  The projector augmented wave (PAW) method of Bl\"ochl linearly maps smooth
pseudo wavefunctions to the highly oscillatory all-electron DFT orbitals.
Compared to norm-conserving pseudopotentials (NCPP), PAW has the advantage of
lower kinetic energy cutoffs and larger grid spacings at the cost of having to
solve for non-orthogonal wavefunctions. We earlier developed orthogonal PAW
(OPAW) to allow the use of PAW when orthogonal wavefunctions are required. In
OPAW, the pseudo wavefunctions are transformed through the efficient
application of powers of the PAW overlap operator with essentially no extra
cost compared to NCPP methods. Previously, we applied OPAW to DFT. Here, we
take the first step to make OPAW viable for post-DFT methods by implementing it
in real-time time-dependent (TD) DFT. Using fourth-order Runge-Kutta for the
time-propagation, we compare calculations of absorption spectra for various
organic and biological molecules and show that very large grid spacings are
sufficient, 0.6-0.8 Bohr in OPAW-TDDFT rather than the 0.4-0.5 Bohr used in
traditional NCPP-TDDFT calculations. This reduces the memory and propagation
costs by up to a factor of 5. Our method would be directly applicable to any
post-DFT methods that require time-dependent propagations such as GW and BSE.
