Exercises

Chemical shifts¶

Calculate the chemical shift between the two -CH$_2$ and -CF$_2$ ionization energies, $1s \rightarrow \pi^{\ast}$ core-excitation energies (XAS), and $\pi \rightarrow 1s$ core-decay energies (XES) of 1,1-difluoroethene (structure below). How do the chemical shifts of the different spectroscopies compare, and what do you think is the reason for any (dis)similarlities?

c2h2f2 = '''
C       0.0000000000     0.0000000000     1.3836545197
C       0.0000000000     0.0000000000     0.0624718520
H       0.9374006976     0.0000000000     1.9085904157
H      -0.9374006976     0.0000000000     1.9085904157
F       1.0780878284     0.0000000000    -0.6951077256
F      -1.0780878284     0.0000000000    -0.6941077256
'''

Basis set augmentation¶

Consider the ionization energy of neon, as calculated with $\Delta$SCF. Using a 6-31G* basis set, add a single s-function at a time with different exponents. Plotting the resulting IE as a function of the exponent, where is the resulting IE closest to experiment? What does this tell you?

Ground-state model for XAS¶

Starting with the ground-state model used to calculate X-ray emission spectra, adapt this to instead consider X-ray absorption spectra of 1,1-difluoroethene (energies from $\epsilon_c - \epsilon_v$, intensities from $| \langle \psi_c | \hat{\mu} | \psi_v \rangle |^2$). How does the absolute energies compare to experiment? What about relative features?

The Tamm-Dancoff approximation¶

Adapt the full-space versus CVS-space comparison of X-ray absorption spectra calculated with TDDFT to one using the Tamm-Dancoff approximation. How does the full- versus CVS-space solutions compare? How does the TDA results compare to the full RPA results?