[DFTB-Plus-User] Fw: DFTB+ 18.2 NPT atoms going out of cell

[Jorge Laranjeira]

Dear DFTB+ programmers

I'm a new user of DFTB+ (using version 18.2) and I'm trying to use NPT molecular dynamics on some polimeryzed C60 phases but I'm having some problems.

Using some particular cells the atoms start getting out of the cell  even if the tag "KeepStationary = Yes" is present. A lot of times when this happens the final temperature is higher than the given threshold. I  tried different thermostats however the outcome was always the same (so it shouldn't be the ice cube effect). Although if I stop the calculation at some step, then translate the atoms back to the cell (making all fractional positions to be between 0 and 1), and then continue the calculation (without giving the velocities and charges) it will go as it should (making the temperature profile as required). Furthermore I tried to restart the calculation without changing the positions to be sure that it was the problem and indeed the threshold of temperature was again overshoot (I'm attaching this input). Normally this overshoot of temperature happens with a huge deformations of the cell. If you could elucidate me on why the keep stationary tag is not working properly and on whats happening afterwards it would be much appreciated.

Besides this when I try different description of the system (all of them with the same number of atoms) the transformation is deferent. Should this happen since it is exactly the same structure just described in a different cell?

Finally I would like to ask how is the pressure applied to the cell. It seems that if we give XXgpa of pressure it applies XXgpa in each lattice vector although in the provided literature it isn't clear how the program works.  Furthermore I would like to request the inclusion of a pressure matrix in future versions.

 Best Regards,

Jorge Laranjeira




________________________________
From: Jorge Laranjeira
Sent: Monday, March 4, 2019 6:16 PM
To: dftb-plus-user at mailman.zfn.uni-bremen.de
Subject: DFTB+ 18.2 NPT atoms going out of cell


Dear DFTB+ programmers

I'm a new user of DFTB+ (using version 18.2) and I'm trying to use NPT molecular dynamics on some polimeryzed C60 phases but I'm having some problems.

Using some particular cells the atoms start getting out of the cell  even if the tag "KeepStationary = Yes" is present. A lot of times when this happens the final temperature is higher than the given threshold. I  tried different thermostats however the outcome was always the same (so it shouldn't be the ice cube effect). Although if I stop the calculation at some step, then translate the atoms back to the cell (making all fractional positions to be between 0 and 1), and then continue the calculation (without giving the velocities and charges) it will go as it should (making the temperature profile as required). Furthermore I tried to restart the calculation without changing the positions to be sure that it was the problem and indeed the threshold of temperature was again overshoot (I'm attaching this input). Normally this overshoot of temperature happens with a huge deformations of the cell. If you could elucidate me on why the keep stationary tag is not working properly and on whats happening afterwards it would be much appreciated.

Besides this when I try different description of the system (all of them with the same number of atoms) the transformation is deferent. Should this happen since it is exactly the same structure just described in a different cell?

Finally I would like to ask how is the pressure applied to the cell. It seems that if we give XXgpa of pressure it applies XXgpa in each lattice vector although in the provided literature it isn't clear how the program works.  Furthermore I would like to request the inclusion of a pressure matrix in future versions.

 Best Regards,

Jorge Laranjeira

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