from __future__ import print_function
import sys
import functools
import numpy as np
[docs]def get_band_gap(calc, direct=False, spin=None, output=sys.stdout):
"""Calculates the band-gap.
Parameters:
calc: Calculator object
Electronic structure calculator object.
direct: bool
Calculate direct band-gap.
spin: int or None
For spin-polarized systems, you can use spin=0 or spin=1 to look only
at a single spin-channel.
output: file descriptor
Use output=None for no text output.
Rerurns a (gap, k1, k2) tuple where k1 and k2 are the indices of the
valence and conduction k-points. For the spin-polarized case, a
(gap, (s1, k1), (s2, k2)) tuple is returned.
Example:
>>> gap, k1, k2 = get_band_gap(silicon.calc)
Gap: 1.2 eV
Transition (v -> c):
[0.000, 0.000, 0.000] -> [0.500, 0.500, 0.000]
>>> print(gap, k1, k2)
1.2 0 5
>>> gap, k1, k2 = get_band_gap(atoms.calc, direct=True)
Direct gap: 3.4 eV
Transition at: [0.000, 0.000, 0.000]
>>> print(gap, k1, k2)
3.4 0 0
"""
kpts_kc = calc.get_ibz_k_points()
nk = len(kpts_kc)
ns = calc.get_number_of_spins()
e_skn = np.array([[calc.get_eigenvalues(kpt=k, spin=s)
for k in range(nk)]
for s in range(ns)])
e_skn -= calc.get_fermi_level()
N_sk = (e_skn < 0.0).sum(2) # number of occupied bands
e_skn = np.array([[e_skn[s, k, N_sk[s, k] - 1:N_sk[s, k] + 1]
for k in range(nk)]
for s in range(ns)])
ev_sk = e_skn[:, :, 0] # valence band
ec_sk = e_skn[:, :, 1] # conduction band
if ns == 1:
gap, k1, k2 = find_gap(N_sk[0], ev_sk[0], ec_sk[0], direct)
elif spin is None:
gap, k1, k2 = find_gap(N_sk.ravel(), ev_sk.ravel(), ec_sk.ravel(),
direct)
if gap > 0.0:
k1 = divmod(k1, nk)
k2 = divmod(k2, nk)
else:
k1 = (None, None)
k2 = (None, None)
else:
gap, k1, k2 = find_gap(N_sk[spin], ev_sk[spin], ec_sk[spin], direct)
if output is not None:
def sk(k):
"""Convert k or (s, k) to string."""
if isinstance(k, tuple):
s, k = k
return '(spin={0}, {1})'.format(s, sk(k))
return '[{0:.3f}, {1:.3f}, {2:.3f}]'.format(*kpts_kc[k])
p = functools.partial(print, file=output)
if spin is not None:
p('spin={0}: '.format(spin), end='')
if gap == 0.0:
p('No gap!')
elif direct:
p('Direct gap: {0:.3f} eV'.format(gap))
p('Transition at:', sk(k1))
else:
p('Gap: {0:.3f} eV'.format(gap))
p('Transition (v -> c):')
p(' ', sk(k1), '->', sk(k2))
return gap, k1, k2
def find_gap(N_k, ev_k, ec_k, direct):
"""Helper function."""
if N_k.ptp() > 0:
return 0.0, None, None
if direct:
gap_k = ec_k - ev_k
k = gap_k.argmin()
return gap_k[k], k, k
kv = ev_k.argmax()
kc = ec_k.argmin()
return ec_k[kc] - ev_k[kv], kv, kc