I want to calculate dipole of one water molecule in siesta. I need to find electronic and ionic partitions.there is electric dipole (debye) in output file but electronic and ionic partitions has not displayed.
can every one help me in case?
thanks
Dear Sabike,
I have never worked with Siesta, but I know it uses pseudopotentials. From that information alone, one can infer the bare charge of the ions in the molecule, that is +4 for O and + 1 for H. I assume that the program also computes the angle at the vertex of the V-shaped molecule and the optimum O-H distance. The total bare ionic dipole is given by the sum of the two dipole vectors directed from H-O,
with the charges +1 on H and +2 on O (since the latter appears twice). We are not used to compute the electric dipole moment when there are two charges of the same sign at the beginning and at the end of the vector. This can be transformed into an (anti-) symmetric situation by subtracting the charge 1.5 from both sides.
- (i.e. adding a vector with dipole moment zero). So now you have two dipole vectors with charges (+1/2) and (-1/2) separated by the O-H distance and with a known angle betwee them. Their sum is the bare ionic dipole of the molecule.
Dear Sabike,
I owe you an apology. The number I gave you for the bare ionic charge of oxygen is wrong. The bare ion is constituted of the nucleus, with charge +8 and the two 1s electrons, so that it is +6 and not +4. So the two dipoles have a charge +1 on the hydrogen and +3 on the oxygen, and one has to subtract the charge 2 from both ions to get an (anti-) symmetric situation with charge +1 on the oxygen and -1 on the hydrogen. Each of the two dipoles is directed from H to O and has the size 1 x d, where d is the H-O distance. The total ionic dipole is obtained by summing the two vectors and is equal to 2x1xd cos (α/2), where α is the angle between the two O-H bonds.
Mea maxima culpa!
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