# questions regarding Monte Carlo simulations in CP2K

lar... at lbl.gov lar... at lbl.gov
Thu Jan 14 01:08:02 UTC 2016

```Dear all,

I have a few very basic questions regarding the Monte Carlo simulations in
CP2K that I hope more experienced users will be able to answer. I have gone
over most of the papers by McGrath and understand the theory associated to
the different MC methods. The capabilities and implementation in cp2k are
very impressive but I have questions regarding the practical use in cp2k:

1) *The concept of steps and cycles.* As far as I understand 1 cycle
corresponds to 1 energy evaluation (i.e. a SCF run when using DFT). During
a cycle cp2k attempts NMOVES sampled with probability PMHMC,etc.
1a) What does it mean to use LSTOP yes? (i.e. what do you gain from moving
from cycles to steps?)
1b) When I run a simulation setting NSTEP=100, in the output files (e.g.
mc_energies) the cycles go up to 185 (it stopped running due to time
limit). How's that possible?

2) *Output of the simulations.* In order to calculate the acceptance ratio
of the different moves I am guessing that the info can be found in the
output file inputname.inp.moves.
2a) What do the two columns in inputname.inp.moves mean?

3) *HMC simulations.* I want to use HCM as an efficient method to relax a
water box. For this purpose, I set PMHMC=0.5 and PMVOLUME=0.5, and add an
MD section to the input file (following the example hmc.inp included in the
cp2k distribution). This should, in theory, only allow HMC and volume
moves. However, when I look at the input.inp.moves file I find that the
columns of the entries bias_rot, rot, and Quickstep show non-zero numbers.
3a) What does Quickstep means as a move?
3b) Why is HMC all zeros?
3c) Why rot and bias_rot are not zero when they are moves that shouldn't be
attempted?

4) *GEMC simulations. *When I try to run the GEMC example from
https://www.cp2k.org/howto:gemc by submitting the input file
GEMC_NVT_box1.inp while having in the same folder the files
GEMC_NVT_box2.inp, bias_template.inp, and topology_atoms_WAT.psf, the
simulation doesn't run correctly (the output file is attached). I don't get
any error messages but the simulation gets stuck after it outputs
" Beginning the Monte Carlo calculation." I haven't modified any of the
input files so I don't know what can possibly be wrong.

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