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Dear all,

I am trying to do berry-phase calculation of polarization with strain effect. I read through the manual but I still get couple of questions about ionic part.

1. I looked into papers and I find that people usually just give a number of ionic contribution. In VASP, we have to set up DIPOL as the center. The ionic contribution actually varies with different DIPOL choice. The example given in manual is comparing undistorted and distorted structure, so there is a subtraction will cancel this effect. But what should I do if there is no comparison? I mean I just need one number of ionic contribution for one structure, then what DIPOL should I set. Since I am doing strain effect, there is actually not possible for a comparison like the manual example.

2. What will happen to ionic contribution if I just simply double the unit cell? Intuitively I think it might just be doubled, but I cannot find a easy way to prove it. Anybody has any idea?

Thanks in advance.

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