Born effective charges (property)

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    "description": "The Born effective charge tensors of the sites of the referenced structure, in units of the elementary charge e.\n\n**Requirements/Conventions**:\n\n- The value is a list over the dimension `dim_sites` of 3x3 matrices over `dim_spatial` x `dim_spatial`.\n  The list MUST have one matrix per site of the referenced structure (the entry the record is a product of), in the order of its sites.\n- The matrix of site kappa holds Z*_{kappa,alpha beta} = (Omega / e) dP_alpha / du_{kappa,beta} at zero macroscopic electric field, where P is the macroscopic polarization, u_{kappa,beta} the displacement of the atom at site kappa, Omega the volume of the referenced cell and e the elementary charge.\n  The first index alpha (the row) is the polarization direction, equivalently the direction of the electric field; the second index beta (the column) is the direction of the atomic displacement.\n- The directions refer to the Cartesian frame of the referenced structure (the frame of `lattice_vectors` and `cartesian_site_positions`).\n- The values are in units of the elementary charge e: a value of 1 means a charge of one elementary charge.\n- The definition does not require the charge-neutrality (acoustic) sum rule, that the matrices sum to zero over the sites, to be enforced; values MAY be given as computed or after a sum-rule correction applied by the producer.\n\nA null value means the quantity is not available or not recorded."
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