Fixed LR bracketing for rotational invariance - #184
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mjv500
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Looks clean and simple. Single file modified, complex arithmetic correct.
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Since the LR dynamical matrix gradient is purely imaginary at q=0, the rotational invariance bracketing was always zero previously due to neglecting the imaginary part. For some 3D polar materials this broke things. I fixed this while trying to understand things for 2D materials. I tested this on HfO2:
I have seen people report negative phonons for some polar materials. Maybe they could verify this fixes things for them before fully merging this.