# [Pw_forum] Bonding Energy

Paolo Giannozzi giannozz at nest.sns.it
Wed May 21 11:04:07 CEST 2003

Hi

> There is no disagree, sure more k-points for small
> systems as I quoted in my previous mail, but \gamma
> point for a big cell (supercell) with large cell
> parameters even there is just one atom.

for molecular systems, the choice of Gamma is justified
even if the supercell is not especially big. If Michal wants
to calculate the energy of the Ni_2 dimer (is this correct?)
Gamma is fine, and a large supercell is not required.

> you should better use a simple cubic lattice (ibrav=1)
> or even better a free lattice (ibrav=0, and take a look to example3)

in principle, however, one can use whatever form of the
supercell, provided that it is big enough to keep periodic
replicas at a sufficient distance.

> remember that atomic coordinatives are  written scaled
> to the lattice parameter ( celldm(1) )

this is the default, but it is overridden by the option in the
ATOMIC_POSITIONS card:

> ATOMIC_POSITIONS{angstrom}

means that input atomic positions are in Angstrom
(they are reprinted in a0=celldm(1) units on output)

Warning: transition metals are nasty beasts. You may have
trouble with both pseudopotentials and DFT. LDA badly
overestimates cohesive energies.

Paolo

--
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
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