[Pw_forum] confused about nonzero total charge

[Axel Kohlmeyer]

On Tue, 2 Sep 2008, Paolo Giannozzi wrote:

PG> Fan Yang wrote:
PG> 
PG> >  I want to calculate the charge of atoms in cell, and I'm not sure 
PG> > whether the process is correct. [...]
PG> > Once I obtain the charge of every atom in unit cell, the total charge of 
PG> > each atom does not equal 0, which confused me because the system of 
PG> > total charge is set as 0.
PG> 
PG> As explained approximately 1001 times in this mailing list,

paolo,

does that mean, we have to start calling you "scheherazade" now?

<g>,
   axel.

PG> "the charge of an atom in a molecule/solid" typically depends on a
PG> particular choice of atomic orbitals over which wavefunctions are
PG> projected. The true total charge is zero, the charge you are
PG> talking about is not. From code projwfc.f90 :
PG> 
PG>      psum = SUM(charges(:,:,:)) / nelec
PG>      WRITE( stdout, '(5x,"Spilling Parameter: ",f8.4)') 1.0d0 - psum
PG>      !
PG>      ! Sanchez-Portal et al., Sol. State Commun.  95, 685 (1995).
PG>      ! The spilling parameter measures the ability of the basis provided
PG>      ! by the pseudo-atomic wfc to represent the PW eigenstates,
PG>      ! by measuring how much of the subspace of the Hamiltonian
PG>      ! eigenstates falls outside the subspace spanned by the atomic basis
PG> 
PG> Paolo
PG> 

-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.