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import numpy as np
from ase.eos import birchmurnaghan
def delta(v1: float, B1: float, Bp1: float,
v2: float, B2: float, Bp2: float,
symmetric=True) -> float:
"""Calculate Delta-value between two equation of states.
.. seealso:: https://github.com/molmod/DeltaCodesDFT
Parameters
----------
v1,v2: float
Volume per atom.
B1,B2: float
Bulk-modulus (in eV/Ang^3).
Bp1,Bp2: float
Pressure derivative of bulk-modulus.
symmetric: bool
Default is to calculate a symmetric delta.
Returns
-------
delta: float
Delta value in eV/atom.
"""
if symmetric:
va = 0.94 * (v1 + v2) / 2
vb = 1.06 * (v1 + v2) / 2
else:
va = 0.94 * v2
vb = 1.06 * v2
npoints = 100
dv = (vb - va) / npoints
v = np.linspace(va + dv / 2, vb - dv / 2, npoints)
e1 = birchmurnaghan(v, 0.0, B1, Bp1, v1)
e2 = birchmurnaghan(v, 0.0, B2, Bp2, v2)
return (((e1 - e2)**2).sum() * dv / (vb - va))**0.5
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