doing are primarily to analyze 
structure, reactivity, and the initial evaporation processes at the 
interface, so it's largely a qualitative study meant to supplement other 
work.

Here is the &DFT input section:
   &DFT
     BASIS_SET_FILE_NAME BASIS_MOLOPT
     POTENTIAL_FILE_NAME GTH_POTENTIALS
     MULTIPLICITY  1
     CHARGE  0
     &SCF
       SCF_GUESS ATOMIC
       MAX_SCF 50
       EPS_SCF 1.0E-6
       &OT
         PRECONDITIONER FULL_SINGLE_INVERSE
         MINIMIZER DIIS
       &END OT
       &OUTER_SCF
         MAX_SCF 10
         EPS_SCF 1.0E-6
       &END
       &PRINT
         &RESTART OFF
         &END
       &END
     &END SCF
     &QS
       EPS_DEFAULT     1.0000000000000000E-10
       EXTRAPOLATION  ASPC
       METHOD  GPW
     &END QS
     &MGRID
        CUTOFF 400
        NGRIDS 5
        REL_CUTOFF 30.0
     &END
     &XC
       DENSITY_CUTOFF     1.0000000000000000E-10
       GRADIENT_CUTOFF     1.0000000000000000E-10
       TAU_CUTOFF     1.0000000000000000E-10
       &XC_GRID
         XC_SMOOTH_RHO  NN10
         XC_DERIV  NN10_SMOOTH
       &END XC_GRID
       &XC_FUNCTIONAL  NO_SHORTCUT
         &GGA_X_PBE_R  T
         &END GGA_X_PBE_R
         &GGA_C_PBE  T
         &END GGA_C_PBE
       &END XC_FUNCTIONAL
       &VDW_POTENTIAL
         POTENTIAL_TYPE  PAIR_POTENTIAL
         &PAIR_POTENTIAL
           R_CUTOFF     1.6000000000000000E+01
           TYPE  DFTD3
           PARAMETER_FILE_NAME dftd3.dat
           REFERENCE_FUNCTIONAL PBE
         &END PAIR_POTENTIAL
       &END VDW_POTENTIAL
     &END XC
     &POISSON
       PERIODIC  XYZ
     &END POISSON
   &END DFT

Thanks,
Daniel

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