# A quick question about input of potentials

Hi, everyone

I have a simple question about input of potentials and need to make sure my understanding is correct. Assume we want to construct a bulk silicon material which composed of two different isotopes: Si28 and Si29. We can define mass for atoms as follows:

element
mass Si28 28.0
mass Si29 29.0
end

In sw2 and sw3 potentials, we should consider all possible combinations of interactions which are shown below:

sw2:
Si28 - Si28
Si28 - Si29
Si29 - Si29

sw3:
Si28 - Si28 - Si28
Si28 - Si28 - Si29
Si28 - Si29 - Si29
Si29 - Si28 - Si28
Si29 - Si29 - Si28
Si29 - Si29 - Si29

If we use the same parameters to simplify the problem, we just need to copy and paste the input of potential many times and then change pivot and terminal atoms. For example, the input of sw2 would be:

sw2 atom1 atom2 A rho B rmin rmax
Si28 core Si28 core 15.2781 2.0951 11.6032 0.0000 3.7712
sw2 atom1 atom2 A rho B rmin rmax
Si28 core Si29 core 15.2781 2.0951 11.6032 0.0000 3.7712
sw2 atom1 atom2 A rho B rmin rmax
Si29 core Si29 core 15.2781 2.0951 11.6032 0.0000 3.7712

Is that a correct way to created the model? If there is any mistake, please let me know. Any suggestion will be greatly appreciated! Thank you.

Best

Jonas

In GULP if you use just Si for the potentials it will apply to all species (i.e. Si27 and Si28) and so
there is no need to use all the different permutations in the input.

Hi Julian,

Thank you for your reply!. Another question just arises to me. After I give the input of sw potential, if I further apply another potential to the interaction between Si27 and Si28, will previous assignment be overwritten or not? If so, how can we use multiple potential? So many thanks.

Best

Jonas

Multiple potentials are fine - they just add to each other and nothing is overwritten & so you can create complex functional forms by overlay as much as you like.