  We evaluate the frequency error from distributed cavity phase in the caesium
fountain clock PTB-CSF2 at the Physikalisch-Technische Bundesanstalt with a
combination of frequency measurements and ab initio calculations. The
associated uncertainty is 1.3E-16, with a frequency bias of 0.4E-16. The
agreement between the measurements and calculations explains the previously
observed frequency shifts at elevated microwave amplitude. We also evaluate the
frequency bias and uncertainty due to the microwave lensing of the atomic
wavepackets. We report a total PTB-CSF2 systematic uncertainty of 4.1E-16.
