[CPMD-list] Test of NPT CPMD simulation

[Juerg Hutter]

Hi

> Dear All:
>
> Now I am trying to understand how to implement a reasonable NPT CPMD simulation. So I took one hydrogen molecule in a cubic box as the simplest test example. Following the turtorial of CPMD (day2.pdf). In order to start a CPMD simulation I followd the instructions to optimize wavefunctions first(same as inp1). Then I added several lines into inp2 to start a NPT CPMD simulation with temperature controlled by rescaling velocities. The modified input file is shown below:
> &CPMD
>  MIRROR
>  MOLECULAR DYNAMICS
>  RESTART WAVEFUNCTION COORDINATES CELL
>  EMASS
>  200
>  TIMESTEP
>    2
>  TEMPCONTROL IONS
>    300  30
>  PARRINELLO-RAHMAN NPT
>  MAXSTEP
>   10000
>  TRAJECTORY SAMPLE
>   5000
> &END
> &SYSTEM
>  SYMMETRY
>    1
>  CELL
>   8.0  1.0    1.0   0.0 0.0 0.0
>  CUTOFF
>   30.
>  PRESSURE
>   0.0
>  ISOTROPIC CELL
> &END
> &DFT
>  FUNCTIONAL LDA
> &END
> &ATOMS
> *H_MT_LDA.psp
>   LMAX=S
>  2
>     0.00000     0.00000     0.00000
>     0.80000     0.80000     0.80000
> &END
>
> But I found several problems when I checked the output file:
> 1. The symmetry reported in the SUPERCELL section is TRICLINIC instead of original SIMPLE CUBIC. I don't know why this change happened because the original cell and positions of two hydrogen atoms will not change during the previous wavefunction optimization step.

See my last mail -- all calculation will be done in a triclinic cell


>
> ************************** SUPERCELL ***************************
> SYMMETRY:                                              TRICLINIC
>
> 2.  The cell size does not change at all during the simulation although the total stress tensor keeps changing. The cell temperature is also zero all the time. It seemed that the barostat does not adjust the cell length at all. I don't know the reason.
>
> ****************************************************************
> *                      AVERAGED QUANTITIES                     *
> ****************************************************************
>                           MEAN VALUE <x>   DEVIATION <x^2>-<x>^2
> ELECTRON KINETIC ENERGY    0.455337E-04             0.148296E-04
> IONIC TEMPERATURE                302.42                    14.65
> CELL TEMPERATURE                   0.00                     0.00
> DENSITY FUNCTIONAL ENERGY     -1.131362             0.477349E-03
> CLASSICAL ENERGY              -1.129926             0.466355E-03
> CONSERVED ENERGY              -1.129880             0.466823E-03
> NOSE ENERGY ELECTRONS          0.000000              0.00000
> NOSE ENERGY IONS               0.000000              0.00000
> ION DISPLACEMENT            1.04871                 0.669774
> CELL VOLUME                 512.000                  0.00000
>
>   CELL PARAMETERS:
>                         8.00000000     0.00000000     0.00000000
>                         0.00000000     8.00000000     0.00000000
>                         0.00000000     0.00000000     8.00000000
>

Your cell temperature is 0 and with a single molecule in a rather big box
no forces on the box variables are induced, hence no movement of the box.


> ****************************************************************
>   TOTAL STRESS (kB)
>             -29.22596123       -1.07079616        0.70578807
>              -1.07079616      -38.53390080       -0.88527846
>               0.70578807       -0.88527846      -30.41372020
> ****************************************************************
>
> Does anybody know the possible reasons?
>
> BTY, I am also interested to know the meaning of fictitious cell mass and how to set it to an appropriate value during NPT simulation. I have some experience on classical MD simulation. In classical MD simulation we don't have to consider the mass of a cell even in NPT simulation since the cell serves only as a boundary for us to calculate the volume and apply the periodic boundary condition. So we are only interested in cell shape and cell size and adjust them accordingly in a NPT simulation. It seemed that CPMD treats the cell as the additional degree of freedom and assign the cell a fictitious mass and solve the special equation of motion for the cell during a NPT simulation. For electrons the situation is the same. A fictitious mass is assigned to the electrons and the equations of motion for electrons are solved together with the equations of motion for ions(nuclus). A larger electronic mass will allow us to use a larger timestep but also brings the stronger coupling !
 be!
> tween electrons and ions. That's the balance we have to choose for electronic mass. What is the effect of cell mass on a NPT simulation then?

For this information you have to look up the original papers,
I suggest that you start with the Papers of M. Bernasconi et al.
on variable cell dynamics in CPMD.

regards

Juerg Hutter


>
>
>
> Best regards,
> Sincerely,
> Jun WANG
>
> Department of Chemistry
> University of Nebraska-Lincoln
> 536 Hamilton Hall
> Lincoln, NE 68588-0304
> U.S.A.
> E-mail:junwang at unlserve.unl.edu
>       junwang at bigred.unl.edu
>
>
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