why are the Ewald energy negative and decreaseing as pressure increases?

[pigguo]

Hi, I am calculating the simple test to compress the system in order to find the lattice constant as in the manual examples. The total energy is of quadratic form and the lattice constant is right. However I found that the Ewald energy(even consider the alpha Z term) is negative which, as coulomb interaction between the positive ions, should be positive.
And the lattice constant decreases, Ewald energy is also decreasing which is also not reasonable for electronical energy.
Thank you.

[admin]

alpha Z and the Ewald energy define the electrostatic interaction of the ions in a compensating (neutralizing) electron gas; alpha(z) deals with the divergent (G=0) parts.

[Modey3]

admin,

That doesn't answer the question why the Ewald Energy is decreasing. In my case, for Al, the sum (Ewald energy + alpha Z) decreases with lattice constant from the equilibrium value of 4.05. Even more disturbing is that an isolated atom has a non-zero Ewald Energy. In terms of the physics how can positive ions decrease the energy of the system if the lattice parameter is also decreased? Even if their interaction is screened by electrons the interaction energy should become more positive as the lattice parameter decreases. Any info would be appreciated.

modey3

[tlchan]

For the ion-ion Coulomb energy, it is divergent. There is a term 4pi*Z^2/V/G^2 where G -> 0 in the Coulomb energy. Z is the total ionic charge of the system and V is the volume of the unit cell. As the unit cell decreases in size, this term increases, so the ion-ion energy does increase. Note that this term is dropped from the total energy evaluation because there is a corresponding divergence in the Hartree energy.

[Modey3]

tlchan,

Concerning the divergent term. Are you referring to the alpha(z) component in the Total Energy? If so, isn't that term added into the Total Energy?

Best Regards

modey3