[DFTB-Plus-User] simulation of a water box with periodic boundary conditions

Ben Hourahine benjamin.hourahine at strath.ac.uk
Mon Mar 26 10:54:10 CEST 2012


Hello Mingjun,

again the gamma point should be sufficient. The use of k-points are required for calculation
of numerical integration across the Brillouin zone, which is in turn a consequence of the
wave-functions for periodic systems being Bloch waves.

These waves are similar in form to the Fourier transform of the molecular orbitals, so that
states which are strongly localised in real space (molecular orbitals for water or peptides
for example) do not show much variation over the Brillouin zone in Fourier (reciprocal) space.
As a result, the integrals can be very well approximated by using a single k-point at the origin.



On 26/03/12 08:44, mjyang wrote:
> Dear Benjamin,
>
> Thanks for your reply.
> What will be the propriate KPointsAndWeights values for a protein solvated into a water box?
> I am not very clear about the relation between the delocalization of MOs in the system and the values of KPointsAndWeights set.
>
> Many thanks.
>
> Mingjun
> ________________________________________
> From: dftb-plus-user-bounces at dftb-plus.info [dftb-plus-user-bounces at dftb-plus.info] On Behalf Of Benjamin Hourahine [benjamin.hourahine at strath.ac.uk]
> Sent: Monday, March 26, 2012 5:15 AM
> To: User list for DFTB+ related questions
> Subject: Re: [DFTB-Plus-User] simulation of a water box with periodic boundary  conditions
>
> Dear Mingjun,
>
> 1) it is only the length of the box sides (and their angles) which matter, so your example is correct.
>
> 2) Water has strongly localised molecular orbitals, so the gamma point is probably sufficient for
> a periodic calculation (as always with numerical parameters, you should test the convergence of the
> property you want to calculate with respect to increasing the grid):
>
> KPointsAndWeights = { 0.0 0.0 0.0 1.0}
>
> Depending on what you are calculating, you may also need to use dispersion, 3rd order and damping.
>
> ________________________________________
> From: dftb-plus-user-bounces at dftb-plus.info [dftb-plus-user-bounces at dftb-plus.info] On Behalf Of mjyang [mjyang at hku.hk]
> Sent: 25 March 2012 11:30
> To: dftb-plus-user at dftb-plus.info
> Subject: [DFTB-Plus-User] simulation of a water box with periodic boundary      conditions
>
> Dear all,
>
>      I am going to simulate a water box under periodic conditions and have some questions:
> 1. can I set the LatticeVectors as follows if the box size is 45.1049*42.5232*45.5762:
>   LatticeVectors [Angstrom] = {
>          45.1049 0. 0.
>          0. 42.5232 0.
>          0. 0. 45.5762
>      }
>
> Or I should set it by the exact value of the minimum and maximum coorinates the box occupied along x, y, z axis?
>
> 2. If I am only interested in the converged energy and charges of the system, how should I set the Hamiltonian, especially the keywords KPointsAndWeights?
>
> Many thanks.
>
>
> Mingjun Yang
>
> _______________________________________________
> DFTB-Plus-User mailing list
> DFTB-Plus-User at dftb-plus.info
> http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
> _______________________________________________
> DFTB-Plus-User mailing list
> DFTB-Plus-User at dftb-plus.info
> http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
> _______________________________________________
> DFTB-Plus-User mailing list
> DFTB-Plus-User at dftb-plus.info
> http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user

-- 
      Dr. B. Hourahine, SUPA, Department of Physics,
    University of Strathclyde, John Anderson Building,
           107 Rottenrow, Glasgow G4 0NG, UK.
     +44 141 548 2325, benjamin.hourahine at strath.ac.uk
    The University of Strathclyde is a charitable body,
registered in Scotland, with registration number SC015263



More information about the DFTB-Plus-User mailing list