# Import the KohnSham module from ATK
from ATK.KohnSham import *

# Load molecule from VNLFile
h2o = VNLFile("h2o.vnl").readAtomicConfigurations()["h2o"]

# Set up Kohn-Sham for two exchange correlation functionals
gga_calculation = KohnShamMethod(exchange_correlation_type=GGA.PBE).apply(h2o)
lda_calculation = KohnShamMethod(exchange_correlation_type=LDA.PZ).apply(h2o)

# Perform the actual calculations
gga_spectrum = calculateMolecularEnergySpectrum(gga_calculation).energies()
lda_spectrum = calculateMolecularEnergySpectrum(lda_calculation).energies()

# eV = electronVolt

print '   Energy eigenvalues for water (in eV)\n'
print '-----------------------------------------'
print '  LDA\t\t  GGA\t\t  diff\t'
print '-----------------------------------------'

for (E1,E2) in zip(lda_spectrum,gga_spectrum):
    e1 = E1.inUnitsOf(eV)
    e2 = E2.inUnitsOf(eV)
    print '%7.3f\t\t%7.3f\t\t%7.3f' % (e1,e2,(e1-e2))