<div>Hi,</div><div><br></div><div>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).</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div><a href="https://groups.google.com/g/cp2k/c/VmDctR1A1Jg">This post</a> suggests that I'm not computing the forces accurately enough, so I went back to check grid convergence for the first step. The total <a href="https://www.cp2k.org/faq:cutoff">charge density</a> is on the order 10^-8 and I'm unable to <a href="https://www.cp2k.org/howto:converging_cutoff">converge the cutoff</a> 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.</div><div><br></div><div>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?</div><div><br></div><div>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.</div><div><br></div><div>Here is the &DFT input section:</div><span style="font-family: Courier New;"> &DFT<br> BASIS_SET_FILE_NAME BASIS_MOLOPT<br> POTENTIAL_FILE_NAME GTH_POTENTIALS<br> MULTIPLICITY 1<br> CHARGE 0<br> &SCF<br> SCF_GUESS ATOMIC<br> MAX_SCF 50<br> EPS_SCF 1.0E-6<br> &OT<br> PRECONDITIONER FULL_SINGLE_INVERSE<br> MINIMIZER DIIS<br></span><div><span style="font-family: Courier New;"> &END OT<br></span></div><div><span style="font-family: Courier New;"> &OUTER_SCF<br> MAX_SCF 10<br> EPS_SCF 1.0E-6<br> &END</span></div><div><span style="font-family: Courier New;"></span></div><span style="font-family: Courier New;"> &PRINT<br> &RESTART OFF<br> &END<br> &END<br> &END SCF</span><div><span style="font-family: Courier New;"> &QS<br> EPS_DEFAULT 1.0000000000000000E-10<br> EXTRAPOLATION ASPC<br> METHOD GPW<br> &END QS<br> &MGRID<br> CUTOFF 400<br> NGRIDS 5<br> REL_CUTOFF 30.0<br> &END<br> &XC<br> DENSITY_CUTOFF 1.0000000000000000E-10<br> GRADIENT_CUTOFF 1.0000000000000000E-10<br> TAU_CUTOFF 1.0000000000000000E-10<br> &XC_GRID<br> XC_SMOOTH_RHO NN10<br> XC_DERIV NN10_SMOOTH<br> &END XC_GRID<br> &XC_FUNCTIONAL NO_SHORTCUT<br> &GGA_X_PBE_R T<br> &END GGA_X_PBE_R<br> &GGA_C_PBE T<br> &END GGA_C_PBE<br> &END XC_FUNCTIONAL<br> &VDW_POTENTIAL<br> POTENTIAL_TYPE PAIR_POTENTIAL<br> &PAIR_POTENTIAL<br> R_CUTOFF 1.6000000000000000E+01<br> TYPE DFTD3<br> PARAMETER_FILE_NAME dftd3.dat<br> REFERENCE_FUNCTIONAL PBE<br> &END PAIR_POTENTIAL<br> &END VDW_POTENTIAL<br> &END XC<br> &POISSON<br> PERIODIC XYZ<br> &END POISSON<br> &END DFT</span></div><div><br></div><div>Thanks,</div><div>Daniel<br></div>
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