[Pw_forum] Atomic-positions VS nat, ntyp

[Paolo Giannozzi]

On May 3, 2011, at 19:07 , Amin Torabi wrote:

> the Atomic-Position input card, which is mandatory, specifies
> the type and coordinates of each atom in the unit cell. Then,
> my question is why we need to specify the number of atoms,
> number of type of atoms and even celldm in the system input card?

celldm specifies the unit cell: a piece of information that is not
contained in atomic_positions card.

The number of atoms and the number of species could in principle
be deduced from the contents of the atomic_positions card.
This would however require a sizable amount of programming
and introduce a number of undesirable complications into the
code. There is a compromise to be struck between ease of use
by ordinary users and ease of maintainance by developers.

P.
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Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222